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[gill-I‘M“? 9341311.; a]: .1..u1.:.afiUt.:r. “Uplift! Ahcu‘anlrl): - 44.; ‘ H r 14.- I: gt in": 1.. in. I blefHuHa‘J‘fldlc .‘ 03:312.; v‘ q 111‘ nflal” ‘ . 1 '1’}! vi if '\ 1 0,. D " ”H i “I. I'M. .{Jif It}! .?ftsqlffuxm¢flflflluhf » - .. . . u b ‘ ‘I v ‘ ‘ ’ ' I I y- F*.§g!‘ (15> 4» 111111111 111117111111111| u- L’ 3 1293 01572 2071 This is to certify that the dissertation entitled FACTORS AFFECTING IMAZAQUIN AND AC-263.499 PERSISTENCE IN SOIL AND SUBSEQUENT CORN RESPONSE presented by Karen Ann Renner has been accepted towards fulfillment of the requirements for Ph.D. Crop & Soil Sciences degree in ‘ k) Leaps)... OWW Mch'g :’ Major professor \A.) Date 3/31/86 MS U i: an Affirmative Action/Equal Opportunity Institution 0-12771 LIBRARY Michigan State Unlverslty PLACE IN REI'URN BOX to remove this checkout from your record. TO AVOID FINES return on or before date duo. DATE DUE DATE DUE DATE DUE 1.1710102 I M3 :1 I) 1999 I I MSU loAn Affirmative Action/Equal Opportunity Inotltuton WWI FACTORS AFFECTING INAZAQUIN AND AC-263,499 PERSISTEICE IN SOIL AND SUBSEQUENT CORN RESPONSE By Karen Ann Renner AN ABSTRACT OF A DISSERTATION Suhnitted to Michigan State University in partial fulfill-ent of the require-eat for the degree of DOCTOR OF PHILOSOPHY Department of Crop and Soil Sciences 1986 ABSTRACT FACTORS AFFECTING INAZAQIIN AND AC-263J99 PERSISTENCE IN SOIL AND SDBSEQUENT CORN CULTIVAR RESPONSE By Karen Ann Renner Nine corn (Q1 ma_Ls L.) cultivars differed in their sensitivity to imazaqui n (2-(4,5-di hydro-4-methyl -4-(1—methyl ethyl )-5-oxo-1_H_-imidazol - 2-yl)-3-quinolinecarboxylic acid). Cargill 921, Great Lakes 422, and Great Lakes 5922 were among the more tolerant cultivars. while Pioneer 3737 and Stauffer 5650 were among the less tolerant cultivars. \Corn injury was 26% greater across all cultivars the first year of study. Preplant incorporated applications were significantly more persistent than preemergence surface applications of imazaquin as determined by soil extraction and analysis and corn bioassays. There was no effect of Spring tillage on injury to corn planted into imazaquin residue. Imazaquin dissipated rapidly during the first 30 days after application. A decreased dissipation rate occurred in the next 120 days. Adsorption of imazaquin and AC-263,499 (2-(4,5-dihydro-4-methyl-4- (1-methylethyl)-5-oxo-lflfimidazol—2-yl)-5-ethyl-3-pyridinecarboxylic acid) on five different soils was very lowu Kd values ranged from 0.056 to 0.113 for imazaquin, and 0.060 to 0.761 for Ail-263,499. The soil that contained the most organic matter, smectite clay, and iron adsorbed the greatest amount of imazaquin. The soil with the highest silt percentage and lowest pH had the greatest AC-263,499 adsorption. Removal of organic matter had no effect on imazaquin adsorption, but increased AC-263,499 adsorption. Ph was lowered by organic matter removal, which could have effected AC-263,499 adsorption. Adsorption of imazaquin and AC-263,499 decreased as the pH of one soil was increased from 3.0 to 8.0. Imazaquin was 42% more phytotoxic to corn than Ail-263,499 at the same application rate. Soil pH appeared to have no effect on AC- 263,499 phytotoxicity to corn. As soil pH increased, phytotoxicity to corn from imazaquin decreased in greenhouse and field studies. Imazaquin injury to corn decreased as the time period between application and corn planting increased. Injury to corn planted in June was less at soil pH 5.8 to 6.2, but pH had no effect on imazaquin injury to corn planted in July and August. Injury to corn planted in four soils decreased over time, with corn grown on the soil containing the highest percentage of smectite clay, organic matter, and iron having the least injury, possibly due to greater soil adsorption. ACKNONLEDGENENTS The author wishes to express her appreciation to her major professor, Dr. William F. Meggitt, fOr his support in completing this dissertation. Sincere thanks is extended to Dr. Donald Penner for his laboratory assistance, and to Dr. Alan Putnam for his guidance and support in all aspects of this study. A special thanks is extended to Dr. Matthew Zabik and Dr. Stephen Boyd fur their assistance as Guidance Committee members, and to Drs. Max Mortland and Richard Leavitt for their guidance and patient review of the results. A sincere thanks is extended to Carla Billings and Jeff Bunkelmann, whose laboratory assistance in extraction of imazaquin residues from soil was greatly appreciated. Finally, a special thanks is extended to Gary Powell and Jackie Schartzer, whose experience in the weed control program made this thesis attainable, and the experience enjoyable. ii TABLE OF CONTENTS PAGE LIST OF TABLES.. ................................ ..... ......... vi LIST OF FIGURES ooooooooooooooooooooooooooooo ooooooo oooooooooo VIII CHAPTER 1: LITERATURE REVIEN ................................. I THE IMIDAZOLINONES ........................................ l CHEMICAL ANTIDOTES ..... ... ................................ 6 VARIETY TOLERANCE T0 HERBICIDES ........ . ..... .... ......... 8 PESTICIDE PERSISTENCE ........... ....... . ..... ...... ....... ll TRANSFER PROCESSES .. ..... ................................. l5 Mechanical dilution ............ . .............. .. ....... l5 Leaching and mobility .... ...... ........................ 15 Volatilization ......................................... 17 Plant uptake and release ............... .... ............ 18 $01] adsorption O...O.......OOOOOOOOOI......OOOOOIOOOOI. ‘9 Adsorption to organic matter ... .......... ...... ...... . 27 Adsorption by amorphous iron and aluminum .............. 28 Soil pH effect on adsorption of ionic compounds.... ..... 29 Desorption ............................................. 32 TRANSFORMATION PROCESSES .0.........OOOOOOOOOOOOIOOOOO ..... 32 Biological transformation .............................. 33 Nonb101091ca] tranSformtion ....OOOOOOOOOIIOOOOOOOIOOOO 34 Hydro‘ys‘s 0.0.0.... ...... O...OOOOIOOOIIOOOOOOOOOOOO. 34 PhOtOIySis 00............OOOOOOOO0.0.000000000000000. 35 Rate of herbicide transformation ....................... 37 ENVIRONMENTAL CONDITIONS AFFECTING TRANSFER AND TRMSFORMATION PROCESSES ......OOOOOOO......OOO... ...... O. 39 $01.] mi5ture ....OOOOOOOOOOOO ...... O ..... O ......... O... 39 Soil temperature ....................... ........ ........ 42 STUDYING PERSISTENCE 0F HERBICIDES ........................ 43 Residue sampling .................... .............. ..... 43 LITERATURE CITED ..... . ....... ..... ....... .... ............. 46 TABLE OF CONTENTS (cont.) PACE CHAPTER 2: RESPONSE OF CORN CULTIVARS TO IMAZAQUIN .......... 62 ABSTRACT .................................................. 62 INTRODUCTION . ............................................. 63 MATERIALS AND METHODS . .................................... 64 RESULTS AND DISCUSSION .................................... 65 LITERATURE CITED ..... ..................................... 78 CHAPTER 3: INFLUENCE OF HERBICIDE APPLICATION METHOD, RATE, AND TILLAGE 0N IMAZAQUIN PERSISTENCE IN SOIL ................ 81 ABSTRACT .................................................. 81 INTRODUCTION ............................... ............... 82 MATERIALS AND METHODS ................... . ............ ..... 84 RESULTS AND DISCUSSION .................................... 88 LITERATURE CITED .......................................... 121 CHAPTER 4: INFLUENCE OF SOIL PROPERTIES ON ADSORPTION, PERSISTENCE, AND AVAILABILITY OF IMAZAQUIN AND AC-263,499 ...125 ABSTRACT .. ................................... . ........... ...125 INTRODUCTION ...... . ......................................... 126 MATERIALS AND METHODS ...................................... .130 RESULTS AND DISCUSSION ...................................... I34 LITERATURE CITED ............................................ 16] iv TABLE OF CONTENTS (cont.) PAGE CHAPTER 5: EFFECT OF SOIL PM ON IMAZAQUIN AND AC-263,499 ADSORPTION T0 SOIL AND PHYTOTOXICITY TO CORN (222_mgy§ L.) ...... I65 ABSTRACT ...................................................... .165 INTRODUCTION ....... ............................................ 166 MATERIALS AND METHODS .......................................... I70 RESULTS AND DISCUSSION ....................................... ..174 LITERATURE CITED ............ ... ................................ 203 CHAPTER 6: SUMMARY AND CONCLUSIONS ............................... 207 CHAPTER 2: TABLE CHAPTER 3: TABLE 5 CHAPTER 4: TABLE 6 LIST OF TABLES PAGE The influence of imazaquin application method and application rate on corn shoot length in 1984 ..... 66 Corn cultivar response to imazaquin in 1984 across all herbicide rates and application methods........................................ ...... 70 The influence of corn cultivar and imazaquin application method on corn shoot length in 1985...... 7l Rainfall summary for 1984 and 1985. Corn was planted and imazaquin applied on May 16, 1984, and May 8, 1985. .............. ... .................... 75 Summary of rainfall in 1984 and 1985 occurring between each of the imazaquin soil sampling periods...l12 Characteristics of soils utilized in soil adsorption and availability studies............................. 131 Calculated distribution coefficients (Kd) for imazaquin and AC-263,499 on five soils ......................... l40 vi LIST OF TABLES (cont.) PAGE CHAPTER 4 (cont.): TABLE 8 Correlation coefficients (r) of soil properties to the availability and persistence of imazaquin and AC-263,499 ........................................... 156 CHAPTER 5: TABLE 9 Calculated distribution coefficients (Kd) for imazaquin and AC-263,499 on a Hillsdale sandy loam adjusted to three pH levels ...................................... 174 LIST OF FIGURES (cont.) PAGE CHAPTER 3 (cont.): FIGURE 8 10 ll 12 13 14 Imazaquin distribution and persistence in the soil profile in 1984 where 70 g ai/ha was applied and incorporated.......................................... 99 Imazaquin distribution and persistence in the soil profile in 1984 where 70 g ai/ha was preemergence surface-applied... ....... ....................... ...... 10l Imazaquin distribution and persistence in the soil profile in 1985 where 70 g ai/ha was applied and incorporated..........................................103 Imazaquin distribution and persistence in the soil profile in 1985 where 70 g ai/ha was preemergence surface-applied.......................................105 Imazaquin dissipation in 1984 and 1985 for preplant incorporated and preemergence surface applications of 280 g ai/ha........................................107 Imazaquin dissipation in 1984 and 1985 for preplant incorporated and preemergence surface applications of 70 g ai/ha.........................................lO7 Effect of spring tillage on imazaquin injury to corn across all imazaquin treatments ........... .. ......... .109 ix LIST OF FIGURES (cont.) CHAPTER 3 (cont.): FIGURE 15 CHAPTER 4: FIGURE 16 17 18 19 20 21 22 23 24 PAGE Effect of imazaquin applications from 1984 on corn height in 1985 ........................................ 118 The adsorption of imazaquin on five soils ............. 135 The adsorption of AC-263,499 on five soils ............ 137 The comparison of the adsorption of imazaquin and AC-263,499 on a Decatur silty clay loam soil .......... 141 The adsorption of imazaquin and AC-263,499 on a Capac sandy 10am soil without and with 2.1% organic matter................................ ........ 143 Corn response to imazaquin and AC-263,499 across all four soils and removal dates............ ..... .....146 The chemical structures of imazaquin and AC-263,499...148 Corn height reduction from imazaquin and AC-263,499 across all herbicides and removal dates ............... 150 Corn height reduction from imazaquin and AC-263,499 for each soil across all removal dates ............ .“153 The removal date by herbicide interaction across all four soils for experiments 1 and 2 .................... 158 CHAPTER 5: FIGURE 25 26 27 28 29 30 31 32 33 LIST OF FIGURES (cont.) PAGE The adsorption of imazaquin and Ac-263,499 on a Hillsdale sandy loam soil adjusted to three pH levels..175 Chemical structures of imazaquin and AC-263,499, with possible protonation sites denoted with an *......179 The effect of soil pH on injury to corn from 26 and 53 g ai/ha of imazaquin and AC-263,499 when surface- irrigated..............................................182 The effect of soil pH on injury to corn from 26 and 53 g ai/ha of imazaquin and AC-263,499 when sub- irrigated......................... ..... ................184 Corn height reduction from imazaquin and AC-263,499 across all soil pH levels under surface and sub- irrigation....................................... ...... 187 The effect of planting date on imazaquin injury to corn planted in 1985 across all soil pH levels.........l90 . The comparison of imazaquin injury to corn planted in June 1984 and June 1985 across all pH 1evels...........l92 The effect of soil pH on imazaquin persistence across all imazaquin application rates in 1985................l95 The effect of soil pH on imazaquin application rate across all planting dates in 1985 ................ . ..... 197 xi LIST OF FIGURES (cont.) 1 PAGE CHAPTER 5 (cont.) FIGURE 34 The effect of soil pH on corn injury from imazaquin at both application rates in June 1984 and June 1985...200 xii CHAPTER 1 LITERATURE REVIEH THE IHI DAZIHJHMES Imazaquin (AC-252,214) (2-(4,5-dihydro-4-methyl-4-(1-methyl ethyl )- 5-oxo-lfl-imidazol-2-yl )-3-quinolinecarboxy1ic acid) and AC-263,499 (2- (4,5-di hydro-4-methyl-4- (1-methyl ethyl )-5-oxo-11;1_-imi dazol-Z-yl )-5- ethyl-3-pyridinecarboxyl ic acid) are two compounds in the imidazolinone class of herbicides (Figure 1). Both compounds can be applied to the soil and incorporated or can remain on the surface. They can also be applied foliarly for postemergence weed control. Soybeans (Glycine mag (L.) Merr.) are tolerant to imazaquin. Soybeans, field beans (Phaseolus sp.), peas (£12911! sp.), and seedling and established alfalfa (Medicago sativa L.) show tolerance to AC-263,499 (3, 4, 90). These herbicides control both monocots and dicots, with selectivity apparently achieved by differential metabolism of the compounds to non- herbicidal forms (108). Imazaquin controls common cocklebur (Xanthium strumarium), common lambsquarter (Chenopodium album), smartweed (Polygonum sp.), vel vetleaf (Abutilon theoghrasti), redroot pigweed (Amaranthus retroflexus), and common ragweed (Ambrosia artemisiifolia) when soil-appl ied (4). Giant foxtail (Setaria faberi), vel vetleaf, redroot pigweed, and jimsonweed (Datura stramonium) are very Figure l: The chemical structures of imazaquin and AC-263,499. mmvfimm . 0< EzcmumE. I I 0112 /OMIZ / \ Z \ \Z \ z n _ \ z / O 4 susceptible to soil applications of AC-263,499, while yellow nutsedge (Cygrus esculentus), wild proso millet (Panicum miliaceum), crabgrass (Digitaria sp.), and barnyardgrass (Echinochloa crusgalli) are somewhat susceptible (3, 90). Imazaquin and AC-263,499 are translocated in the xylem and phloem to meristematic regions with only a small percentage remaining in the root after root uptake (90, 109). with foliar application, movement occurs both above and below the treated leaf (90, 109). Injury symptoms first appear in meristematic tissue and growth ceases soon after treatment. Chlorosis and tissue necrosis are seen first, fol lowed by dieback of mature plant parts and death of the entire plant within two to three weeks (108, 110). Imazaquin is rapidly metabolized by soybeans, but slowly metabolized by cocklebur (90, 109). Velvetleaf has shown increasing tolerance to imazaquin with age due to greatly reduced absorption by older leaves and more rapid ,metabolism in older plants. Two-leaf velvetleaf is susceptible while three-leaf velvetleaf appears tolerant to imazaquin (109). The primary mechanism of action of the imidazolinone herbicides is believed to be inhibition of the biosynthesis of the three branched chain amino acids valine, leucine, and isoleucine, via inhibition of acetohydroxyacid synthase (AHAS) (8, 90, 108). AHAS is one of four enzymes catalyzing the biosynthesis of these amino acids from pyruvate. It is the first enzyme in the biosynthesis of leucine and valine, and the second enzyme in the biosynthesis of isoleucine (8). Protein synthesis is disrupted thus interfering with DNA synthesis and cell growth (90). In maize leaves, DNA synthesis was inhibited in 8 hours with a corresponding decrease in the level of soluble proteins and an increase in the level of free amino acids (110). 5 Sulfonyl ureas such as chlorsulfuron (2-ch1oro-fl-(((4-methoxy-6- methyl-l,3,5-triazin-2-yl)amino)carbonyl)benzenesulfonamide) are active at different concentrations but produce similar physiological effects and are similar in their mechanism of action (8, 108). Two forms of AHAS may be present in corn (99). Imazaquin activity is on a different form than for chlorsulfuron (99). Few papers have been published on the persistence and behavior of imazaquin and AC-263,499 in soil. Imazaquin and AC-263,499 are quinoline and pyridine acids, respectively, and are not considered to be volatile (American Cyanamid personal communication). They are both weak acids with pKa values of 3.8 and 3.9, respectively (3, 4). Imazaquin is not chemically hydrol ized in the presence of distilled water. In the presence of light under aqueous conditions, rapid photolytic hydrolysis to nonactive metabolites occurs (American Cyanamid, personal communication). It is not known whether this photolytic hydrolysis occurs on the soil surface or how soil moisture levels or soil matrix pr0perties affect this reaction. Liu found little adsorption of imazaquin on five U.SC surface soils, Ca-montmorillonite, and Ca-organic matter after shaking 20 h at room temperature (72). He found greater adsorption and lesser desorption at low pH levels, with NaCl ULJ M) more effective than water in displacing the adsorbed herbicide. AC-263,499 applications of 35 to 140 g ai/ha have no effect on corn (_Z_e_a mays L.) or wheat (Triticum aestivum L.) as rotational cr0ps, but could effect cotton (Gossypium hirsutum L.), sorghum (Sorghum bicolor Lt), potatoes (Solanum tuberosum L"), rapeseed (Brassica napus L.), sugarbeets (Beta vulgaris L.), and rice (Oryza sativa L.) (90). 6 Imazaquin rotational restrictions have not been published, but corn- injury has been noted the year following imazaquin application (96L Because of the problem of crop injury from carryover of these herbicides, an approach to the persistence»of these compounds can be ' fourfold: (1) avoid planting a sensitive rotational crop, (2) use a seed or chemical protectant to minimize crap injury to a sensitive crop, (3) plant a resistant variety of a sensitive crap, and (4) alter cultural practices such as tillage, application method, or herbicide rate to reduce the persistence of these compounds in the soil. The following chapters focus on the third and fourth approach in determining the potential use of these compounds in Michigan by altering the persistence so that sugarbeets, potatoes, and corn could be planted as rotational crops after imazaquin or AC-263,499 were used for weed control in soybeans. CHEMICAL ANTIDOTES Chemical antidotes (crop safeners) can be used to increase the selectivity of a herbicide so that higher application rates can be used, or the herbicide applied on semi-tolerant crops, or on a crop that has inconsistent tolerance. NA (1H,3H_-naptho(1,85d)-pyran-1,3- dione) is a chemical safener used for many plant species against a wide range of herbicides (58, 99, 111). CDAA (2-chloro-N4N-di-2- propenylacetamide) and R-25788 (LN-dial lyl-2,2-dichloroacetamide) protect corn from EPTC (S-ethyl diprOpyl carbamothioate) injury, although their mode of action is not fully understood (111). R-25788 elevates the level of GSH (glutathione), and the activity of several 7 enzymes (99). R-25788 increased the amount of EPTC required to injure corn ten times, and decreased the influence of temperature and moisture on EPTC injury (25, 26). Cyometrinil ((2)- _a.((cyanomethoxy)imino)benzeneacetonitri1e) and flurazole (phenylmethyl- Z-chloro-4-(trif1uoromethyl)-5-thiazole carboxylate) are two protectants that are applied to seed to protect sorghum from metolachlor (2-chloro-_N_-(2-ethyl-6-methy1phenyl)-N_-(2-methoxy-1- methylethyl)acetamide), alachlor (2-chloro-N-(2,6-diethylphenyl)-N_- (methoxymethyl)acetamide), and acetochlor (Z-chloro-N-(ethoxymethyl)-N—- (2-ethy1-6-methylphenyl)acetamide) (111). The exact mechanism is not known, although they may compete for sites at which acetanilides act and thus prevent injury (111). Recent work by Rubin and Casida (58) and Hatzios (99) has examined the effect of these protectants on corn to chlorsulfuron. Hatzios found NA the most protective, CGA 43089 (_a_- (cyano-methoxy)-imino-benzeneacetonitri1e) and CGA 92194 (141,3- doi xol an- Z-yl -methoxy)-imino-benzeneacetonitri l e) partial 1y protective, and R-25788 the least protective when chlorsulfuron was applied preemergence to corn (58). In contrast, Rubin and Casida found chlorsulfuron injury to corn to be alleviated by both NAand R-25788 (99). They found R-25788 effectively protected corn hybrids from chlorsulfuron, and corn inbreds from EPTC. They suggested that R-25788 elevated AHAS activity to compensate for chlorsulfuron-induced inhibition. Two fOrms of AHAS are present in corn that differ in their sensitivity to chlorsulfuron. R-25788 did not change the pr0portion of sensitive or insensitive forms, did not function in vitro, and the antidote action therefore appeared to involve enzyme synthesis (99). 8 Imazaquin is selective for different AHAS forms than chlorsulfuron (99). Barrett found CGA-92194 to protect sorghum from imazaquin injury in field experiments (15, 16). In greenhouse experiments with CGA-92194, flurazole, NA, and R-25788, he found CGA- 92184 protected sorghum and corn from imazaquin, whereas R-25788 did not (15). Sorghum was protected more than corn by all four chemical antidotes (15, 16). In further laboratory experiments Barrett found that NA and CGA-92194 reversed imazaquin inhibition of AHAS in vivo in both the dark and the light but did not affect activity in vitro (16). In dark-grown plants, NA and CGA-92194 increased extractable levels of AHAS (16). This did not occur in the light (16). The protectants did not decrease imazaquin uptake or alter distribution, but appeared to increase imazaquin metabolism. Corn varieties varied in their response to imazaquin and degree of imazaquin metabolism (16). VARIETY TOLERANCE T0 HERBICIDES Differential tolerances of genotypes to the same herbicide have been reported for several crops (7, 16, 25, 34, 45, 82, 96, 98, 101, 138). Tolerant genotypes are useful in physiology studies to determine the basis for herbicide selectivity, the inheritance of resistance to herbicides, the herbicide tolerance in inbred lines in corn breeding, and the develOpment of resistant lines for specific herbicides. Potato and soybean cultivars were found to vary in tolerance to .metri buzin (4-amino-6-(1,1-dimethy1 ethyl )-3-(methyl thio)-1,2 ,4-tri azi n- 5(4H)-one) (38, 54, 88). 0plinger found differences of 3-55% leaf kill in soybean injury response to metribuzin (88). Andersen found 9 80% of the variation in soybean injury response to atrazine (6-chloro- N-ethyl-N_'-(1-methylethyl)-1,3,5-triazine-2,4-diamine) was attributable to seed size variation (6). Brinkman found tolerance differences among oat (Avena sativa) cultivars to atrazine residues, but concluded that differences did not appear great enough to justify development of tolerant varieties (21). Differential response of corn varieties to herbicides has been extensively reported in the literature (16, 25, 34, 37, 40, 45, 82, 96, 138). In early studies, resistance of corn plants to simazine (6-chloro- LIP-Methyl-1,3,5-triazine-2,4-diamine) and atrazine was related to stalk rot and corn borer resistance (5). As herbicide resistance increased, the resistance to these insects increased and was correlated to the glucoside level present in the corn plant (5). The MS selection of the maize inbred line GT 112 was very sensitive to atrazine and , simazine with plant death occurring before plants reached the height of 15.24 cm. The reaction was controlled by a single recessive gene (34, 45). In contrast, corn tolerance to alachlor has been studied and differential tolerance among inbreds and hybrids occurred (37, 82). However, researchers were unable to use inbred tolerance to predict hybrid tolerance, and no conclusive pattern of alachlor tolerance inheritability was found (37, 82). Application of diclofop (HOE 23408) ((1)-2-(4-(2,4- dichlorOphenoxy)phenoxy) propanoic acid) for volunteer corn (F2 generation) control produced a range of injury responses due to the difference in susceptibility of the inbred lines (7, 40). The gene action was believed to be additive, and control led by more than one locus (7, 40). 10 Certain corn lines vary in sensitivity to EPTC and to butylate (S- ethyl bis(2-methylpropyl)carbamothioate) (25, 26, 101, 138, 139). XL 22, XL 43, and XL 80A were found to be the most sensitive to EPTC. Wright found tolerant.P3030(N)|ilants absorbed less and metabolized more butylate than sensitive PAG 644(N) (138). Environmental conditions were found to also be important, with certain hybrids injured more at higher temperatures and others at lower temperatures (139). Burt found EPTC and butylate reduced corn growth more at 30°C than at 20° C (26). An interaction occurred between moisture and temperature with greater injury at 33% moisture than at 15% moisture at 30°C, and the opposite response occurring at 20°C (26). The days prior to corn coleoptile emergence were the most critical time for injury (26), with shallow planted corn exhibiting less injury because the cole0ptile emerged quicker (25, 139). Trifluralin (2,6-dinitro-N_,_N- dipropyl-4-(trifluoromethyl)benzenamine) injured corn with hybrid sensitivity varying with environmental conditions (98). Certain hybrids were more sensitive to trifluralin at 15°C than 25°C, with soil moisture having a lesser but significant effect (98). An isolation of a maize cell line which is resistant to the imidazolinones has been identified (110). This resistant cell line contains an altered AHAS which is I“) longer inhibited by the imidazolinones. Resistance is expressed on the whole plant level and appears to be inherited as a single, co-dominant trait (110L American Cyanamid Co. has signed a research agreement with Pioneer Hybrids for further development of the cell line (110). PESTICIDE PERSISTENCE The persistence of'a herbicide in the soil is a function of the chemical properties of the herbicide, soil properties, and prevailing environmental conditions (2, 23, 29, 60, 61, 64, 80). Ideally a compound should persist long enough to control weeds during the critical weed interference period, but not long enough to lead to herbicide residue problems and injury to rotational craps (11, 61, 74). When herbicide activity is no longer evidenced the herbicide is believed to be non-persistent, when in fact it can still be present in the plant root zone and be unavailable due to immobilization, or be present at a concentration that does not injure the crop or weed species (61). Because of the exceptional bioactivity to corn by the imidazolinones, and the large number of acres rotated to corn from soybeans, an understanding of imazaquin and AC-263,499 persistence in the soil and how it can be altered by environmental and cultural practices is important. Man can alter the persistence of pesticides by changing cultural practices. Changing til‘Lage methods, applying a lower application rate, and using a different application technique are three ways that the persistence of a compound can be altered. Herbicide persistence can be mechanically altered by tillage that dilutes the herbicide in the plant root zone (111). The extent of detoxification depends on the resulting herbicide concentration and the sensitivity of the rotational crop. Although acute toxicity may be 11 12 avoided, the prolonged exposure may result in injury as the plants no longer have untreated soil for the roots to grow in (111). Aaberg found no difference in visual injury to alfalfa, soybean, and oats from ethofumesate ((1)-2-ethoxy-2,3-dihydro-3,3-dimethyl-5-benzofuranyl methanesulfonate) when the soil was spring plowed or disked prior to planting (1). Trifluralin was found to persist longer where the soil was moldboard plowed instead of disked (36). Triazines remain active longer under conventional plow and chisel plow systems than under no- till situations (17,113). This was attributed to a lower pH under no-till that causes triazines to bind and become chemically'hydrolized, an increase in moisture under no-till that increases hydrolysis, and an increase Hiorganic matter under no-till that increases adsorption (113). However a decrease in soil pH under no-till conditions is not always observed as shown by studies conducted by Lowder and Weber (73). The acidic soil surface in no-till from the application of acidifying nitrogen fertilizer that increases atrazine adsorption and hydrolysis did not occur because the soil surface was limed and the acidic fertilizer effects were negated (73). Kells et. al. found unextractable 14C residues were higher under no-till situations in Kentucky and attributed this to either greater breakdown of atrazine or increased reversible adsorption (66). Applying minimal rates of a herbicide reduces the concentration of herbicide initiallyu and increases tolerance to slower degradation rates. Oliver and Frans found the amount of trifluralin remaining in soil was directly correlated to the initial application rate after six months (87). Regardless of environmental conditions, herbicide degradation rates are believed to be independent of initial 13 concentration (142). Buchanan and Hiltbold found no differences in the half life of atrazine at different rates in the south (22). However Hance and McKone found the degradation rate of atrazine and linuron (N'-(3,4-dichlorophenyl)-_N_-methoxy-N-methylurea) decreased as their concentration in the soil increased, possibly due to a limited number of highly active enzyme and chemical reaction sites where degradation of these herbicides in soil could occur (53). The effect of initial concentration on degradation was greater under sterile conditions where microbial degradation was not a factor. They speculated that microbial degradation is not as affected by initial concentration as nonbiological processes (53). Hurle and Walker found a lengthening of the lag phase prior to degradation at higher herbicide concentrations which they attributed to either herbicide toxicity to the degrading organism or inhibition of enzymes necessary fOr degradation (64). In contrast to this, multiple applications of certain compounds can gradually increase the degradation rate in the soil due to microbial adaptation to the herbicide (111). Low levels of substrate may be insufficient to induce adequate enzyme production or sustain microbe activity, and thus at low concentrations of pesticide, degradation could be very slow (69, 111). Persistence of some herbicides can increase when other pesticides are applied at the same time that are a preferred energy source (111). Application method can have a significant effect on pesticide persistence. If a compound is volatile or subject to photodecomposition, preemergence surface applications (PES) are less persistentt Walker and Bond found pendimethalirl(AC-92,553)(Nf(1- ethylpropyl)-3,4-dimethyl~2,6-dinitrobenzenamine) more persistent under 14 dry conditions and where incorporated (130). Twenty weeks after application, 80% of the incorporated herbicide could be detected, while only 28% where it was applied PES. They believed this difference would be less in a wetter year. Hilliams and Eagle found the persistence of dichlobenil (2,6-dichlorobenzonitrile) to be much greater when incorporated due to decreased codistillation and volatility (136). Oliver found 25% of the trifluralin remained four months after application when incorporated to a IIL16 cm depth, but only five percent when incorporated to a five cm depth, and zero when applied on the soil surface (87). In other research where trifluralin was applied at 0.84 and 4.48 kg/ha, 11% remained after forty weeks when incorporated to a five cm depth, and 40% remained where incorporation was to a ten cm depth (103). In contrast, Buchanan found the persistence of atrazine not to differ between PES applications and . atrazine applications incorporated to a 10-15 cm depth with two passes of a disk (22). Hal ker found no difference in herbicide loss between surface and incorporated applications of propyzamide (3,5-dichloro (N: 1,1-dimethy1-2-propynyl)benzamide)(127). Persistence under different application methods can be affected by soil type. Fluridone (I-methyl-3-pheny1-5-(3-(trif1uoromethyl) phenyl)-4(1H)-pyridinone) persistence on a Miller clay showed no difference between PPI and PES applications, but was significantly more persistent when incorporated compared to PES applications on a fine sandy loam (14). The authors gave no explanation for this occurrence. Herbicides are inactivated by biological, chemical, and physical means (9, 11, 23, 31, 60, 61, 80, 111). Inactivation can result from transfer of the pesticide molecule to a location that is unavailable to 15 plants, or transformation of the herbicide to nontoxic forms by degradation. The means and rate of these processes is dependent on the nature of the herbicide and on environmental conditions. Transfer processes that alter the persistence of a herbicide include mechanical dilution or tillage as previously mentioned, leaching, volatilization, plant uptake or release, and soil adsorption (11, 23, 111). Transformation processes include biological transformation by microbes, and nonbiological degradation by chemical and photochemical means. Each of these processes will now be discussed. TRANSFER PROCESSES Mechanical Dilution Tillage was mentioned previously as a cultural method that alters soil persistence of herbicides by removing or diluting the herbicide in the plant root zone. Leaching and Mobility Herbicides move in soil by molecular diffusion or by the mass flow of water. Leaching results in herbicide movement out of the plant root zone. Herbicide movement is dependent on water flux, soil-water content, soil texture, and the herbicide concentration in solution. Herbicides may leach under saturated conditions because water is a competitor for soil adsorption sites. Acidic herbicides are more readily leached than neutral or cationic herbicides due to the decreased number of sites in the soil that are positively charged and available for ionic binding of these herbicides (41, 47, 48, 56, 119k 16 The balance between capillary movement upward and the downward hydraulic gradient create the area in the soil profile where the herbicide will be distributed (41L. Herbicide distribution obtained by laboratory leaching experiments is seldom seen under field conditions because frequent changes in water movement direction in the field occur, and/or under laboratory studies adsorption equilibrium is not reached (41, 119). Stoller et. al. found bentazon (3-(1-methylethyl)- (1H)-2,1,3-benzothiadiazin-4(3fl)-one 2,2-dioxide) to move with the solvent front in soil thin layer chromatography and soil columns in the laboratory (119). However in the field less than 2% was leached from the tap 15 cm of soil. He concluded that less than 1.0 meg/100 kg of soil anion exchange would be required to satisfy the exchange capacity of 3.36 kg/ha of bentazon applied, and therefore the herbicide did not leach in the field (119). In laboratory studies, Grover found picloram (4-amino-3,5,6-trichloro-Z-pyridinecarboxylic acid) to be readily leached in a wave form with the greatest mobility in a fine sandy loam (47, 48). He found picloram to leach farther on sandy soils when the soil column was dry prior to herbicide application because of faster water movement, and on clay soils to leach farther when the column was initially wet because of the increased percolation rate (47). Grover also found picloram to move readily upward when soil columns were subirrigated (47), and Harris had similar results with dicamba (3,6- dichloro-Z-methoxybenzoic acid), another acidic herbicide (56). Herr et.1al. applied picloram at five rates to three soils and sampled three times (59). Herbicide residues were greatest on heavy textured soils, and little movement below the soil surface occurred. 0n lighter textured soils movement was detected down to a 61 cm depth, with 17 increasing concentration with increasing depth. Application rate can alter pesticide movement. Grover found picloram movement in leaching columns did not change when applied at a 1 or 211 rate (47). However Duseja and Holmes found in field studies less trifluralin movement below the application zone at higher application rates (33L They attributed this to the limited solubility of the compound. Volatilization Herbicide persistence can decrease immediately if volatility occurs, and therefore herbicides susceptible to volatilization are more effective when incorporated. Soil moisture is important in herbicide 'volatilization (12, 23, 92, 130). ‘Vapor pressures increase under moist conditions, and herbicide loss from dry soils is less (12, 23, 92, 130L. 0n dry soils volatile herbicides can be adsorbed, but the binding energy is low enough that the herbicide is biologically available» At higher moisture levels, less herbicide is adsorbed resulting in more being susceptible to vapor loss (12,23). Trifluralin was readily volatilized from wet soil, and weed control decreased when incorporation was delayed 96 hours (117). Walker and Bond found trifluralin and pendimethalin to be volatile from a metal surface with a 70% loss in 14 days, and a 95% loss in 28 days (130). (Mia film of soil particles wetted daily, 95% of the trifluralin and 60% of the pendimethalin were lost in 28 days, but when the soil surface remained dry, only 30% of both compounds were lost in a four week period (139). EPTC volatilization from soil increased as temperature increased when the soil was moist, but temperature had no effect on dry soils (92). 18 EPTC losses also increased with increased air movement across the soil surface, and was dependent on soil composition (92). Beestman found 50% of applied alachlor was lost in 12 to 29 days when exposed to 3.2 hn/h air movement, with loss dependent on soil type (18). Parochetti found 50% of the alachlor and metolachlor volatilized in 8 days when applied to a glass surface, but only 0.1% volatilized when applied to the soil surface (89). Plant Uptake and Plant Release Plant uptake may also remove persistent herbicides from the soil. Once in the plant the herbicide can be metabolized or stored. Plant uptake may cause only temporary removal of a herbicide, as conjugated herbicides can be hydrolytically released during plant decomposition and thus available to the next year's cr0p(23,111). Tolerant plant species can be planted to absorb and degrade herbicides when a tolerant crop can be planted into the field containing the persistent herbicide. In contrast, plant roots can release bound herbicide residues, which results in the herbicide appearing to increase in persistence. In the rhizosphere, plant roots excrete a variety of organic and inorganic compounds that can influence the growth of fungi and bacteria, and change the soil pH (55, 67, 105, 141). Plant roots may thus alter the amount of herbicide available fOr uptake by controlling the ionic state of the herbicide and penetration into the plant, or by indirectly changing the herbicide's adsorption to the soil colloid, and thus altering herbicide availability (67, 105, 141). All plants can reduce rhizosphere pH when (NH4)zso4 is applied (55). This could increase herbicide adsorption to soil colloids, and/or plant absorption 19 of herbicides that become undissociated at low soil pH (55). Lowering soil pH also disperses humic materials into fibrous networks, which could release herbicides entrapped in internal voids of the organic matter structure (141). lhonocots are more effective than dicots at raising the soil pH when CaN03 is applied (55). This can result in rapid desorption of adsorbed herbicides to the soil. Applications of NH4+ can also disrupt ionic bonds and release herbicides. Yee, Heinberger, and Khan studied the release of bound residues of prometryne (LE-bi s(1-methyl ethyl )-6-(methyl thio)-1,3,5-triazine-2,4- diamine) by oats, soybean, and wheat roots (141). Soybeans released 37% of the bound residues and wheat 10% of the bound residues. They attributed this release of bound residues to increased microbial activity in the rhizosphere, because altering soil pH and adding fertilizer did not correlate with the release of bound prometryne , (141). Soil Adsorption Soil adsorption affects the amount of herbicide available for effective weed control, the amount of herbicide to be leached through the soil profile, and the ability of the herbicide to be microbially degraded (43). As soil adsorption increases the availability and subsequent phytotoxicity to plants decreases (23). Soil adsorption can increase or decrease pesticide persistence (23). Clay has an infinite number of adsorption sites, and the pesticide molecules can be distributed across the clay surface so that interaction with microbes is delayed. Also if pesticides are adsorbed in interlamellar spaces they can be protected from enzymatic attack (43). In contrast, soil 20 adsorption can accelerate degradation by concentrating the enzyme and substrateeat one site, and catalyzing nonbiological and biological reactions (43). ‘Therefore soil adsorption can result in less chemical available for plant uptake, but if the herbicide becomes desorbed at a later date, extensive injury to rotational craps may occur (23, 43). Adsorption of herbicides to soil can be divided into three categories; chemical adsorption, physical adsorption, and hydrogen bonding (2, 12, 27, 43, 80, 91, 118). Types of low energy bonds involved in physical adsorption include van der Haals, charge transfer, and charge dipole bonds. Van der Haals forces are additive and occur by an interaction between polar and polarizable moieties in the pesticide>molecule and the ionic charges on soil and the hydration water around the soil micelle (2, 12, 13, 91). Attraction between large herbicide molecules and soil constituents can be great. Charge- transfer bonds between electron deficient aromatic rings and electron rich aromatic rings of organic matter also increase physical adsorption (41). Physical adsorption results in low heats of adsorption and low binding strength (43). Chemical adsorption is due to coulombic forces resulting from charge interactions (shared electrons) and results in higher heats of adsorption and higher binding strength than dipole interactions (2, 43, 91). High energy bonds include ionic bonds and ligand exchange. Ionic chemical bonds occur with cationic herbicides such as paraquat (lgr-dimethyl-4-4A-bipyridinium ion), weak acids such as benzoic acids, and weak bases such as the triazines (91). Ligand exchange is another type of chemical bonding that can occur between herbicides and transition metals in clays or humics (27). Anions can penetrate the coordination shell of an iron or aluminum atom in the 21 surface of the hydroxide and get incorporated into the surface hydroxyl layer (106). Hydrogen bonding is sometimes considered a third method of adsorption as the forces are intermediate between physical and chemical adsorption (2L. Hydrogen bonding involves a bond between two highly electronegative atoms through the medium of a hydrogen atom (2). Hydrogen bonds can occur between adsorbed water and organics or between soil surface groups and organics (41). Adsorption isotherms are a very useful technique used to characterize properties of herbicide adsorption in the presence of an adsorbing surface such as soil (41, 42, 44, 122). A pesticide solution of known concentration is equilibrated with a given quantity of soil, and the amount of pesticide adsorbed is calculated from the decrease in pesticide concentration in solution. It is usually'expressed in umoles adsorbed per gm of soil (x/m). By repeating the measurement at several concentrations, an adsorption isotherm can be obtained by plotting the quantity adsorbed (x/m) versus the herbicide concentration remaining in solution (C) at equilibrium, expressed in umoles/ml of solution. The shape of the isotherm is diagnostic to the mechanism underlying the adsorption process (41, 42). In most instances a straight line can be obtained from the isotherm plot if both x/m and C are plotted as log transformations. The Freundlich adsorption equation is then calculated as x/m = KC to the 1/n power, where K and n are constants. 1/n is the slope of the line and the constant K provides a measure of the extent of adsorption (41, 42, 44, 76%. N is usually less than one because the amount of herbicide adsorbed increases less rapidly than the concentration remaining in solution at equilibrium. When n is near unity, the constant K (the distribution coefficient) can be used to 22 describe the partitioning of the herbicide between soil and solution. The adsorption of a chemical can be described as a partitioning of the chemical between the soil and aqueous medium and is expressed by a partition soil sorption constant Kd (76, 118). The coefficient Kd is = to pesticide adsorbed (umoles/kg of soil) / pesticide in solution (umoles/liter). 'This constant can be used to provide a measure of the extent of soil adsorption, and determine the importance of various soil parameters on adsorption. Kd is related to the organic content of the soil by the equation Koc - (Kd / % organic carbon) x 100. Soil adsorption is dependent on experimental conditions, herbicide properties, and soil pr0perties such as the clay content and type of clay, organic matter content, amorphous minerals such as aluminum and iron hydroxides, and soil pH (2, 12, 13, 27, 28, 43, 80, 91, 135, 137). Because the imidazol inones are weak acids, only adsorption of ionic compounds will be discussed. Herbicide pr0perties such as electronic structure, molecular volume, and water solubility affect herbicide adsorption to the soil matrix (27). Functional and substituting groups are important in soil adsorption, and related positioning can enhance or hinder intramolecular binding (27). Organic compounds are preferential 1y adsorbed on clays instead of inorganic compounds because of their large size and high molecular weights (27). Acidic herbicides exhibit specific types of binding to soil. Acidic compounds can physicallyradsorb to soil. ‘They also can bind with soil through a CFO group, or associate with soil through a water bridge on an exchangeable cation, or through a hydrogen bonding of a carboxyl group to clay via proton association (12, 13). Three 23 mechanisms by which phenoxyalkanoic acids were thought to be bound to soil included van der Naals forces, hydrogen bonds at low soil pH between the carbonyl group of the herbiciderand the amino groups of humics or the carboxyl group of the herbicide and the carbonyl or NH groups of organic matter, or via salt linkages with polyvalent cations linking the carboxyl group of humics in the soil with that of the alkanoic acids (83). Frissel found no adsorption of 2.4-0, 2,4,5-T, MCPA ( 4-chloro-2-methylphenoxy)acetic acid), and DNBP (2-(1- methyl propyl )-4,6-dinitr0phenol) to montmoril lonite clays, implying none of these types of binding occurred (39). Carringer et. al. summarized the soil adsorption of acidic compounds, and stated that moderate adsorption to organic matter may occur, and very low adsorption to clays and hydrous metal oxides may also occur (28). Experimental or environmental conditions such as temperature, soil and water ratios, and ionic composition, can alter soil aggregates and change the physical and chemical properties of the soil matrix, thus affecting adsorption (27). Adsorption behavior in the field may differ from herbicide adsorption in the laboratory due to temperature, water, soil pH, or concentration effects (35, 41, 49, 70). Changing the ratio of soil to solution can change the amount of herbicide adsorbed to soil, and thus change the Kd values. In laboratOry studies, increasing the salt concentration and water content of the system increased the adsorption of 2,4,5-T (2,3,5- trichlorOphenoxy)acetic acid) on a Palouse silt loam (70). Farmer and Aochi found picloram adsorption to decrease when soil and solution ratios in the laboratory were widened from 1:2 to 1:5 (35). Gerber and Guth increased precision when a soil to water ratio of 1:1 instead of 24 1:10 was used (41). Grover increased linuron and atrazine adsorption when he changed the soil : water ratio from 1:10 to 4:1 (49). Polar organic chemicals and water compete for soil adsorption sites in ion-dipole and coordination reactions, and with water for ligand positions around soil cations (122). Some organic compounds won't be adsorbed by clay from water suspensions because they cannot compete with the large number of water molecules present. In air dry soils they may adsorb and may or may not be diSplaced from soil when water is added (136). Moisture has a large effect on pesticides if polarizable moieties such as C=0, NHZ, and coon are present (91). Amitrole (lfl-l,2,4-triazol-3-amine) can become protonated by highly polarized water molecules that directly coordinate with aluminum, magnesium, and calcium cations, and when protonated are stable to leaching (100). Clay can bind pesticides through bonds that are , resistant to leaching, while organic matter may form bonds under dry conditions that are released when moisture is added (81,91). Hhen soil drys it is never completely dehydrated. The increased dissociation of the water remaining at the clay surface is an important aspect of clay- water interactions from the standpoint of pesticide adsorption. Burnside and Fenster found carryover of atrazine, prOpazine (6-chloro- M'bisU-methyl ethyl )-1,3,5-triazine-2,4-diamine), and linuron to be greatest on coarse textured soils in drier regions of Nebraska (24). Research on fl uridone under field and laboratory conditions showed persistence in a Miller clay to be significantly less than on a coarse textured Lufkin sandy loam (14). Although there were little differences in fluridone concentrations remaining after 60 days, thereafter residues decreased much more rapidly on the Miller clay, 25 where 10% remained one year after treatment compared with 20% on the sandy loam (14). Soil properties therefore play an important role in the adsorption and persistence of pesticides to soil. The mechanism of pesticide binding to the clay fraction of the soil has been studied in greater detail than soil organic matter or other portions of the soil matrix. Silicate clay minerals consist of stacked layers of silica and alumina sheets (122). Clay minerals including montmoril lonite and vermiculite are considered expanding and limited-expanding clays, respectively. ‘Their specific surfaces are 500-750 mzlg and have cation exchange capacities (CEC) of 80-200 meq/lOO 9. Other clays such as kaolinite and illite are non-expanding clays and have specific surfaces of 25-125 m2/g and cam of 2-40 meq/IOO g (135). Kaolinitic clays consist of one sheet of silica and one sheet of aluminum with the tips of the silica oxygen tetrahedral s projecting into the hydroxyl plane of the aluminum octahedrals and replacing two thirds of the hydroxyl ions (122, 135%. Kaolinitic calys thus have crystal edges containing exposed groups of Al (0H)2+ or Al (0H)+2 that are important sites in adsorption. They make up 10-20% of the total crystal area, while in montmorillonitic soils they make up only 5% (122). Thus surface reactions such as anion exchange are more pronounced on kaolinite, and organic compounds having functional groups that can capture an electron via a hydrogen bond such as nitrite or carbonyl would preferentially adsorb on these edges of kaolinite (2, 104). However only 2.4-0 and chloramben (3-amino-2,5-dichlorobenzoic acid) have exhibited this behavior and conflicting reports have found 2.4-0 phytotoxicity to be similar on kaolinitic clays and 26 montmorillonitic clays, reflecting no increase in adsorption (107). Montmorillonite is a 2:1 clay containing one layer of alumina between two silica layers. The side by side arrangement of silica - oxygen tetrahedrals results in no hydrogen bonding between layers (122). This weak attraction between oxygens lets this mineral expand as water enters and hydrates the exchangeable interlayer cations (122, 135). Small uncharged polar groups of organic compounds compete for the same ligand sites as water around the cation (122, 136). The extent to which the interlamellar surface can sorb organic molecules depends on the nature of the exchange cation on the clay, the degree of clay hydration, and the properties of the organic molecules (43). ‘The nature of the exchangeable cation is important as it determines the surface acidity. Montmorillonite clays are negatively charged due to isomorphous substitution of magnesium for aluminum, and this resulting negative charge is satisfied by exchangeable cations on the soil surface (122, 127, 136). The greater the affinity of the exchange cations for electrons, the greater the interaction with polar groups of organic molecules capable of donating electrons (135%. The most acidic are in order H+ > Fe+3 >A1+3 >Mg+2 > Ca+2 > Na+2 >Kt. The compensating cations are also important because they compete with herbicides for binding sites and act as sites in coordination bonds (27). Picloram is believed to form exchange complexes with copper, and to a lesser extent with iron and zinc, and form chelate rings (10). Exchangeable cations also have a predominant influence on the degree of disassociation of'water molecules on the soil surface, and cation-water relationships influence the mechanism of adsorption of pesticides such as ion exchange and protonation (136). 27 Adsorption to Soil Organic Hatter Soil organic matter can be classified into two main groups, nonhumic and humic substances (106, 118, 137). Unaltered plant and animal material, proteins, fats, waxes, and resins are non-humic substances (118). Humic substances are chemically and biologically modified substances consisting of a series of highly acidic, colored, high molecular weight substances, with a high content of oxygen- containing groups such as carboxylic, phenolic, and aliphatic groups (106, 118, 134, 137). These acidic groups have pH dependent ionization and can form ionic bonds with ionized molecules, and participate in cation-dipole bonds (27). The aromatic portion of the soil organic matter can form charge transfer complexes or hydrogen bonds with pesticides (118). Most organic matter is colloidal in nature, and associated with the inorganic fraction to form a clay-humus complex (23, 118%. llfis complex is the site of soil adsorption and biological and nonbiological degradation (23, 118). The large surface area and chemical nature explain the high adsorptive capacity of organic matter (118). Two major adsorptive surfaces are available to a herbicide; the clay fraction and the clay-humus fraction (118). When soils contain 8% organic matter or less, both the clay and organic fractions contribute to adsorption (118). Soils having greater than 8% organic matter, probably adsorb pesticides exclusively on organic matter surfaces (118). Upchurch found a 5X herbicide rate increase required on a soil containing 20% organic matter compared to a soil with 4% organic matter for twelve cotton herbicides (124L. For soils with similar textures and organic matter contents, the organic matter 28 contribution to adsorption would be greater on kaolinitic soils because of the lower CEC of kaolinitic clays (118). Soils differ in their organic matter quantitatively and qualitatively. The percentage of organic matter as fats, waxes, and resins can range from 2 to 20%, the protein from 15 to 45%, and the carbohydrates from 5 to 25% (118). These fractions have different affinities for herbicides. Organic matter is often cited as the most important soil parameter in correlating pesticide persistence in the soil (23, 118, 124, 134, 137). Upchurch investigated soil factors affecting diuron (N'-(3,4-dichlor0phenyl )-N_,_N_-dimethyl urea) phytotoxicity and concluded that soil organic matter was the most important factor (124). However anionic chlorinated aliphatic acids and benzoic acids including chloramben have shown limited adsorptive response to organic matter (23). Organic anion adsorption by anion exchange on organic matter is not believed to occur because anions are readily released by water. Adsorption to organic matter must therefore be by weak physical forces or hydrOgen bonding, and not by anion exchange (134). Adsorption byTAlorphous Fe and Al Amorphous sesquioxides such as aluminum and iron oxides serve as cementing agents and coat clay and organic matter (2L. They have similar surface areas to montmorillonite, are positively charged, and have a high anion exchange capacity (12). These iron and aluminum hydrous oxides exist in crystalline and amorphous forms and are in intimate contact with the surface of soil colloids. Positive adsorption sites exist on aluminum and iron hydroxides below soil pH of 29 8.0. Organic anions such as acidic pesticides can be associated with these charges by coloumbic attraction.. The anions can be di5placed by raising the soil pH to 8 or 9 (27, 106, 133). In adsorption isotherm studies using different adsorbents, iron oxides adsorbed 6% of the picloram, and organic matter only 1.9% (19). Chlorsul furon had very low adsorption on four soils with Kd values less than 0.28, but had greater adsorption on aluminum oxide resins (123). Soil pH Effect on Adsorption of Ionic Coupounds Soil pH may influence the activity and detoxification of a herbicide by changing the ionic character of the herbicide and the soil colloids, or changing the microbial p0pulation (10, 19, 30, 32, 35, 46, 62, 64, 70, 84, 107, 121, 122, 123, 128, 132). Organic colloids, and to a lesser extent clay, have a pH-dependent charge (118, 134, 137). The pKa of the acidic groups in soil humus is approximately 5.2, and at a pH of 7.0, 85% of these are ionized (118). The organic matter contribution to the soil CEC increases as soil pH increases, and at a pH of 7.0, 50% of the CEC is due to these pH dependent charges, while at a soil pH of 5.0, only 25% of CEC is due to these charges (118). At high soil pH values, the H+ disassociates, and the acidic functional groups can react with divalent cations to form chelate bridges (137). Adsorption of many compounds is dependent on soil pH, and because soil acidity can be altered by common agronomic practices such as liming and fertilizing, the effect of soil pH on herbicide adsorption and degradation is important (78, 84). Ionizable herbicides are affected by soil pH changes. The pH of the soil surface is one to two units below the measured pH of a soil- 30 water suspension (12). Basic herbicides adsorb at 1-2 pH units above their pKa values, and acidic herbicides become undissociated 1—2 units above their pKa value (12). An assumption is made that the pKa of a molecule does not change when the molecule comes in close proximity to the clay mineral surface. This may not be val id as the magnitude of the force fields on the surface or highly active protons may induce a change in the pKa of the molecule (13). Friessel and Bolt found that the adsorption of ionic herbicides increased as the soil pH decreased, with the soil pH of maximum and minimum adsorption being a function of the specific compound (39). Fluridone is a weak base with a pKa of 1.7, and adsorption increased at lower soil 1»! values due to protonation (112). ‘§-triazines were shown to be less phytotoxic at low soil pH values due to protonation and resulting adsorption (77). Heber found triazine adsorption to increase as soil pH decreased, with maximum adsorption near the pKa of each Specific triazine herbicide (132%. Soil adsorption of most weak acid herbicides is relatively low. Dicamba, picloram, and 2,4-0 have Kd values of 0.08, 0.04 to 0.49, and 0.14 to 3.38, respectively (48). Harter and Ahlrichs found 2,4-0, aniline, urea, and chloramben to protonate and adsorb 2-3 units higher than what would be predicted by their pKa values (57). At soil pH values greater than pKa values, acidic herbicides exist in the anionic form, and negative charges on the soil colloids repel anionic herbicides and increase the concentration available for plant root uptake (78, 84). Activity in soils of undissociated anionic pesticides is determined by the interaction with organic materials, and hydrogen bonding to undissociated acid and carbonyl groups of organic matter can occur (137). Increasing the soil pH may make organic matter 31 structure more compact. Herbicides become excluded, and phytotoxicity to plants is increased (112). Hiltbold and Buchanan found the persistence of atrazine to increase with increasing soil pH and heavier texture due to decreased hydrolysis (62). Chlorsulfuron is a weak organic acid with a pKa of 3.8. Phytotoxicity to corn is greater at a pH of 6.9 than at a pH of 5.9 to 4.2 (78, 84). Similar results of increased phytotoxicity at higher soil pH values have been found for 2,4-0 and dicamba (30). Corbin et. al. found no change in phytotoxicity from soil pH of 4.3 to 7.5 for chloramben and picloram, and found dalapon phytotoxicity to increase as the soil pH decreased (30). Activity in soils of anionic pesticides at soil pH values where they are associated is determined by the interaction with organic materials (137). Hydrogen bonding to undissociated acid and carbonyl groups of organic matter can occur (137). Decreased activity above the pKa value of an acidic herbicide may reflect adsorption or complexation with hydroxides or oxides of iron and aluminum, or alternatively, an increase in polarity and dissociation making the chemical unfavorable for uptake (137). Picloram had some adsorption at soil pH values of 6.5 to 7.5 due to bridging to divalent metal ions, or because of hydrogen bonding, or because of some other mechanisms (83). Soil pH can can change herbicide solubility and possibly change soil adsorption. Bailey and Hhite found adsorption of ureas and E' triazines increased as the solubility increased (13). Burns however found the adsorption of ureas to increase as the solubility decreased (23). Harris and warren found no relationship between solubility and adsorption of diquat (6,7-dihydrodipyrido(l,2-a:2',1'-g)pyrazinediium ion), CIPC (l-methyl ethyl 3-chlorophenylcarbamate), DNBP, and atrazine 32 (56). Adams stated that as water solubility decreased, the hydrOphobic nature of pesticides should increase (2), and Carringer et. al. stated that nonionic pesticides with low water solubilities should adsorb the most (28). Soil (Hi can affect the degradation of herbicides (123). Dalapon degraded most rapidly at a soil pH of 6.5 (30). 2.4-0 and dicamba degraded most rapidly at a soil pH of 5.3 (30). Ph had no effect on chloramben and picloram degradation. Increased hydrolysis can occur at low soil pH values, and decreased degradation could occur due to increased soil adsorption, hydrolysis, or because of soil pH influence on the microbial papulations (64). Desorption Hhen compounds are released from soil the process is called desorption. The extent of desorption can be partial, completely reversible, or completely irreversible (118, 137%. Desorption is important in longterm persistence of compounds in soil. Chemicals that are completely'desorbed from clays are often incompletely'desorbed from organic materials (118). TRANSFORllATIm PROCESSES Transformation processes occur by biotic and abiotic means, and it is often difficult to distinguish between the two (11). Commonly used methods of sterilization such as autoclaving produce chemical and physical changes in the soil that may alter the degradation of organic molecules by nonbiological means (11). 33 Biological Transfbrlotion Herbicides can be degraded by algae, actinomycetes, bacteria, fungi, and possibly extracellular enzymes (11, 60, 61, 65, 69). Microbes degrade herbicides by a number of reactions including oxidation, reduction, hydrolysis, decarboxylation, deamination, deal kylation, and conjugation (11, 60, 61, 65, 69). Conditions that promote the growth of microorganisms often accelerate the rate of degradation. These factors include temperature, moisture, soil pH, cation exchange capacity, soil type and structure, organic matter content, oxidation-reduction potential, and dissolved oxygen levels. (11, 61, 65, 69, 111). Microbial degradation is dependent on the threshold level of pesticide required for optimum utilization by microbes (69, 111). If there is no microbial degradation of an applied pesticide, degradation is either dependent on concentration and not enough herbicide was applied, or the pesticide fails to induce the enzymes required by soil microbes to utilize the pesticide carbon for growth, or the pesticide is unable to penetrate the microbial cells, or the steric configuration prevents or hinders microbial attack (60, 61). Organisms that are naturally present often cannot produce the enzymes required to transfonm the herbicide to an intermediate that can enter into common metabolic pathways and be completely mineralized (69). Certain chemical groups appear to increase chemical persistence, and include amines, methoxys, sulfonates, nitro groups,'m-chlorines, ether linkages, and branched carbons (69). Usually a lag phase is seen prior to microbial degradation. If there is none, it may be that microbial breakdown is not involved, or alterrnatively, there are already 34 organisms present in abundance that posess.the constitutive enzymes required for degradation, and no lag phase occurs (60, 65). Adsorption of herbicides to soil components can reduce biodegradation, since the pesticide would be unavailable to microorganisms (11, 60). Organic matter can enhance microbial degradation by enhancing the growth and population of the microbes. However large amounts of organic matter can retard degradation by increasing pesticide adsorption and reducing the availability to microbes by providing a preferred carbon source that is utilized for energy instead of the herbicide (60, 111). Honbiological Degradation Hydrolysis Nonabiotic degradation of herbicides can occur in the air, water, and soil by both chemical and physical processes (9, 11, 23, 31). Initial degradative reactions may be nonbiological and further degradative processes biological, cu: vice versa (11). Herbicides can be oxidized, reduced, hydrolyzed,.and epoxidized,1and nucleOphilic displacement reactions or free radical induced reactions can occur (9, 31). Hydrolysis reactions are more rapid in soil than aqueous systems due to catalysis of reactions by sorption, especially at more acidic soil (HI levels (9). High organic matter content can increase hydrolysis due to additional nucl e0phil ic sources, and because more acidic surfaces are available for hydrolytic adsorption (111). Chemical hydrolysis occurred with the chlorofig-triazines and organo-pr insecticides (9), and more recently with the sulfonylureas (115). 35 Photolysis Two types of photolysis occur in nature; direct and indirect photolysis (74, 79%. Direct photolysis occurs when the herbicide absorbs the radiation from the sun and undergoes a chemical reaction. This type of transformation can occur in aqueous systems (79). In indirect photolysis, another material such as a soil component, absorbs the sunlight and initiates a chemical reaction that transforms the pesticide (74, 79). Many organic chemicals can sensitize, induce, or enhance photochemical reactions, while other organics can slow down or quench reactions (74, 79) Photolysis is influenced by the light intensity and wavelength. UV light of 290-340 nm is important in photolysis reactions, as UV light less than 285 nm is absorbed by ozone and not available to cause photolysis (9%. Only'pesticides that absorb light above 285 nm are expected to undergo photodecomposition unless a photosensitizer induces photolysis (9). PhotOproducts and photolysis rates of several pesticides are affected by the soil particle size, the organic matter content, the mineral base, the light absorbing character of the soil, and the soil- moisture content (79). Herbicide photolysis is reduced on soils compared to aqueous solutions or on a glass surface (23, 79, 89, 130). The major limiting factors to photolysis in soils is the adsorption of light energy by other soil components that decreases the energy available to the herbicide, and the non-penetration of light below the soil surface (9). In soils and natural waters, light attenuation can significantly reduce photolysis rates as the herbicide is mixed vertically in the soil and water profile, and light is completely adsorbed in the upper soil or water layer (79%. Klehr found 80% of 36 thidiazuron (1-phenyl-3-(1,2,3-thiadiazol-S-yl)-urea) applied to soil thin layer plates was photolyzed (68%. He attributed the other 20% that did not undergo photolysis to penetration of the applied herbicide solution into deeper soil layers impenetrable to sunlight (68). llilles and Zabik found the photolysis of fluchloralin (N-(z-chloroethyl)-2,6- dinitroeN-propyl-4-(trifluoromethyl)benzenamine) was reduced on soil thin layer plates compared to photolysis in a methanol:water solution (85). They also found that photolysis of bentazon on soil resulted in formation of a dimer that they didn“t find as a photOproduct in aqueous solution, and concluded that soils contained transition metals that coordinated two bentazon molecules and held them in direct proximity for coupling (86). Liang and Lichtenstein found the most and least loss of azinphosmethyl (OLQ-Dimethyl _S_-((4-oxo-1,2,3-benzo-triazin- 3(4fl)-yl)methyl)phosphorodithioate) on glass plates and on soils, _ respectively. Increased unextractable bound residues were produced by irridating soils having high organic matter and moisture content, showing the role these parameters played in hydrolysis (71%. When soil moisture was increased to 12%, the photolysis of profenofos (Q-(4- bromo-Z-chlorOphenyl )-Q-ethyl _S_-propyl phosphorothioate) and diazinon _ (_0;Q-Diethyl Q-(Z-iSOpropyl-4-methyl-6-pyrimidinyl )phosphorothioate) increased, photolysis of carbaryl (l-napthyl N-methylcarbamate) decreased, and there was no change in parathion (9,9-diethyl Q-p— nitrophenyl phOSphorothioate) photolysis (79). Increased rates of a pesticide for weed control when surface applied may indicate that photolysis is occurring. Hright found that three to five times the rate of trifluralin was needed for chemical weed control when applied PES than when incorporated, and he concluded that photodecomposition or 37 volatility must be occurring (140). Rate of Herbicide Degradation By summing all transformation processes for a given pesticide, a degradation rate can be determined and used to predict persistence. Conflicting degradation rates in the literature may be due to environmental conditions. It is difficult in field studies to distinguish the contribution of degradation from other processes in the disappearance of a chemical (29). Surface runoff, leaching, plant uptake, and volatility must be assessed before degradation can be quantified (29). Soil moisture and temperature can alter the rate and the transformation processes and thus change the persistence of a pesticide in the soil. Field measured hal f-l i ves are often shorter than in the laboratory due to multiple degradative pathways and the influence of soil nunsture content, temperature and substrate concentration (125). Regardless of environmental conditions, herbicide degradation rates are believed to be independent of initial concentration (111, 142). However Hamaker and Goring (51), and Cheung and Lehmann (29) stated that it would be unreasonable to assume a simple rate law, and at very high and very low'concentrations the kinetics of herbicide degradation are very different. Hamaker and Goring felt pesticide turnover was a complex process, and when the initial concentration of herbicide is added to soil there is a period of rapid degradation (51). Over time, a: greater percentage of herbicide becomes 'less available', and degradation slows until it reaches a steady state and the degradation rate is determined by a labile pool. Hurle and Walker agreed that the rate of pesticide 38 degradation is disprOportionatelyr slower at ‘low residual concentrations, and proposed a two compartment soil model with available and nonavailable areas for pesticide location, with only available areas subject to degradation (64). Dissipation of fluridone was found to be very slow following initial rapid disappearance during the first 90 days (14%.Zimdahl and Gwynn saw initial rapid loss of tri f1 uralin followed by a much slower reaction rate (142). Duseja and Holmes found that trifluralin persistence in the field dr0pped rapidly within 9 days after application to one third and one fifth remaining in the 0 to 10 cm depth on a loam and clay soil, respectively (33). Five months after application, only 2.1% and 0.4% of the initial amount applied remained on the loam and clay soils, respectively. In another study trifluralin concentrations decreased rapidly to 10 to 15% of the initial application within one-half year after application, with only 20% remaining after 43 days, followed by a more gradual decrease over time (93). Savage showed similar results on two different soils using trifluralin and nitralin (4-(methylsulfonly)-2,6-dinitro-N_,_N_- dipr0pylbenzenamine) with 60 ppbw remaining after eight months, and 30 ppbw remaining after 18 months (102%.In contrast, Hance and McKone found the degradation rate of atrazine and linuron to increase at low concentrations, suggesting a saturation of active degradation sites at high herbicide concentrations (53). ENVIRONMENTAL CONDITIONS AFFECTING TRANSFER AND TRANSFORMATION PROCESSES Soil Moisture Soil moisture influences transfer and transformation processes, thus altering the persistence of herbicides in soil (2, 20, 23, 24, 75, 126). Excessive soil moisture may leach the compound out of the soil profile and reduce damage to susceptible crops, but this may result in other environmental problems. Soil moisture can alter the vapor pressure of compounds, making them more volatile under moist conditions (23, 130). Adequate soil moisture increases plant uptake during the growing season which may or may not result in decreased persistence, depending on whether the compound is degraded within the plant prior to plant residues being returned to the soil (23). Increased plant phytotoxicity can occur as moisture increases due to an increased amount of herbicide being taken up with water and accumulating in the plant (126%. Yearly variations in plant response to herbicides reflect the difference in herbicide availability to the plant. Changing moisture levels in the soil alter the distribution between the sorbed and solution phases, change the rate of herbicide movement by molecular diffusion or mass flow, and change the amount of herbicide translocated in the plant (126). Adsorption of pesticides to soil colloids may be affected by soil moisture (12, 126). More EPTC is adsorbed in the air dry state than 39 40 at field capacity (12). For non-volatile herbicides, phytotoxicity increases under moist conditions as less herbicide is adsorbed to the soil making more available for plant uptake (23, 24). Dry soils increase herbicide persistence as there is less competition for adsorption sites by water. Hater is very polar and strongly adsorbed to the soil matrix. At low moisture levels the number of water molecules competing for soil adsorption sites decreases, and less polar organic compounds can then adsorb to soil (23). Secondly, higher concentrations of pesticides due to low moisture results in herbicide precipitation out of solution (23,91). At low soil moisture levels the herbicide concentration per unit volume increases, and crystallization can occur that results in decreased herbicide bioactivity (23). Sandy soils have lower water-holding capacities, which may result in herbicide precipitation occurring. Loss of material to the solid phase can be termed sorption to avoid the implication of precipitation or adsorption (106). The soil moisture content and the method in which water is added to soil is critical in determining herbicide availability (20, 50, 52). Hhen i50proturon (M-dimethyl-N_'-(4-(1-methyl ethyl )phenyl )urea) was applied to soil, plants were severely injured after watering from above, but not after sub-irrigation (20). Hhen the soil surface was very moist prior to herbicide application, adsorption was 'normal' and the herbicide leached, especially if watered frequently from above. Hall et al found that metsul furon (2-(((((4-methoxy-6-methyl-1,3,5- triazi n-2-yl )amino)carbonyl )amino)sul fonyl )benzoic acid), chlorsulfuron, picloram, and clapyralid (3,6-dichloro-2- pyridinecarboxylic acid) all showed little activity in sub-irrigated 41 pots when the herbicides were applied to the surface of dry soils (50). when applied to moist soils and sub-irrigated all herbicides showed activity on Canada thistle (Cirsium arvense) except for chlorsulfuron (50). They concluded that either the herbicide was adsorbed on dry soil surfaces and couldn't be displaced by water, or the herbicide precipitated on the dry soil surface and failed to leach. Hance and Embling applied linuron, metribuzin, and simazine to soil that was wetted to 20% field capacity, and to dry soil that was wetted to 20% field capacity 10 minutes or 24 hours after herbicide application (52). When they extracted samples of the soil solution with a pressure membrane cell they found much lower concentrations of herbicide where the soil had remained dry for 24 hours after herbicide application prior to rewetting. They concluded that application to air dry soil makes a herbicide inaccessible to some of the water that is applied . later, and the soil moisture content at the time of application is therefbre a very important factor in performance variability of soil- applied herbicides (52). Soil moisture can affect biological and nonbiological transformation (23, 75, 79, 111, 127). Microbial activity is regulated by soil conditions, as 50-75% of field capacity and 25-35°C are optimum for microbial growth (111). Pronamide degradation at 23°C increased as soil moisture increased from 3.5 to 12.0% with the half-life decreasing from 78 to 29 days (127). Pesticide degradation in soils at 15 bars was slower than at one bar, and more bound residues were formed in moist soils than under very dry conditions at 15 bars (94). Nonbiological transformations are slower under dry conditions (23, 24, 31, 74, 79). Dry soils result in reduced photolysis for some 42 herbicides, and chemical breakdown is decreased because water is needed for hydrolytic transformations and oxidation reactions (23). However in dry soils water is more dissociated, and this may lead to acid- catalyzed hydrolysis (23). Generally pesticides persist longer in dry cool regions than moist warm regions (24). However in a study with ethofumesate, there was a 90% loss.in activity'over a 12 day period when applied to dry soil, and no loss when applied to a wet soil (75). The authors felt this activity loss could be from volatilization, photodecomposition, increased microbial degradation, adsorption, or chemical decomposition (75%. They concluded that volatilization from a dry soil was less likely and microbial degradation on dry soils was also less likely. The ethofumesate loss occurred in the greenhouse where UV light couldn't penetrate, so photodecomposition was not believed to be the cause. Chemical adsorption was ruled out because compounds weren“t released when refluxed with methanol. They concluded that ethofumesate chemical decomposition increased on dry soil. Soil adsorption was greater so chemical hydrolysis increased, and this coupled with the pH of the water film being lower under dry conditions caused increased hydrolysis under dry conditions (75). Soil Temperature Soil temperature affects transfer processes such as volatilization, and transformation processes such as chemical degradation and microbial decomposition (23, 91). Higher temperatures increase herbicide volatilization as vapor pressure is temperature dependent. Higher temperatures can either increase or decrease plant uptake. Under adequate moisture conditions, increased temperature will 43 increase plant uptake. However under drouth conditions, higher temperatures can cause reduced plant uptake or plant death. Transformation processes are dependent on soil temperature, and the degradation rates of most soil-appl ied herbicides increase with temperature (94, 128, 142). Zimdahl and Gwynn found the degradation of three dinitroanilines in the soil to be directly correlated to soil temperature and soil moisture content (142). They found first order degradation at 15°C,but not at 30°C where they had initial rapid loss, and then a slower dissipation rate (142). Rao et al found degradation rates for nine pesticides on seven soils weren't associated with soil, organic carbon, soil pH, cation exchange capacity, percent clay, or total bacteria or fungal propagules, but rather the soil-water content and soil temperature (94). Soil temperatures less than 25°C had a greater influence on degradation, causing a rapid decrease in the degradation rate. When soil temperatures were above 25°C a slight increase in the degradation rate occurred (94). Temperatures may also affect pesticide binding by altering the solubility of the herbicide. As temperature increases the solubility of some pesticides increases, and sorption decreases (91). STUDYING PERSISTENCE OF HERBICIDES Residue Sapling Field residue sampling results show significant variability in the amount of pesticides persisting in the soil which limits precision (63, 95, 97, 120, 129, 131). Variability may be from application method, soil variability, sampling procedure, extraction efficiencies, and 44 other analytical error (63, 95, 97, 114, 116, 120, 129, 131). The herbicide application method can be a major factor in causing increased variability in detectable residue levels (97, 120, 129, 131). Hauchope et. al. found disk harrow incorporation resulted in redistribution of trifluralin, producing areas of poor weed control and other areas of crop damage (131). Robinson found a coefficient of variation (CV) of 42-114% when 0.84 kg/ha of nitralin was applied and incorporated with a disk harrow (97). Concentrations varied widely in soil samples five cm apart, which explained why plant response varied in the treated field. In other research, the wide variability found in simazine recovery persisted throughout the 112 day period that soil cores were taken (129). The CV was reduced to 14% when simazine was applied to smaller plots with a knapsack sprayer. Taylor et. al. found measurements of dieldrin (hexachloro—epoxy-octahydro-endo, exo-dimethanonapthalene) residues to have a 50 -fold variation, and felt it was due to either Spray coverage or redistribution of herbicide by disking to a 7.5 cm depth (120). By increasing the number of cores taken per plot from eight to thirty, the CV was lower (120). They attributed this to the irregular herbicide distribution. Hormann et. a1. concurred, and felt that analytical error was only a small fraction in overall variability, with sampling error giving the greatest variability (63). Twenty cores per plot taken systematically reduced the CV to 20% (63). In the soil, physical, biological, and chemical processes which transport and transform the pesticide in the field vary with the differing composition of the soil area (95). These varying soil properties lead to different dissipation rates, resulting in increased sample variation over time (95). 45 Residue sampling is also complicated by the fact that pesticide extraction efficiency is difficult to determine. Nhen pesticide residues age they become more resistant to solvent extraction due to adsorption to soil colloids and diffusion into the interior of the humic colloids (114, 116%. This makes it difficult to determine a true extraction efficiency as fortified soil results could be largely different than the extraction efficiency from field treated soils (114, 116). LITERATURE CITED Aaberg, D. A. 1981. Environmental factors and the activity of ethofumesate on subsequent crapping systems. Ph.D. Thesis. Michigan State University. pp. 124. Adams, R. S. Jr. 1973. Factors influencing soil adsorption and bioactivity of pesticides. Pages 1-54 in Francis A. Gunther, ed. Residue Reviews. Vol. 47. Springer Verlag, New York. American Cyanamid Co. 1985. Technical information report. Scepter (AC-252,214). American Cyanamid C0. Technical information report. AC-263,499 Experimental Herbicide. 1985. Andersen, R.1L 1964. Differential response of corn inbreds to simazine and atrazine. Heed Sci. 12:60-61.’ Andersen, R.1L 1969. Influence of soybean size on response to atrazine. Heed Sci. 18:162-164. Andersen, R. N. and J. L. Geadelmann. 1982. The effect of parentage on the control of volunteer corn 1ZEE.EE¥§) in soybeans (Glycine max). Heed Sci. 30: 127-131. Anderson, P. c. and K. A. Hibbard. 1985. Evidence for the interaction of an imidazolinone herbicide with leucine, valine, and isoleucine metabolism. Need Sci. 33: 479-483. 46 9. 10. 11. 12. 13. 14. 15. 16. 17. 47 Armstrong, 0. E. and J. G. Konrad. 1973. Nonbiological degradation of pesticides. Pages 123-131 in H. D. Guenzi, ed. Pesticides in The Soil and Hater. Soil Sci. Soc. of Am. Madison, Hi. Arnold, J. S. and H. J. Farmer. 1979. Exchangeable cations and picloram sorption by soil and model adsorbents. Heed Sci. 27:257-262. Ashton, F. M. 1982. Persistence and biodegradation of herbicides. Pages 117-131 in Fumio Matsumara and C. R. Krisha Murti, eds. Biodegradation g: Pesticides. Plenum Press, New York. Bailey, G. H. and J. L. Hhite. 1964. Review of adsorption and desorption of organic pesticides by soil colloids with implications concerning pesticide bioactivity. J. Agr. Food Chem. 12:324-332. Bailey, G. H., J. L. Hhite, and T. Rothberg. 1968. Adsorption of organic herbicides by montmoril lonite: Role of pH and chemical character of adsorbate. Soil Sci. Soc. Am. Proc. 32:222-234. Banks, P. A., M. L. Ketchersid, and M. G. Merkle. 1979. The persistence of fluridone in various soils under field and controlled conditions. Heed Sci. 27:631-633. Barrett, M. 1984. Potential safeners for imazaquin. Proc. NCHCC. 39:39-40. Barrett, M. 1985. Studies on protectant mode of action. Abstr. HSSA. (210). Bauman, T. T. and M. A. Ross. 1983. Effect of three tillage systems on the persistence of atrazine. Heed Sci. 31:423-426. 18. 19. 20. 21. 22. 23. 24. 25. 26. 48 Beestman, G. B. and J. M. Deming. 1974. Dissipation of acetanilide herbicides from soil. Agron. J. 66:308-311. Biggar, J. H., U. Mingel gri, and M. H. Cheung. 1978. Equilibrium and kinetics of adsorption of picloram and parathion with soils. J. Agr. Food Chem. 26(6):1306-1312. Blair, A. M. 1983. Some problems associated with studying effects of climate on the performance of soil-acting herbicides. Pages 379-388 in Aspects of Applied Biology 4, Influence of Environmental Factors o_n_ Herbicide Performance and Crop and Heed Biology. Academic Press, New York. Brinkman, M. A., D. K. Langer, R. G. Harvey, and A. R. Hardie. 1980. Response of oats to atrazine. Crop Sci. 20:185-189. Buchanan, G. A. and A. E. Hiltbold. 1973. Performance and persistence of atrazine. Heed Sci. 21:413-416. Burns, R. G. 1975. Factors affecting pesticide loss from soil Pages 103-141 in E. A. Paul and A. D. McLauren, eds. S_o__i_1_ Biochemistry vol. 4. Marcel Dekker, Inc., New York. Burnside, O. C., G. R. Fenster, G. A. Hicks, and J. V. Drew. 1969. Effect of soil and climate on herbicide dissipation. Heed Sci. 17:241-245. Burt, G. H. 1976. Factors affecting thiocarbamate injury to corn II. Soil incorporation, seed placement, cultivar, leaching, and breakdown. Heed Sci. 24:326-330. Burt, G. H. and A. O. Akinsorotan. 1976. Factors affecting thiocarbamate injury to corn. 1. Temperature and soil moisture. Heed Sci. 24:319-321. 27. 28. 29. 30. 31. 32. 33. 34. 35. 49 Calvert, R. 1980..Adsorption-desorption phenomena. Pages 1-30 in R. J. Hance, ed. Interactions between Herbicides and the S211. Academic Press, New York. Carringer, R. 0., J. B. Heber, and T. J. Monaco. 1975. Adsorption-desorption of selected pesticides by organic matter and montmorillonite. J. Agr. Food Chem. 23:568-572. Cheung, H. H. and R. G. Lehmann. 1985. Characterization of herbicide degradation under field conditions. Heed Sci. 33 (Suppl. 2):7-10. Corbin, R. T., R. P. Upchurch, and F. L. Selman. 1971. Influence of pH on the phytotoxicity of herbicides in the soil. Heed Sci. 19:233-239. Crosby, D. G. 1976. Nonbiological degradation of herbicides in the soil. Pages 65-97 in L. J. Audus, ed. Herbicides: Physiology, Biochemistry, and Ecology. 121; _1. Academic Press, New York. Doehler, R. H. and H. A. Young. 1961. Some conditions affecting the adsorption of quinoline by clay minerals in aqueous suspensions. Clay Min. 9:468-483. Duseja, D. R.iand E. E. Holmes. 1978. Field persistence and movement of trifluralin in two soil types. Soil Sci. 125:41-48. Eastin, E.ii, R. D. Palmer, and C. 0. Grogan. 1964. Mode of action of atrazine and simazine in susceptible and resistant lines of corn. Heed Sci. 12:49-53. ‘ Farmer, H. J. and Y. Aochi. 1974. Picloram sorption by soils. Soil Sci. Soc. Am. Proc. 418-423. 36. 37. 38. 39. 4D. 41. 42. 43. 44. 45. 50 Fink, R. J. 1972. Effects of tillage method and incorporation on trifluralin carryover and injury. Agron. J. 64:75-77. Francis, T. R. and A. S. Hamill. 1980. Inheritance of maize seedling tolerance to alachlor. Can. J. Plant Sci. 60:1045-1047. Friesen, G. H. and D. A. Hall. 1984. Response of potato (Solanum tuberosum) cultivars to metribuzin. Heed Sci. 32:442-444. Frissel, M. J. and G.1L Bolt. 1962. Interactions between certain ionizable organic compounds (herbicides) and clay minerals. Soil Sci. 94:284-291. Geadelmann. J. L. and R. N. Andersen. 1977. Inheritance of tolerance to HOE 23408 in corn. Crop Sci. 17:601-603. Gerber, H. R. and J. A. Guth. 1973. Short theory, techniques, and practical importance of leaching and adsorption studies. Proc. Eur. Heed Res. Counc. Symp. Herbicides in Soil. p.51-68. Giles, C. H. and D. Smith. 1974. A general treatment and classification of solute adsorption isotherms. 1% of Colloid and Interface Science. (47:755-765. Greene, R. E. 1974. Pesticide clay-water interactions. Pages 3-37 in H. D. Guenzi, ed. Pesticides _i_n Soil and Hater. Soil Sci. Soc. of Am., Madison, Hi. Greene, R. E., J. M. Davidson, and J. H. Biggar. 1980. An assessment of methods for determining adsorption and desorption of organic chemicals. Pages 67-72 in A. Barin and U. Kafkafied, eds. Agrochemicals _i_n _t_l_1_e Soil. Pergamon Press, New York. Grogan. C. 0., E. F. Eastin, and R. D. Palmer. 1963. Inheritance of susceptibility of a line of maize to simazine and atrazine. Crop Sci. 3:451. 46. 47. 48. 49. 50. 51. 52. S3. 54. 51 Grover, R. 1971. Adsorption of picloram by soil colloids and various other chemical adsorbents. Heed Sci. 19:417-428. Grover, R. 1973. Movement of picloram in soil columns. Can. J. Soil Sci. 53:307-314. Grover, R. 1977. Mobility of dicamba, picloram, and 2,4-0 in soil columns. Heed Sci. 25:159-162. Grover, R. and R. J. Hance. 1970. Effect of ratio of soil to water on adsorption of linuron and atrazine. Soil Sci. 109:136- 138. Hall, J. C., H. B. Bestman, M. D. Devine, and H. H. Vandenborn. 1985. Contribution of soil Spray deposit from postemergence applications to control of Canada thistle (Cirsium arvense). Heed Sci. 33:836-839. Hamaker, J. H. and C. A. I. Goring. 1976. Turnover of pesticide residues in the soil. Pages 219-243 in D. 0. Kaufman, G. G. Still, C. D. Paulson and S. K. Bandal , eds. Bound and Conjugated Residues. ACS Am Chem Soc. Symp. Ser.29, Hashington, D. C. Hance, R. J. and S. J. Embling. 1979. Effect of soil water content at the time of application on herbicide content in soil solution extracted in a pressure membrane apparatus. Heed Res. 19:201-205. Hance, R. J. and C. E. McKone. 1971. Effect of concentration on the decomposition rates in soil of atrazine, linuron, and picloram. Pesticide Sci. 2:31-34. Hardcastle, H. S. 1974. Differences in the tolerance of metribuzin by varieties of soybeans. Heed Res. 14:181-184. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 52 Harris, G. R. and K. Hurle. 1979. The effect of plant induced changes of pH upon the adsorption and phytotoxicity of s-triazine herbicides. Heed Res. 19:343-349. Harris, C. I. and G. F. Harren. 1964. Adsorption and desorption of herbicides by soil. Heed Sci. 12:120-126. Harter, R. D. and J. L. Ahlrichs. 1969. Effects of acidity on reactions of organic acids and amines with'montmorillonite clay surfaces. Soil Sci. Soc. Am. Proc. 33:859-863. Hatzios, K. K. 1984. Interactions between selected herbicides and protectants on corn (Leg EELS). Heed Sci. 32:51-58. Herr, D. E., E. H. Stroube, and D. A. Ray. 1966. The movement and persistence of picloram in soil. Heed Sci. 14:248-250. Hill, I. R. 1980. Microbial transformation of Pesticides. Pages 136-202 in Pesticide Microbiology. Academic Press, New York. Hiltbold, A. E. 1974. Persistence of pesticides in soil. Pages 203-222 in w. o. Guenzi, ed. pesticides in Soil and Hater. Soil Sci. Soc. of Am. Madison, Hi. Hiltbold, A. E. and G. A. Buchanan. 1977. Influence of soil pH on persistence of atrazine in the field. Heed Sci. 25:515-520. Hormann, H. 0., B. Karlhuber, K. A. Ramsteiner, and D. D. Eberle. 1973. Soil sampling for residue analysis. Proc. Eur. Heed Res. Counc. Symp. Herbicides in Soil. p.129-140. Hurle, K. and A. Hal ker. 1980. Persistence and it's prediction. Pages 84-122 in R. J. Hance, ed. Interactions between Herbicides and the Soil. Academic Press, New York. 65. 66. 67. 68. 69. 7D. 71. 72. 73. 53 Kaufman, D. D. 1970. Pesticide metabolism. Pages 74-86 in Pesticides lg the Soil: Ecolggh Degradation, and Movement. International Symp. on Pesticides in the Soil. Michigan State U. Kells, J. J., C. E. Rieck, R. L. Blevins, and H. M. Muir. 1980. Atrazine dissipation as affected by surface pH and tillage. Heed Sci. 28:101-104. Khan, S. U. and K. C. Ivarson. 1982. Release of soil bound (nonextractable) residues by various physiological groups of microorganisms. J. Environ. Sci. Health. 817:737-749. Klehr, H., J. Iwan, and J. Riemann. 1983. An experimental approach to the photolysis of pesticides adsorbed on soil:thidiazuron. Pestic. Sci. 14:359-366. Kobayashi, H. M. and B. E. Rittman. 1982. Microbial removal of hazardous organic compounds. Environ. Sci. Tech. 16:170A-180A. Koskinen, H. C. and H. H. Cheung. 1983. Effects of experimental variables on 2,4,5-T adsorption/desorption in soil. J. Environ. Oual. 12:325-330. Liang, T. T. and E. P. Lichtenstein. 1976. Effects of soil and leaf surfaces on the photodecomposition of (14C) azinphosmethyl . J. Agr. Food. Chem. 24:1205-1210. Liu, S. L. and J. B. Heber. 1985. Retention and mobility of prometryn, AC-252,214, chlorosulfuron, and SD 95481. Abstr. Heed Sci. Soc. Am. (17). Lowder, S. H. and J. B. Heber. 1982. Atrazine efficacy and longevity as affected by tillage, liming, and fertilizer type. Heed Sci. 30:273-280. 74. 75. 76. 77. 78. 79. 80. 81. 54 Lykken, L. 1972. Role of photosensitizers in alteration of pesticide residues in sunlight. Pages 449-469 in F. Matsumara, G. Mallory Roush, Tomoma Misato, eds. Environmental Toxicology 91 Pesticides. Academic Press, New York. McAul iffe, D. and A. P. Appleby. 1981. Effect of preirrigation period on the activity of ethofumesate applied to dry soil. Heed Sci. 29:712-717. McCall, P. J., D. A. Laskowski, R. L. Swann, and H. J. Dishburger. 1983. Estimation of environmental partitioning of organic chemicals in model ecosystems. Pages 231-244 in Residue Reviews. Vol. 85. Springer-Verlag, New York. McGlamary, M. D. and F. H. Slife. 1966. The adsorption and desorption of atrazine as affected by pH, temperature, and concentration. Heed Sci. 14:237. Mersie, H. and C. L. Foy. 1985. Phytotoxicity and adsorption of chlorsulfuron as affected by soil pr0perties. Heed Sci. 33:564- 568. Miller, G. C. and R. G. Zepp. 1983. Extrapolating photolysis rates from the laboratory to the environment. Pages 89-110 in Residue Reviews. Vol. 85. Springer-Verlag, New York. Morril l , L. G., B. C. Mahilum and S. C. Mohiuddin. 1982. Organic Compounds 1!.Efl£.§211i Sorption, Degradation and Persistence. Ann Arbor Sci., Buttersworth group. pp. 267. Mortland, M. M. and H. F. Meggitt. 1966. Interaction of ethyl M-di-n-propyl thiol carbamate (EPTC) with montmoril lonite. J. Agr. Food Chem. 14:126-129. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 55 Narsaiah, D. Bala and R. G. Harvey. 1977. Differential responses of corn inbred and hybrids to alachlor. Crop Sci. 17:657-659. Nearpass, D. C. 1976. Adsorption of picloram by humic acids and humins. Soil Sci. 115:272-277. Nichol ls, P. and A. A. Evans. 1985. Adsorption and movement in soils of chlorsulfuron and other weak acids. Proc. 1985 British Crop Protection Conf. Heeds 1:333-339. Nilles, G. P. and M. J. Zabik. 1974. Photochemistry of bioactive compounds. Multiphase photodegradation of basalin. J. Agr. Food. Chem. 22:684-688. Nilles, G. P. and M. J. Zabi k. 1975. Photochemistry of bioactive coupounds. Multi phase photodegradation and mass spectral analysis of basagran. J. Agr. Food Chem. 23:410-445. Oliver, L. and R. E. Frans. 1968. Inhibition of cotton and soybean roots from incorporated trifluralin and persistence in the soil. Heed Sci. 16:199-203. 0plinger, E. 1982. Soybean variety and sencor rate study. Parochetti, J. V. 1978. Photodecomposition, volatility, and leaching of atrazine, simazine, alachlor, and metolachlor from soil and plant material. Abstr. Heed Sci. Soc. Am. (17). Peoples, T. R., T. Hang, R. R. Fine, P. L. Orwick. S. E. Graham and'K. Kirkland. 1935. AC-263,499: A new broadSpectrum herbicide for use in soybeans and other legumes. Proc. British Crap Protection Conference 1:99-106. Pierce, R. H. Jr., C. E. Olney, and G. T. Felbeck Jr. 1971. Pesticide adsorption: soils, sediments, humic acids, soil lipids. Environmental Letters 1(2):157-172. 92. 93. 94. 95. 96. . 97. 98. 99. 100. 101. 56 Plimmer, J. R. 1976. Volatility. Pages 891-934 in P. C. Kearney and D. D. Kaufman eds., Herbicides, Chemistry, Degradationy, and HEP. 93 m Vol. 2. Marcel Dekker, Inc., New York. Probst, G. H., Tomas Goolab, R. J. Hersberg, F. J. Holzer, S. J. Parka, C. VanderSchans, and J. B. Tepe. 1967. Fate of trifluralin in soil and plants. J. Agr. Food Chem. 15:592-599. Rao, P. S. C., V. E. Berkheiser, and L. T. Du. 1984. Estimation of parameters for modeling the behavior of selected pesticides and orthophOSphates. EPA-600/53-84-019. Rao, P. S. C. and R. J. Hagenet. 1985. Spatial variability of pesticides in field soils. Methods for data analysis and consequences. Heed Sci. 33(Suppl.2):18-24. Renner, K. A. and H. F. Meggitt. 1984. Corn variety response to imazaquin residue in the soil. Proc. NCHCC. 39:105-106. Robinson, E. L. 1976. Herbicide distribution in a block of soil. Heed Sci. 24:420-421. Roggenbuck, F. 1984. The influence of environmental and genetic factors on corn (Leg _m_gy_s_) tolerance to trifluralin. M. S. thesis, Michigan State University, pp.94. Rubin, B. and J. E. Cassida. 1985. R-25788 effects on chlorsulfuron injury and acetohydroxyacid synthase activity. Heed Sci. 33:462-468. Russell, J. 0., M. L. Cruz, and J. L. Hhite. 1968. Adsorption of 3-aminotriazole by montmorillonite. J. Agr. Food Chem. 16:21-24. Sagaral, E. G. and C. L. Foy. 1982. Response of several corn (23 ESE.) culti vars and weed Species to EPTC with and without the antidote R-25788. Heed Sci. 30:64-69. 102. 103. 104. 105. 106. 107. 108. 109. 110. 111. 57 Savage, K.EL 1973. Nitralin and trifluralin persistence in soil. Heed Sci. 21:285-289. Savage, K. E. and H. L. Barrentine. 1969. Trifluralin persistence as affected by depth of soil incorporation. Heed Sci. 17:349-352. Schliebe, K. A., O. C. Burnside, and T. L. Lavy. 1965. Dissipation of amiben. Heeds 13:274-276. Schmidt,lL R.and H.Pestemer. 1980. Plant availability and uptake of herbicides from soil. Pages 179-201 in R. J. Hance ed., Interactions Between Herbicides and the Soil. Academic Press, New York. Schnitzer, M. and S. U. Khan. 1972. Humic Substances i the Environment. Marcel Dekker, Inc., New York. pp.327. Scott, D. C. and J. B. Heber. 1967. Herbicide phytotoxicity as influenced by adsorption. Soil Sci. 104:151-158. Shaner, D. L., P. C. Anderson, and M. A. Stidham. 1984. Potent inhibitors of acetohydroxyacid synthase. Plant Physiol. 76:545- 546. Shaner, D. L. and P. A. Robson. 1985. Adsorption, translocation, and metabolism of AC-252,214 in soybean (Glycine 913;), common cocklebur (Xanthium strumarium), and vel vetleaf (Abutilon thegphrasti). Heed Sci. 33:469-471. Shaner, 0., M. S. Stidham, M. Muhitch, M. Reider, P. Robson, and P. Anderson. 1985. Mode of action of the imidazolinones. Proc. British Crap Protection Conference Heeds. Vol. 1:147-154. Shea, P. J. 1985. Detoxification of herbicide residues in soil. Heed Sci. 33(Suppl.2):33-41. 112. 113. 114. 115. 116. 117. 118. 119. 120. 121. 58 Shea, P. J. and J. B. Heber. 1983. Fluridone adsorption on mineral clays, organic matter, and modified Norfolk soil. Heed Sci. 31:528-532. Slack, C. H., R. L. Blevins, and C. E. Rieck. 1978. Effect of soil pH and tillage on persistence of simazine. Heed Sci. 26:145-147. Smith, A. E. 1978. Comparison of solvent systems for extracting herbicide residues from weathered soils. Pesticide Sci. 9:7-11. Smith, A. and A. I. Hsiao. 1985. Transformation and persistence of chlorsulfuron in prairie field soils. Heed Sci. 33:555-557. Smith, A. and L. J. Milward. 1983. Comparison of solvent systems for the extraction of diclof0p acid, picloram, simazine, and trial late from weathered field soils. J. Agr. Food Chem. 31:633-637. Smith, D. T. and A. F. Hiese. 1973. Delayed incorporation of trifluralin and nitralin. Heed Sci. 21:163-165. Stevenson, F. J. 1972. Organic matter reactions involving herbicides in soil. .1. Environ. Quality. 1(4):333-343. Stoller, E. H., L. M. Hax, L. C. Haderlie, and F. H. Slife. 1975. Bentazon leaching in four Illinois soils. J. Agr. Food Chem., 23:682-683. _ Taylor, A. H., H. P. Freeman, and H. M. Edwards. 1971. Sample variability and the measurement of dieldrin content of a soil in the field. J. Agr. Food Chem. 19:832-836. Terce, M. and R. Cal vet. 1978. Adsorption of several herbicides by montmoril lonite, kaolinite, and illite clays. Chemosphere. 4:365-370. 122. 123. 124. 125. 126. 127. 128. 129. 130. 59 Theung, B. K. G. 1979. Formation and Properties 2f Clay-polymer Complexes. Elsevier Scientific Publishing Co. New York, pp. 361. Thirunarayanan, K., R. Zimdahl, and D. E. Smika. 1985. Chlorsulfuron adsorption and degradation in soil. Heed Sci. 33:558-563. Upchurch, R. P.and D. D. Mason. 1962. The influence of soil organic matter on the phytotoxicity of herbicides. Heeds 10:9- 14. Hagenet, R. J. and P. S. C. Rao. 1985. Basic concepts of modeling pesticide fate in the crap root zone. Heed Sci. 33(suppl. 2):25-32. Halker, A. 1971. Effects of soil moisture content on the availability of soil-applied herbicides to plants. Pestic. Sci. 2:56-59. Halker, A. 1973. Use of simulation model to predict herbicide persistence in the field. Proc. Eur. Heed Res. Counc. Symp. Herbicides in soil. p.240-249. Halker, A. and P. A. Brown. 1983. Measurement and prediction of chlorsulfuron persistence in soil. Bull. Environ. Contam. Toxicol. 30:365-372. Halker, A. and P. A. Brown. 1983. Spatial variability in herbicide degradation rates and residues in soil. Crop Prot. 2:17-25. Hal ker, A. and H. Bond. 1977. Persistence of the herbicide AC 92,553 (N-(I-ethylprOpyl)-2,6-dinitro-3,4-xy1idine) in soils. Pestic. Sci. 8:359-365. 131. 132. 133. 134. 135. 136. 137. 138. 60 HauchOpe, R. 0., J. M. Chandler, and K. E. Savage. 1977. Soil sample variation and herbicide incorporation uniformity. Heed Sci. 25:193-196. Heber, J. B. 1970. Adsorption of _s_-triazines by montmorillonite as a function of pH and molecular structure. Soil Sci. Soc. Am. Proc. 34:401-404. Heber, J. B. and S. B. Heed. 1973. Effects of soil on the biological activity of pesticides. Pages 223-256 in H. D. Guenzi, ed. Pesticides _i_g the Soil and Hater. Soil Sci. Soc. of Am. Madison,Hi. Heed, S. B. and J. B. Heber. 1973. Pesticide-organic matter interactions. Pages 39-66 in H.[L Guenzi, ed. Pesticides in Soil and Hater. Soil Sci. Soc. of Am. Madison, Hi. Hhite, J. L. and M. M. Mortland. 1970. Pesticide retention by soil minerals. Pages 95-100 in Pesticides i_n the Soil: Ecology, Degradatiory, and Movement. International Symp. on Pesticides in the Soil. Michigan State University. Hilliams, J. H. and D. J. Eagle. 1979. Persistence of dichlobenil in sandy soil and effects of residues on plant growth. Heed Res. 19:315-319. Holcott, A. R. 1970. Retention of pesticides by organic materials in soils. Pages 128-138 in Pesticides lfl.£fl£H§211i Ecology, Dggpadatiop, and Movement. International Symp. on Pesticides in Soil. Michigan State University. Hright, T. H. and C. E. Rieck. 1973. Differential butylate injury to corn hybrids. Heed Sci. 21:194-196. 139. 140. 141. 142. 61 Hright, T. H. and C. E. Rieck. 1974. Factors affecting butylate injury to corn. Heed Sci. 22:83-85. Hright, H. L. and G. F. Harren. 1965. Photochemical decomposition of tri fl uralin. Heed Sci. 13:329-331. Yee, D., P. Heinberger, and S. U. Khan. 1985. Release of soil- bound prometryne residues under different soil pH and nitrogen fertilizer regimes. Heed Sci. 33:882-884. Zimdahl , R. L. and S. M. Gwynn. 1977. Soil degradation of three dinitroanilines. Heed Sci. 25:247-251. CHAPTER 2 RESPONSE OF CORN CULTIVARS TO IMAZAQUIN ABSTRACT Nine corn (Leg m L.) cul ti vars differed in their response to imazaquinl (2-(4,5-dihydro-4-methyl -4-(1-methyl ethyl )-5-oxo-1H_- imidazol-Z-yl )-3-quinol inecarboxyl ic acid) applied from 35 to 280 g ai/ha, as measured by fresh Shoot and adventitious root weight and length. Cargill 921, Great Lakes 422, and Great Lakes 5922 cultivars were among the more tolerant cultivars in both years of study, while Pioneer 3737, Northrup King 9410, DeKalb-Pfizer Trojan 1100, and Stauffer 5650'were among the less tolerant varieties both years. There was no cultivar tolerant to imazaquin across all application rates in both years of the study. There was less corn injury from imazaquin in the second year, and preplant incorporated applications caused significantly more injury than preemergence surface applications. Lack of rainfall the second year limited movement of imazaquin in the soil profile. Increased sorption to soil and decreased availability to plant roots especially'from preemergence surface applications, resulted in reduced corn injury. 1AC-252,214 Code Number. American Cyanamid Co. Princeton, NJ 08540. 62 INTRODUCTION Imazaquin is a selective herbicide for grass and broadleaf weed control 'hi soybeans (Glypine max (L.) Merr.%. Plant uptake of imazaquin is by both the root and shoot, with apoplastic and symplastic translocation to meristematic regions (13%. Only a small percentage of imazaquin remains in the root after root uptake (13%. Plant growth ceases, tissue chlorosis and necrosis develOp, and plant dieback and death occur in two to three weeks (14). The primary mechanism of action of imazaquin is believed to be inhibition of the enzyme acetohydroxyacid synthase (AHAS) that catalyzes the biosynthesis of the three branched chain amino acids valine, leucine, and isoleucine (2, 14). Differential metabolism of imazaquin apparently determines crap and weed selectivity (13). Little information is available regarding the behavior and persistence of imazaquin in soil (8, 9, 10). Rotational crop restrictions for imazaquin have not been published, but injury to corn (Z_e_gi_n_a£ L.) has occurred when planted the year fol lowing imazaquin application in soybeans (9, 10). The persistence of imazaquin in soil has not been related to any specific soil or edaphic factor, and cultivar tolerances have not been correlated to instances of corn injury. Differential tolerance of crop cultivars, including corn, to several herbicides has been reported. These herbicides include atrazine 63 64 (6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine), simazine (6-chloro-N,l_l_'-diethyl-1,3,5-triazine-2,4-diamine), alachlor (2-chl oro-N-(2,6-di ethyl phenyl )-l_V_-(methoxymethyl )acetamide), di c1 ofop ((t)-2-(4-(2,4-dichlorophenoxy)phenoxy)propanoic acid), trifl ural in (2,6-dinitro-N,N-diprOpyl-4-(trifluoromethyl)benzenamine), butylate (S- ethyl bis(2-methylpropyl)carbamothioate), and EPTC (_S_-ethyl dipr0pyl carbamothioate) (1, 4, 5, 6, 7, 11, 17, 18). Temperature and moisture conditions alter hybrid sensitivity (5, 11, 18). Field studies were conducted in 1984 and 1985 to determine if corn cultivars varied in response to imazaquin, and if environmental conditions alter corn varietal response. MATERIALS AND METHODS Field plots were prepared on May 16, 1984 and May 8, 1985 on a Capac sandy loam (Aeric Ochraqual fs, fine-loamy, mixed, mesic) with 2.1% organic matter, a pH of 6.4, 57% sand, 26% silt, 17% clay, a cation exchange capacity of 10.1, and <1 and 43 ppm of extractable aluminum and iron, respectively. The experimental design was a three factor factorial arranged in a randomized complete block design with four replications. The factors were imazaquin rate, application method, and corn cultivar. Eight plots (9.14 by 12.2 m) (two per replication) were treated with imazaquin at 35, 53, 70, 140, and 280 g ai/ha with a tractor-mounted boom sprayer at 215 L/ha and 206 kPa. Four plots ( one per replication) were incorporated immediately with 65 one pass using a Triple K Danish s-tine field cultivator2 set at an incorporation depth of 6 cm. One plot in each replication did not receive imazaquin and served as the control. Nine corn varieties per plot were planted at a 4 cm depth. The nine hybrids were Great Lakes 422, Great Lakes 5922, Cargill 921, Pioneer 3901, Pioneer 3737, Northrup King 9410, DeKalb-Pfizer Trojan 1100, Stauffer 5650, and Funks G-4342. Twenty-eight days after planting, four plants per hybrid per plot were dug from the field, and fresh shoot and adventitious root length and weight measured. All measurements were converted to a percentage of the control plants for the respective cultivar to eliminate growth differences among cultivars. Results were not combined over years due to significant interactions of years with herbicide rate, application method, and cultivar. All measured parameters gave similar results. Shoot length measurements had greatest precision, so when data is presented for one parameter it will be for shoot length. RESULTS AND DISCUSSION In 1984, the main effects of application method, imazaquin rate, and corn cultivar were highly significant. A highly significant interaction occurred for application method by imazaquin application rate (Table 1). The interaction can be explained by the preplant incorporated (PPI) applications causing less injury than preemergence 2Manufacturer. Kongskilde, Box 88, Exeter, Ontario. 66 Table 1: Application method and imazaquin rate interaction in 1984. Application Method Imazaquin Preplant incorporated Preemergence surface 9 :175a shoot lengtha shoot length ------------------- (% of control)---------------- 35 67 a 73 a 53 59 b 69 a 70 56 bc 51 c 140 36 de 41 d 280 27 f 31 ef aValues followed by the same letter are not Significantly different at_ the 5% level according to Duncan's multiple range test. Statistical comparisons are valid across rows and between columns. 67 surface (PES) applications at 70 g ai/ha, while at all other rates PPI applications caused 4 to 10% more injury. The magnitude of the significance of the interaction was considerably less than the significance of the main effects. The main effect of application method was highly significant in 1984. PPI applications of imazaquin reduced corn height 4% more compared to PES applications. Corn injury increased significantly as imazaquin application rate increased (Figure 2). Corn cultivars in 1984 Showed great differences in tolerance to imazaquin across all application rates and methods (Table 2). Cargill 921, Pioneer 3901, and Great Lakes 422 were among the more tolerant cultivars for all measured parameters. In 1985, all main effects were highly significant, and a highly significant interaction occurred between application method and corn cultivar (Table 3). ‘This interaction was significant because of the 33 to 39% increase in injury to Stauffer 5650 and Northrup King 9410 when imazaquin was incorporated compared to surface applications, and a lesser difference in injury'due toiapplicationinethod found for the cultivars Pioneer 3737 and Dekal b-Pfizer Trojan 1100. There was no significant difference in the effect of application method on cultivar injury to the other five hybrids, and this difference in response among cultivars in 1985 caused the corn cultivar by application method interaction. Hhen analysis of variance was evaluated including the control plants, no significant difference in injury to the nine cultivars at any imazaquin application rate compared to the controls for PES applications occurred. .All nine varieties at all application rates for PPI applications, exhibited injury as measured by all four 68 Figure 2: The effect of imazaquin application rate on fresh Shoot and root weight and length in 1984. 69 .mrSm a. mwmc c_30mNmE_ DV— - Dhmm q . mm 39m? ¢DOL< Mam—cm. 30.. 4 E E? 3930 50:2 yoozm .- www— IOJiuoo 1° X 70 Table.2: Corn cultivar response to imazaquin in 1984 across all herbicide rates and application methods. Cultivar Shoot lengtha ---(% of control)--- Cargill 921 61 a Great Lakes 422 55 bc Pioneer 3901 59 ab Great Lakes 5922 52 cd Northrup King 9410 49 de DeKalb-Pfizer Trojan 1100 41 f Stauffer 5650 46 ef Funks G 4342 51 c-e Pioneer 3737 45 ef aValues followed by the same letter are not significantly different at the 5% level according to Duncan's multiple range test. 71 Table 3: ‘The influence of corn cultivar and imazaquin application method on corn shoot length in 1985. Application Method Preplant incorporated’ Preemergence surface Cultivar Shoot lengtha Shoot length ---------------- (% of control)------------------- Cargill 921 84 a-d 93 ab Great Lakes 422 75 c-f 88 a-d Pioneer 3901 71 d-f 82 a-d Great Lakes 5922 89 a-c 93 ab Northrup King 9410 66 ef 100 a DeKalb-Pfizer Trojan 1100 71 d-f 92 a-c ' Stauffer 5650 so 9 90 a-c Funks G 4342 75 c-f 81 b-e Pioneer 3737 63 f-g 85 a-d a‘Values followed by the same letter are not significantly different at the 5% level according to Duncan's multiple range test. comparisons are valid across rows and between columns. Statistical 72 parameters. Therefore evaluation of culti var response to imazaquin in 1985 can be made only with PPI appl ications. PES applications caused 18% less injury than PPI applications, which resulted in a highly significant main effect. As the imazaquin application rate increased, injury to corn decreased (Figure 3), but there was less corn injury across all imazaquin application rates in 1985 than 1984. Less corn injury occurred in 1985 compared to 1984 across all imazaquin application rates, and PPI applications gave significantly more corn injury than PES applications. In 1984, 9.14 cm of rain fell in the 28 days after imazaquin application and corn planting, with 7 cm of this rain falling during the first 10 days after planting (Table 4). In 1985, a total of 3.96 cm of rain fell in the 28 days after application and corn planting, with only 0.91 cm of this rain falling during the first 10 days. The increased moisture level in 1984 appeared to increase the availability of imazaquin to corn by decreasing soil sorption. Imazaquin did not leach out of the corn root zone and was available for uptake. In 1985 rainfall was very limited, PES applications failed to move into the corn root zone to be available for root uptake, resulting in decreased corn injury in 1985, especially for PES applications. Changing moisture levels in the soil alter the herbicide distribution between the sorbed and solution phases, and change the rate of herbicide movement by diffusion and mass flow (3). At low moisture levels the number of water molecules competing for soil adsorption sites decreases so herbicides can adsorb, and a higher concentration of herbicide per water volume at low soil moisture levels Figure 3: The effect of imazaquin application rate on fresh Shoot and root weight and length in 1985. .74 Hm£\_m a. mamc CEUmnmE DmN 0v. DB mm mm D _ _ _ _ _ . 0 50cm. ~00£m O l D. e 39m; Eden 0 1 au as 50cm. “an: 4 1 am ”a 0 E99: “00.. 4 F 0v 1 0m 1 0m 1 DR 1. 0m 1 on 9.. r 0...: Dw._.mn omp . om cm on 3 o 5 4E L L g . rr . N 2 Eu 0.3 I 0.3% . Eu of i 0.38 2 :8 ed. .. o.m § :8 o.m I EH I Eu end I 0D I O IO N L o co O N o o o 2‘3 E'— .9 (qdd) ugnbezeuu O N N . 000 98 Soil cores were also extracted and analyzed from 1984 and 1985 for PES (Figures 9 and 11) and PPI (Figures 8 and 10) imazaquin applications of 70 g ai/ha. In 1984 imazaquin was not detected below 5 cm in the soil profile for either application technique. The most imazaquin was detected in the top 2.5 cm when surface-applied, and it was more evenly distributed across the tap 5.0 cm when imazaquin was incorporated. Soil samples from 1985 depict some movement of imazaquin to the 5.0 to 10.0 cm depth under both application methods. More imazaquin was detected in 1985 (Figures 10 and 11) than in 1984 (Figures 8 and 9), regardless of application method. In 1985, PPI applications of imazaquin has greater persistence than PES applications during the first 30 d.a.a.. This trend was reversed at the 60 d.a.a. sampling date. However at 150 d.a.a., PES application residues were lower in the top 2.5 cm compared to PPI applications. A summary of imazaquin persistence in the soil profile was made by adding the ppb remaining in each depth increment. The differences in soil volume of the depth increments were accounted for. Results of the 280 g ai/ha applications are given in Figure 12. In 1984 there was no significant difference between PPI and PES applications, and no statistical difference in residues remaining from 60 to 150 days after application. In 1985, the application method and sampling date were highly significant. PPI applications persisted significantly more than PES applications at each sampling date. There was no statistical decrease in imazaquin remaining for both applications methods from 60 to 150 days after application. Hhen imazaquin was applied at 70 g ai/ha (Figure 13) the main effects of application method and sampling date were signficant in both 1984 and 1985. In 1984 there was a 99 Figure 8: Imazaquin distribution and persistence in the soil profile in 1984 where 70 g ai/ha was applied and incorporated. 100 00.. :o_«mo_.an< .32 £60 1% — q . mm— om 0 Mt low low Eu 0.3 I 0.0.. a. . =8 0.0.. I 00' Q -00 Eo 0.09 I 0.m§ . Eu 0.0 l vm.~% -00 so «Ru 1 cm . .00 (qdd) ugnbezeun F 101 Figure 9: Imazaquin distribution and persistence in the soil profile in 1984 where 70 g ai/ha was preemergence surface-applied. 102 5:50:33 .32 £30 cm. on on o 1.. . . H. . .8 L L .3 Eu c.3193 a. . :8 o.mT Eu o. o. M 6m Eco..opiom§ . seed 1 cm. «H -8 Eu 3d 108 . -8. (qdd) ugnbezetul 103 Figure 10: Imazaquin distribution and persistence in the soil profile in 1985 where 70 g ai/ha was applied and incorporated. 104 00.9 c2523.? .32 $30 00 1% ... . Eo 0.0N10.m...a. Eo 0.0..10.OF ...m... Eu 0.0w 1 06$ Eu 0.0 I vmfl 7% Eu vm.N I 0U u m 0 a r. Ctr; 00.. (qdd) ugnbezeuu 105 Figure 11: Imazaquin distribution and persistence in the soil profile in 1985 where 70 g ai/ha was preemergence surface-applied. 106 00.. 9.0.50.3“; .32 «>00 00 00. 0.. 1% . Eu 0.0m I 0.0 ......— Eo 0.0..10.0Fm. :8 0.3 i 00$ EoninN% Eu «0.0 .. o_n_ d .. 0 N (qdd) ugnbezewl 0 v A'O‘ O 6 co 0 O F 107 Figure 12: Imazaquin dissipation in 1984 and 1985 for preplant incorporated and preemergence surface applications of 280 g ai/ha. 108 cozmoznn< .32 0.60 oomtam 000093009... «00 w 0 0320900.... E0303 #00 .. 4 000.30 oocomeoEoEn 000; . 03039005 0:209... 000 _. 4 00 00.. 00.. 8 (qdd)ugnbezeu1| 000 000 001 ..Nv 109 Ifigure L3: Imazaquin dissipation in 1984 and 1985 for preplant incorporated and preemergence surface applications of 70 g ai/ha. 110 00:00:32 .32 0.60 00F 00 on m p 0 1 d J . fi 4 o o o. 111111111111 I10. vol/l o / 0 3 ll, ..I-II ow. to 41 114 // no \ / Ill. / on D 4 III.‘\ 90 / .8 e. /; 00 0 on a / / 0s. / oomtam 03.090500... v00 9 o no 00 0390900... «20.00:. 000 p a 000.50 03.092000... 000 .. o a 0 : 0303903.. 3.0.030 000 _. 4 a 00.. (qdd) ugnbezetuI 111 significant interaction between application method and sampling date because of the significant increase in imazaquin remaining at 150 days after the PPI application, and no correSponding increase in the PES application. In 1985, PPI applications persisted significantly more than PES applications at all sampling dates except at 60 d.a.a.. There was an initial rapid decrease in persistence, followed by a Slower dissipation rate for all application rates and techniques. The estimated half-lives in 1984 of PPI applications were 30 days for 280 g ai/ha and 36 days for 70 g ai/ha. PES applications estimated half- lives were < 30 days for 280 g ai/ha and 32 days for 70 g ai/ha. In 1985, the estimated PPI half-lives were 52 and 48 days for 280 and 70 g ai/ha, respectively, and 20 and 23 days for PES applications of 280 and 70 g ai/ha, respectively; Half-life estimations differed more with the year and application technique, than the initial herbicide rate applied. The pesticide residues remaining in the field reflect the combined influence of transfer processes such as leaching, plant uptake, volatility, and soil adsorption,.and transformation processes including microbial and photolytic degradation (3, 6, 7,12,14, 20, 27). These processes can be affected by environmental conditions such as soil moisture and temperature, and cropping systems and soil management (7). Imazaquin dissipation in the field is the result of a combination of the above mentioned processes. ‘Very little imazaquin was detected below the tap 10 cm of the soil in either year for both application techniques and rates. Imazaquin could have leached below the 23 cm sampling depth and not been present for detection. Imazaquin is an acidic herbicide, and acidic herbicides 112 are more readily leached than cationic or nonionic pesticides due to a small number of sites in soil that are positively charged and available for anionic binding (1, 9, 10, 11). However only yearly rainfall totals within the first month after application correlate with decreased persistence due to leaching (Table 5). Less imazaquin remained in 1984 where 11.33 cm of rain fell in the first 30 d.a.a., compared to 1985 rainfall of 4.27 cm in the first 30 d.a.a.. Yet, over 15.8 cm of rainfall occurred both years between 90 and 150 d.a.a., and imazaquin residues did not decrease substantially either year during this time period. Table 5. Summary of rainfall in 1984 and 1985 occurring between each of the imazaquin soil sampling periods. Rainfall Si-ary Time increment 1984 1985 (d.a.a.) .............. cm ----------------- 0 to 15 11.20 1.47 15 to 30 0.13 2.79 30 to 60 1.35 5.82 60 to 90 6.25 5.56 90 to 150 15.77 19.28 113 Plant uptake may have been an important factor in decreasing imazaquin persistence over time (7). In 1984, the 11.33 cm of rainfall received in the 30 days following imazaquin application and corn planting would have resulted in increased movement of the herbicide to the plant root and increased plant uptake. This may have resulted in the lower herbicide residues remaining in the soil cores in 1984. In 1985, only 4.34 cm of rain fell in the 30 days after application and planting, and greater imazaquin residues remained in the soil cores. The removal of the herbicide by corn would be permanent if metabolized within the plant, or temporary if it was only conjugated and then hydrolytically released later during plant decomposition (6, 27% Application method may have a significant effect on pesticide persistence, as persistence decreases immediately if the herbicide is volatile or subject to photodecomposition (2, 6, 8, 18, 19, 22, 23, 30). Imazaquin is not believed to be volatile, but is subject to photolytic hydrolysis 2. The two major limiting factors to photolysis in soils is adsorption of light by other soil components that decreases available energy, and the non-penetration of light below the soil surface (2, 3, 6, 18, 19). Herbicides such as pendimethalin and trifluralin that are subject to volatility and photodecomposition have shown increased persistence when incorporated and when incorporated to deeper depths (21, 25, 30%. Photolytic hydrolysis may be a factor causing the decreased persistence of PES imazaquin applications. Rainfalls within 30 dJLa. could cause photolytic hydrolysis of imazaquin to occur on the soil surface, and to a lesser extent in the top 2.5 cm soil depth. Capillary water movement during the growing season could move imazaquin applied by either application method to the 114 soil surface and subject it to continued photolytic hydrolysis. The extent of capillary water and imazaquin movement is not known. Imazaquin rapidly dissipated during the first 60 d.a.a. under both application rates and techniques in 1984 and 1985. A slower rate of loss occurred during the remainder of the growing season. The level of imazaquin at which dissipation slowed varied with the year and application method. The dissipation rate slowed at 100 ppb and 50 ppb for PPI and PES applications, respectively, at 280 g ai/ha in 1985, and approximately 55 ppb for 1984 PPI and PES applications, respectively, of 280 g ai/ha. The slower degradation rate occurred at lower concentrations for the 70 g ai/ha applications. These results agree with Hamaker and Goring (12) and other researchers who found biphasic degradation rates occurred with numerous pesticides (4, 8, 15, 23, 24, 32). A greater percentage of the herbicide remaining is believed to become less available over time. Degradation Slows until a steady state is reached, and the degradation rate determined by a labile pool (12%. Pesticide degradation was disprOportionately slower at low residual concentrations, and Hurle and Halker pr0posed a two compartment model containing available and non-available compartments, with only herbicides in available areas subject to degradation (15). The non-available areas appear to be dynamic and change with environmental conditions, as evidenced by the differences in the imazaquin level at which degradation appeared to slow. The validity of the hypothesis of too low a concentration of imazaquin to support microbial p0pulations is questionable. The hypothesis that imazaquin became increasingly less available over time due to increased adsorption may be a better explanation for the experimental results. 115 Decreased persistence of imazaquin during the growing season may result from decreasing extraction efficiency of imazaquin over time on field treated soils. Hhen pesticide residues age they become more resistant to solvent extraction because of adsorption to soil colloids and diffusion into the interior of humic colloids (28). It is difficult to determine true extraction efficiency as fortified soils in the laboratory can have extraction efficiencies different than field treated soils, and the difference is difficult to quantify. Spring tillage. There was no significant difference in corn cul ti var response to imazaquin, so results were pooled over cultivars and the main effects of tillage and herbicide treatment, and their interaction evaluated. There was no interaction between tillage and treatments (Figure 14). There was no signifiant effect of spring tillage on corn injury, as neither moldboard plowing or disking altered the amount of injury to corn planted in 1985. Corn roots growing for 60 days after planting had proliferated throughout the 23 cm soil profile and could not grow in untreated soil (27). The effect of herbicide treatment was highly significant (Figure 15). Imazaquin residues of 42 and 25 ppb remained in October 1984 from PPI and PES applications of 280 g ai/ha, respectively. Twenty-seven and 4 ppb remained from PPI and PES applications, respectively, of 70 g ai/ha. A reduction in corn height occurred from both PPI and PES applications of 280 g ai/ha, and injury was significantly greater from PPI applications of 140 g ai/ha compared to surface application. Hhere 70 g ai/ha was applied, there was no difference in plant height between PPI and PES applications, but corn height was reduced 116 Figure 14: Effect of Spring tillage on imazaquin injury to corn across all imazaquin treatments. 117 .053 0. 003 0. 02.0.2 savanna. 00m 0.: 00' 0h 00 q) A . 03.0.0 0:0 000.50 oocomLoEooE 0 0950.0 0.02.0.3: 0:0 ooutam 00:090—003.. 0 03.0.0 0:0 0303900... «:0...an 0 0026.0 0.03.0.3: 05... 0390903... “20.00:. 4 1 . . 1 00w (IOJlUOO 10 %) 1461914 1110:) 118 Figure 15: Effect of imazaquin applications from 1984 on corn height in 1985. 119 Corn Height (% of Control) ..001 00 1 00 r 40 i b 030.03 30303030 0 0303080300 3.1000 00 i 00 T Lt . p . _ . . _ L 0 mm mm 00 .00 .A0 n00 .3335: >0_.on_ 21000.0 0.25. 120 significantly from the control. Possibly more imazaquin remained than the 6 ppb detected by gas chromatography. ‘Therefore, 42 and 25 ppb remaining in the soil in early October 1984 significantly injured corn planted on May 8, 1985 regardless of tillage. Increased corn injury occurred where imazaquin was incorporated at 140 and 280 g ai/ha, reflecting increased persistence of incorporated imazaquin treatments. ACKNONLEDGEMENTS The authors appreciate the imazaquin supplied and the soil extraction and analysis procedure provided by American Cyanamid Co. LITERATURE CITED Adams, R. S.'Jr. 1973. Factors influencing soil adsorption and bioactivity of pesticides. Pages 1-54 in Francis A. Gunther, ed. Residue Reviews. Vol. 47. Springer-Verlag, New York. Armstrong, 0. E. and J. G. Konrad. 1973. Nonbiological degradation of pesticides. Pages 123-131 in H. D. Guenzi, ed. Pesticides _i_g the Sell _a_n_d_ 13.1221:- Soil Sci. Soc. of Am., Madison, Hi. Ashton, F. 1982. Persistence and biodegradation of herbicides. Pages 117-131 in Fumio Matsumura and C. R. Krisha Murti, eds. Biodegradation g_f_ Pesticides. Plenum Press, New York. Banks, P. A., M. L. Ketchersid, and M. G. Merkle. 1979. The persistence of fluridone in various soils under field and controlled conditions. Heed Sci. 27:631-633. Buchanan, G. A. and A. E. Hiltbold. 1973. Performance and persistence of atrazine. Heed Sci. 21:413-416. Burns, R. G. 1975. Factors affecting pesticide loss from soil. Pages 103-141 in E. A. Paul and A. D. McLauren, eds. _S_o_i_]_ Biochemistrgy. Vol. 4, Marcel Dekker, Inc., New York. Cheung, H. H. and R. G. Lehmann. 1985. Characterization of herbicide degradation under field conditions. Heed Sci. 33(Suppl. 2):7-10. 121 10. 11. 12. 13. 14. 15. 122 Duseja, D. R. and E. E. Holmes. 1978. Field persistence and movement of trifluralin in two soil types. Soil Sci. 125:41-48. Gerber, H. R. and J. A. Guth. 1973. Short theory, techniques, and practical importance of leaching and adsorption studies. Proc. Eur. Heed Res. Counc. Symp. Herbicides in Soil. p. 51-68. Grover, R. 1973. Movement of picloram in soil columns. Can J. Soil Sci. 53:307-314. Grover, R. 1977. Mobility of dicamba, picloram, and 2,4-D in soil columns. Heed Sci. 25:159-162. Hamaker, J. H. and C. A. I. Goring. 1976. Turnover of pesticide residues in the soil. Pages 219-243 in D. D. Kaufman, G. G. Still, C. D. Paulson, and S. K. Bandal, eds. Bound and Conjugated Residues. ACS Am Chem Soc. Symp. Ser. 29, Hashington, D. C. Hance, R. J. and C. E. McKone. 1971. Effect of concentration on the decomposition rates in soil of atrazine, linuron, and picloram. Pesticide Sci. 2:31-34. Hiltbold, A. E. 1974. Persistence of pesticides in soil. Pages 203-222 in H. D. Guenzi , ed. Pesticides i_g Soil and Hater. Soil Sci. Soc. of Am., Madison, Hi. Hurle, K. and A. Halker. 1980. Persistence and it's prediction. Pages 84-122 in R. J. Hance, ed. Interactions between Herbicides and the Soil. Academic Press, New York. 16. 17. 18. 19. 20. 21. 22. 23. 123 Kaufman, D. D. 1970. Pesticide metabolism. Pages 74-86 in Pesticides lg the Soil: Ecology, Degradation, and Movement. International symposium on Pesticides in the Soil. Michigan State University. Kobayashi , H. M. and B. E. Rittman. 1982. Microbial removal of hazardous organic compounds. Environ. Sci. Tech. 16:170A-180A. Lykken, L. 1972. Role of photosensitizers in alteration of pesticide residues in sunlight. Pages 449-469 in F. Matsumura, G. Mallory Roush, Tomoma Misato, eds. Environmental Toxicology gl Pesticides. Academic Press, New York. Miller, G. C. and R. G. Zepp. 1983. Extrapolating photolysis rates from the laboratory to the environment. Pages 89-110 in Residue Reviews. Vol. 85. Springer-Verlag, New York. Morrill, L. G., B. C. Mahilum, and S. C. Mohiuddin. 1982. Organic Compounds lg the Soil: Sorptiog, Deggadation, and Persistence. Ann Arbor Sci., Buttersworth group, pp. 267. Oliver, L. and R. E. Frans. 1968. Inhibition of cotton and soybean roots from incorporated trifluralin and persistence in the soil. Heed Sci. 16:199-203. Plimmer, J. R. 1976. Volatility. Pages 891-934 in P. C. Kearney and D. D. Kaufman eds., Herbicideg, Chemistry, Degradatiog, and Mode gl Action. Vol. 2. Marcel Dekker, Inc., New York. Probst, G. H., T. Goolab, R. J. Hersberg, F. J. Holzer, S. J. Parka, C. VanderSchans, and J. B. Tepe. 1967. Fate of trifluralin in soil and plants. J. Agr. Food Chem. 15:592-599. 24. 25. 26. 27. 28. 29. 30. 31. 32. 124 Savage,IL E. 1973. Nitralin and trifluralin persistence in soil. Heed Sci. 21:285-289. Savage, K. E. and H. L. Barrentine. 1969. Trifluralin persistence as affected by depth of soil incorporation. Heed Sci. 17:349-352. Shaner, D. L., P. C. Anderson, and M. A. Stidham. 1984. Potent inhibitors of acetohydroxyacid synthase. Plant Physiol. 76:545-546. Shea, P. J. 1985. Detoxification of herbicide residues in soil. Heed Sci. 33(Suppl. 2):33-41. Smith, A. and L. J. Milward. 1983. Comparison of solvent systems for the extraction of diclof0p acid, picloram, simazine, and triallate from weathered field soils. 1% Agr. Food Chem. 31:633-637. Hal ker, A. 1973. Use of simulation model to predict herbicide persistence in the field. Proc. Eur. Heed Res. Counc. Symp. Herbicides in Soil. p. 240-249. Halker, A. and H. Bond. 1977. Persistence of the herbicide AC 92,553 (N-(I-ethyl propyl )-2,6-dinitro-3,4-xylidine) in soils. Pesticide Sci. 8:359-365. Hilliams, J. H. and D. J. Eagle. 1979. Persistence of dichlobenil in sandy soil and effects of residues on plant growth. Heed Res. 19:315-319. Zimdahl , R. L. and S. M. Gwynn. 1977. Soil degradation of three dinitroanilines. Heed Sci. 25:247-251. CHAPTER 4 INFLUENCE OF SOIL PROPERTIES ON ADSORPTION. PERSISTHCE ANO AVAILABILITY OF IMAZAWIN AND AC-263,499 ABSTRACT Studies were conducted to evaluate the effect of soil type on adsorption, persistence, and availability of imazaquin1 (2-(4,5- dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1fl-imidazol-2-yl)-3-quinoline- carboxylic acid) and AC-263,499 (2-(4,5-dihydro-4-methyl-4-(1- methyl ethyl )-5-oxo- 111-imidazol -2-yl )-5-ethyl -3- pyridinecarboxyl ic acid) to corn (;e_a_ geLs L.% Adsorption of imazaquin and AC-263,499 were low for all five soils tested. Kd values ranged from 0.056 to 0.113 for imazaquin, and 0.060 to 0.761 for AC-263,499. The Kilmanagh soil that contained the most organic matter, smectite clay, iron, and had the highest soil pH adsorbed the most imazaquin. The Decatur soil adsorbed the most AC-263,499, with the Kilmanagh soil having the second greatest adsorption. Hhen organic matter was removed from a Capac sandy loam, there was little change in imazaquin adsorption but a large increase in adsorption of AC-263,499. The soil pH decreased from 6.4 to 4.63 when 1AC-252,214 code number. American Cyanamid Company. Princeton, NJ 08540. 125 126 the organic matter was removed with hydrogen peroxide, which appeared to affect adsorption of AC-263,499 more than imazaquin. AC-263,499 adsorption on the Decatur silty clay loam was much greater than imazaquin adsorption, due to either the lower soil pH or the high silt percentage increasing adsorption of AC-263,499. Corn injury from imazaquin at 105 g ai/ha was 42.5% greater than from AC-263,499 across all soil types. This occurred because of increased availability of imazaquin or increased corn sensitivity to imazaquin. Injury to corn by imazaquin and AC-263,499 decreased over time, with the exception of AC-263,499 in experiment 1. ‘The Kilmanagh sandy clay loam had the least corn injury for both herbicides over time, but corn responded differently on the Greenville sandy clay loam in the two experiments. INTRODUCTION Imazaquin and AC-263,499 are selective herbicides for grass and broadleaf control 'Hl soybeans (Glycine max (L.) Merr.%. Little infbrmation is available regarding the behavior of these herbicides in soil. Soil properties alter the adsorption of herbicides to soil, and their subsequent soil persistence and availability to plants. The persistence of a herbicide in soil is a function of the chemical properties of the herbicide, soil properties, and environmental conditions (1, 3, 5, 7, 14, 23). Hhen herbicide activity is no longer observed the herbicide is believed to be dissipated, when in fact it could be adsorbed to soil material or diluted below the threshold level irf the bioassay species (13). Herbicides are inactivated by transfer mechanisms that result in the herbicide being 127 unavailable to plants, and by transformation mechanisms that change the herbicide to nontoxic forms (2, 3, 5, 8, 13, 23). Transfer processes that alter soil persistence include soil adsorption, mechanical dilution, leaching, volatilization, and plant uptake (3, 5, 8, 10, 11, 12, 23). Transformation processes include microbial degradation and nonbiological degradation by chemical and photochemical means (2, 3, 5, 13, 15, 18). The means and rates of these processes are dependent on the herbicide, soil, and environmental conditions. Soil adsorption affects the amount of herbicide available for effective weed control, the amount that will leach through the soil profile, and the ability of the herbicide to be microbially or nonbiologically degraded (10%. As adsorption increases, the availability and subsequent phytotoxicity to plants decreases. Adsorption can accelerate or Slow degradation dependent on whether the ,pesticide is adsorbed into interlamellar spaces where it may be protected from microbial attack, or adsorption may increase degradation because the enzyme and substrate are concentrated at one site and biological and nonbiOIOgical reactions are catalyzed (10). Soil adsorption in the field may differ from adsorption measured in the laboratory due to temperature, water content, soil pH, or concentration effects (8, 12, 16). Herbicides and water compete for adsorption sites. Hater can occupy positions around soil cations, reducing available binding sites for herbicide molecules (6). Soil moisture has a large effect on polar pesticide groups, such as carbonyls, amines, and carboxylic acids (21). Some organic herbicides are not adsorbed when water is present, but in air dry soils they are adsorbed, and may or may not be diSplaced by the addition of water (19. 128 21, 22, 26). At low soil moisture levels precipitation of herbicides can occur, resulting in less herbicide available for plant uptake (5). Adsorption of acidic compounds can occur on clay, metal oxides, and organic matter, and can be dependent on soil pH (1, 4, 6, 25, 28). Smectite clays have greater surface area than kaolinitic clays and can expand when hydrated due to the weak attraction between oxygen layers (26). Herbicides can adsorb into the expanded inner layers and bind to the substituted interlayer cations (26). The greater the affinity of these exchangeable cations for electrons the greater the interaction with polar groups on the herbicides capable of donating electrons. Hydrogen, iron, and aluminum cations are the most acidic, and have the greatest affinity for these electrons (28%. Kaolinitic soils have exposed aluminum hydroxides on the clay crystal edges that make up a large portion of the cation exchange capacity (cec) and are important sites for anionic adsorption (26, 28). Aluminum and iron hydroxides exist in crystalline and amorphous forms, have similar surface areas as smectite, and have high anionic exchange capacities (1, 4, 6, 27). Organic.matter contains carboxylic and phenolic groups that have pH dependent ionization with pKa values of approximately 5.2 (25). At high soil pH levels, ionized acidic functional groups can react with polyvalent cations to form chelate bridges with acidic herbicides (29% Ph also affects ionic herbicides as they become undissociated when the suspension pH is 1 to 2 units above their pKa or lower (4). At pH values greater than their pKa values, acidic herbicides exist largely in the anionic form and are repelled by negatively charged soil colloids. Increasing concentrations are then available for root uptake. This has been shown recently with the acidic herbicide chlorsulfuron 129 (2-chloro-N-(((4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino)carbonyl) benzenesul fonamide) (17, 20). Acidic herbicides exhibit different types of binding to the soil matrix (4, 25, 28). Physical adsorbtion can occur at low soil pH levels when herbicides are in the molecular form, or hydrogen bonding at low pH levels with the carbonyl, amino, and carboxyl groups of soil humus can occur (4, 25%. Herbicides also bind with polyvalent cations which can bind both the carboxyl group of the herbicide and the carboxyl group of humics. Binding of the herbicide to polyvalent cations on clay may also occur (4, 25). Anionic herbicides can bind via water bridges or by anion exchange on the positive sites of aluminum and iron polyhydroxy complexes, or by penetrating the shell of iron and aluminum in the surface of hydroxides and becoming incorporated into the interlayer of the clay (I, 6, 27). The type of binding that occurs is dependent on the percentage and type of clay, the aluminum and iron content, the percentage of organic matter, and the soil pH of the soil system. The relationship of these variables in different soils is important in determining how each of these factors affect herbicide adsorption, and resulting persistence and availability. Therefore experiments where conducted to determine a) the adsorption of imazaquin and AC-263-499 on five different soils and one soil with organic matter removed, and b) the availability and persistence of imazaquin and AC-263,499 on four of the soils. MATERIALS AND METHODS Characteristics of the five soils used in the following experiments are given in Table 6. Adsorption study. Soils were air dryed and sieved through a 2 -mm Sieve. Organic matter was removed from the Capac sandy loam by a hydrogen peroxide treatment. Hydrogen peroxide was added in 50 ml increments to soil contained in a beaker until all organic matter was oxidized. The soil was then leached with distilled water to remove hydrogen ions. To determine equilibration time, 5 g of soil and 5 ml of a 0.5 ppm 14C-imazaquin (Specific activity of 4.61 uCi/uM) and 14C- AC-263,499 (7.23 uCi/uM) were weighed into 30 -ml glass centrifuge tubes. sealed with parafilm-covered rubber st0ppers, and gently shaken on a reciprocal shaker at 23°C at 70 strokes/min. Tubes were removed at O, 4, 8, 12, 24, and 36 h, and centrifuged for 15 min at 12,350 x 9. Two 1 ml samples were removed from each tube and added to glass scintillation vials containing 15 ml of a water-accepting scintillation solution.2 The samples were radioassayed on a liquid scintillation Spectrometer. To determine adsorption, 5 g of soil were placed in 30 -ml glass centrifuge tubes and 5 ml of the appr0priate concentration of 14C- imazaquin or 14C-AC-263,499 at 2.0. 0.5, 0.25, and 0.02 ppm in 130 131 Table 6: Characteristics of soils utilized in soil adsorption and availability studies. Cation Organic exchange ppm Soil Classification pH Sand Silt Clay Matter capicity --------------- '------------- (meq/lOO g) -AT__F3 Capac Aeric Ochraqualfs, sandy fine-loamy, mixed, loam mesic 6.4 57 26 17 2.1 10.1 < l 43 Kilmanagh Aeric Haplaquepts, sandy clay fine-loamy, mixed, loam honacid, mesic 7.3 51 24 25 3.1 12.3 1 68 Kalamazoo Typic Hapludalfs, sandy fine-loamy, mixed, loam mesic 6.5 58 31 11 1.8 7.3 < l 26 Greenville Rhodic Paleudults, sandy clay clayey, kaolinitic, loam thermic 6.6 59 ll 29 1.2 5.0 4 25 Decatur Rhodic Paleudults, silty clay clayey, kaolinitic loam 5.9 18 53 29 2.0 7.1 l 25 132 distilled water added. Maximum concentrations of both herbicides equilibrated with soil were 30 times below the solubility of the acid form of the compounds to prevent precipitation. ‘Tubes were sealed with parafilm-covered rubber stoppers, taped securely, and gently Shaken at 23°C on a reciprocal shaker at 70 strokes/min for 10 h. Samples were then centrifuged at 12,350 x g f0r 15 min, and two 1 ml aliquots of the supernatent were pipetted into separate glass Scintillation vials containing 15 m1 of water-accepting scintillation solution.2 These were radioassayed on a liquid scintillation spectrometer. Differences in radioactivity between known standard concentrations and the supernatents of the samples were assumed to be due to soil adsorption. The experiments were repeated twice with each treatment replicated 3 times. The herbicide adsorbed (ng per g of soil) was plotted on the y axis, and the equilibrium concentration of herbicide remaining (ng per ' ml of solution) plotted on the x.axis. Plots of each herbicide and soil were linear without log transformation. Regression equations were then calculated for each soil and herbicide treatment without log transfbrmations, and single t-tests were used to compare the s10pes of the lines, and determine if they were different from zero and from each other (24). Distribution coefficients (Kd) were calculated to express the ratio of the amount of 14C-herbicide adsorbed to the amount remaining in solution at equilibrium (8, 9). 2Safety-solve, high-flash point cocktail. Research Products. International Corp. Mount Prospect, IL 60056. 133 Persistence and Availability study. Imazaquin and AC-263,499 were applied at 105 g ai/ha to 1100 -m1 pots containing 500 cc of each of the first four soils listed in Table 1. The herbicides were thoroughly mixed throughout the 7.6 cm soil depth. Sixty ml of water was added to each pot to bring the soils to approximately 25% of their field capacity, and the pots covered with polyethylene wrap3 that is oxygen but not water permeable. Each herbicide was applied to 12 pots of each soil, and 12 pots of each soil did not receive herbicide and were used as controls. Three pots of each herbicide applied to 4 soils plus 3 control pots of each soil were immediately placed in the freezer. The remaining pots were divided into 3 groups that were considered the replications and placed in 3 growth chambers. All chambers were set at 26°C and 16°C for day and night temperatures, respectively; Sixteen h of light was supplied by inflorescent tubes and incandescent bulbs supplying from 120 to 190 uE-m'Z-S'1 of photosynthetically active radiation. Pots were maintained in the growth chambers until removal of 12 pots of each herbicide treatment plus the controls at 30, 60, and 120 days after initial application. Removed pots were immediately placed in the freezer. Sixty ml of water was added to each pot in the growth chambers every 30 days to maintain moisture levels. After 120 days all pots were removed from the freezer, equilibrated to 23°C for 2 days, and then the soil was transferred to 600 -ml pots. Three corn 'Stauffer 5650' seeds per pot were planted to a 3.5 cm depth. Corn was grown for 21 days in the greenhouse and thinned to two plants/pot after 3Sysco Plastic Hrap. Sysco Corporation. Houston, Texas 77002. 134 7 days. Temperature was maintained at a maximum of 27°C during the day and a minimum of 16°C at night. Supplemental lighting was supplied by high pressure metal halide lamps to provide a 16 h photOperiod and 360 uE-lll'z-S'l of photosynthetically active radiation. Plants were watered each day with approximately 120 ml of water, and alternated surface and sub-irrigated. After 21 days, shoot fresh weight and height were measured, averaged for each pot, and then converted to a percentage of the weight or height of the control plants grown on that soil and in that replication that received zero herbicide so that any effect of soil type on plant growth would be eliminated. The experiment was repeated twice, and results were not combined because there was an interaction between experiments and soil type. Injury to corn grown on the Greenville soil was significantly different in each experiment. In both experiments, shoot height and weight gave similar results, and only fresh Shoot height data will be presented. RESULTS AND DISCUSSION Adsorption study. The equilibrium time for soil adsorption of 1"'C- imazaquin and 14C-AC-263,499 was 4 h, with no additional adsorption in the next 32 h (data not shown). Soil adsorption was low on all five soils for both herbicides (Figures 16 and 17). Increasing the volume of solution to 10 ml did not increase adsorption (data not shown). Very low adsorption is typical of acidic herbicides such as chlorsul furon, picloram (4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid), and 2,4-0 ((2,4-dichlor0phenoxy)acetic acid) (11, 12, 27). Low Kd values of 0.03 to 0.49, 0.03 to 0.3, 0.14 to 3.38, and 0.54 to 0.123 135 Figure 16: The adsorption of imazaquin on five soils. 136 :53... cozmzcoocoo E:_5___:am 000 w 000 00a 0 I d d d u d - d d H d d 1 d d - - q u q u q .1 . ‘ i \ ‘0 \ ‘ O L 85800 30 i 2.0. u” .u. 1 32.30020 33 + .60. u .4. . Scones-.3. 00.0 + .60. u M .0. 002.00 000.Ix:..u .3 1 2.02.2.5. 3.3 + x... L .0. .. 00w (Elfin) paqmspv 9910101914 137 Figure 17: The adsorption of AC-263,499 on five soils. 138 000w — 4 fl 0059 1 .2505 00:03—30:00 E:_La._.:cm 000.. d d # 000w I q d d u 4 q q q d openness. 3.0 1 x00. ”.... 02.5520 3.0 + .60. .4. e826 00.0 + x3. ”.4. $022.5. «0.0 1 3. ... 12800 «0.0 + .6 F. 1 . .... I l l l I I l I O 0 GOOD a§a§eeae (IBulp c1109 9 000 000 L.l 139 were found for picloram, dicamba (3,6-dichloro-Z-methoxybenzoic acid), 2,4-D, and chlorsul furon, respectively (11, 12, 27). Calculated Kd values for imazaquin and AC-263,499 are given in Table 7. The first four soils listed adsorbed significantly more imazaquin than the Greenville and Decatur soils. These four soils are smectite based and have increasingly higher Kd values as the percentage of smectite clay is increased. All Kd values for AC-263,499 were significantly different from each other as determined by single t-tests (24). There was no significant difference at the 10% level between imazaquin and AC-263,499 adsorption on the Capac sandy loam and also on the Greenville sandy clay loam soils. The greatest difference in soil adsorption between imazaquin and AC-263,499 occurred on the Decatur silty clay loam, and on the Capac sandy loam with organic matter removed (Figures 18 and 19). Examining the soil characteristics, we found the pH of the Decatur soil was significantly lower and the percentage of silt significantly higher than all other soils. Low soil pH or high silt content may be important in AC-263,499 adsorption to kaolinitic soils. Hhen the organic matter was removed from the Capac sandy loam there was no significant difference in imazaquin adsorption, but a significantly greater increase in AC-263,499 adsorption (Figure‘l9%.Hhen organic matter was removed, the soil pH dropped from 6.4 to 4.63. Adsorption of acidic herbicides has been shown to increase when in the molecular form (4, 17, 20). Increased adsorption of AC-263,499 may have occurred because at the lower pH an increased amount of both herbicides existed in the nonionic form. AC-263,499 in the molecular form may have higher affinity to clays than the molecular form of imazaquin, and result in 140 Table 7. Calculated distribution coefficents (Kd) for imazaquin and AC-263,499 on five soils. Soil Imazaquin AC-263,499 ........... Kda --------- Capac sandy loam .105ab .098d Capac sandy loam w/out organic matter .094ab .761a Kilmanagh sandy clay loam .1134 .145c Kalamazoo sandy loam .081bc .060f Greenville sandy clay loam .osscd .01356 Decatur silty clay loam .066Cd .192b aKd values followed by the same letter are not significantly different from each other at the 5% level using single t-test comparisons. Comparisons of letters between herbicide columns are not valid. 141 Figure 18: ‘The adsorption of imazaquin and AC-263,499