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This is to certify that the
thesis entitled

Response of 'Wakefield' Winter Wheat
(Triticum aestivum) to Dicamba

 

presented by

Matthew J. Rinella

has been accepted towards fulfillment
of the requirements for

M. S . degree in Crop Science

QWQ. AZ/

Major professor
/
Date Mm“, 4A W28

0-7639 MS U is an Affirmative Action/Equal Opportunity Institution

 

 

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RESPONSE OF ‘WAKEFIELD’ WINTER WHEAT (T riticum aestivum)
TO DICAMBA

By

Matthew J. Rinella

A THESIS
Submitted to
Michigan State University

in partial fulfillment of the requirements
for the degree of

MASTERS OF SCIENCE

Department of Crop and Soil Science

1998

ABSTRACT

RESPONSE OF ‘WAKEFIELD’ WINTER WHEAT (T riticum aestivum)
TO DICAMBA

By

Matthew J. Rinella

Dicamba is a herbicide that is used for the control of broadleaf weeds in wheat.
Typically, when dicamba is applied at proper rates and grth stages, little or no crop
injury results. However, the grain yield of ‘Wakefield’ winter wheat is severely
depressed by dicamba applied at typical rates and stages. Two years of research have
confirmed that dicamba has minor effects on the number of spikelets and the seed weight
of ‘Wakefield’. Dicamba drastically reduces the number of normal seeds per spikelet of
‘Wakefield’ at typical rates and timings. Normal seeds are replaced by small, shn'veled
seeds that are lost during mechanical harvest. Greenhouse studies showed that untreated
seed bearing plants hybridized with treated pollen source plants develop very few
abnormal seeds, while treated seed bearing plants hybridized with untreated pollen source
plants develop abnormal seeds. Hence abnormal pollen is not responsible for the

formation of abnormal seeds.

ACKNOWLEDGMENTS

I would like to express my heart-felt thanks to the individuals that aided me in the
work that made this thesis possible. My committee members (Jim Kells, Rick Ward, and
Jack Kelly) gave me invaluable direction. I am indebted to them for taking the time to
share there expertise with me. My adviser (Jim Kells), luckily, has proven to be a very
patient and understanding mentor. I thank him for assuming the risks inherent in taking a
greenhom under ones wing.

I extend my gratitude to Brent Tharp, Christy Sprague, Joe Simmons, and Andy
Chomas. Only people of there indelible character could make possible laughter in the
grave shadow of so many lambsquarters plants. I feel deep anxiety when I contemplate a
world without Christy Sprague, Brent Tharp, Kelly Nelson, and Jason F ausey to entertain
my ignorant software questions. I will never again be able to hold my head high while
asking dumb city boy farm questions like I could when asking Andy Chomas and Joe
Simmons. I want to thank Stephanie Eickholt and Rhonda Bafus for their diligent
attention to detail in the face of the most tedious of tasks. And finally, I would like to
thank Gary Powell for allowing me to make stew from the bunnies that abound on his
lovely farm.

And with a more resounding finality, I would like to thank my family for their loving
support.

iii

TABLE OF CONTENTS

PAGE
LIST OF FIGURES .................................................... vi
CHAPTER 1. REVIEW OF LITERATURE
Introduction ...................................................... 1
Germination, Growth, and Development of Winter Wheat .................. 2
Germination ................................................ 2
Growth and development ...................................... 2
Wheat Production in the Midwest ..................................... 6
Planting ................................................... 6
Fertilization ................................................ 7
Harvest .................................................... 7
Weed Competition and its Effects on Cereals ............................ 8
Competition ................................................ 8
Effects of weeds on yield components ............................ 9
Effects of weeds on seed quality ................................ 9
History and Properties of Dicamba .................................... 9
History .................................................... 9
Site and mode of action and broadleaf weed symptoms ............. 10
Selectivity of dicamba ....................................... lO
Metabolism ............................................... 12
Differential Sensitivity of Wheat Cultivars to Herbicides .................. 12
Herbicide tolerance testing .................................... 13
Testing of cereal varieties for tolerance to dicamba ................ l6
Restrictions ............................................... 1 7
Genetic basis of differential sensitivity .......................... 17

The effects of growth stage on cereal response to grth regulator herbicides . 18
Relation of external stages of cereals to stages of apical development . . 19
Evaluation of the safety of dicamba application timings for cereals . . . . 20

The effects of grth regulator herbicides on cereals ..................... 21
Root effects ............................................... 22
Leaf effects ................................................ 22

iv

TABLE OF CONTENTS (cont)

Maturity effects ............................................ 22
Stem effects ............................................... 23
Head effects ............................................... 24
Seed effects ............................................... 24
Protein, flour, mineral content, and baking quality effects ........... 26
Cytological effects .......................................... 27
‘Wakefield’ wheat ................................................ 28
‘Harus’ wheat .................................................... 28
Literature cited .................................................. 30

CHAPTER 2. RESPONSE OF ‘WAKEFIELD’ WINTER WHEAT
(T riticum aestivum) TO DICAMBA

Abstract ........................................................ 40
Introduction ..................................................... 41
Materials and Methods ............................................. 45
Field study ................................................ 45
Greenhouse study ........................................... 47
Hybridization study ......................................... 48
Results and Discussion ............................................ 50
Field and greenhouse study ................................... 50

Grain yield in field .................................... 50

Seed weight per head in greenhouse ...................... 51

Seeds per spikelet in field .............................. 52

Seeds per spikelet in greenhouse ......................... 52

Spikelets per head in field .............................. 53

Spikelets per head in greenhouse ......................... 53

Seed weight ......................................... 54

Effect of dicamba on seed development in field ............. 54

Effect of dicamba on seed development in greenhouse ........ 56

Hybridization study ......................................... 57
Literature cited ............................................. 6O

LIST OF FIGURES

CHAPTER 2. RESPONSE OF ‘WAKEFIELD’ WINTER WHEAT
(T riticum aestivum) TO DICAMBA

PAGE
Figure 1. 1995 herbicide sensitivity study .................................... 41
Figure 2. ‘Wakefield’ wheat treated Feeke’s stage 5 ............................ 42
Figure 3. F eeke’s scale and growth stages for field and greenhouse studies .......... 46
Figure 4. Observed structures in wheat heads ................................. 54
Figure 5. Effect of dicamba on grain yield and yield components in field 1996 ....... 63
Figure 6. Effect of dicamba on grain yield and yield components in field 1997 ....... 64
Figure 7. Response of yield components to dicamba in greenhouse run 1 ........... 65
Figure 8. Response of yield components to dicamba in greenhouse run 2 ........... 66
Figure 9. The effect of dicamba on seed quality in field 1997 .................... 67
Figure 10. The effect of dicamba on seed quality in greenhouse ................... 68
Figure 11. The effect of dicamba on seed bearing and pollen donating plants ........ 69

vi

CHAPTER 1

REVIEW OF LITERATURE

INTRODUCTION

Dicamba is a herbicide that is used for the control of broadleaf weeds in winter wheat
(T riticum aestivum L.) (4). Research has identified safe rates and growth stages for the
application of dicamba to winter wheat (60). However, some varieties of winter wheat
are injured by dicamba applied at rates and growth stages that are generally considered
safe (28, 88). ‘Wakefield’, a soft red winter wheat (103), is severely damaged by
dicamba applied at labeled rates and growth stages. Many efforts have been made to
evaluate the effects of dicamba on wheat (20, 42, 108). The findings of some of these
reports conflict. The objective of this research was to characterize the sensitivity of

‘Wakefield’ winter wheat and to compare its sensitivity to that of ‘Harus’ winter wheat.

 

Germination. Winter wheat (T riticum aestivum L.) is planted in the fall (77, 15 ).
With proper moisture, oxygen, and temperature, germination quickly follows planting
(96). Germination begins with swelling of the seed coat as it is permeated by water and
oxygen. Once stimulated by water and dissolved oxygen, the scutellum, a specialized
absorptive organ belonging to the embryo, begins to secrete gibberellic acid (99).
Gibberellic acid is absorbed by the aleurone layer, a secretory organ surrounding the
endosperm. Gibberellic acid in turn triggers the release of starch degrading enzymes
from the aleurone layer into the starchy endosperm of the seed (104). These enzymes
break down the starch in the endosperm forming simple sugars. The scutellum, in
addition to supplying its own reserves, proceeds to absorb and transport food materials
(sugars and amino acids) from the endosperm to the growing embryo (56). In this
manner the embryo receives the nutrition necessary for early growth and development.

Upon germination the root sheath (coleoriza) containing the primary root grows
through the outer layers of the seed (pericarp) (77). About one day later the primary root
breaks through the coleoriza and seminal roots begin to develop.

At the same time that the seminal roots are developing the stem is growing upward.
The coleoptile, a protective leaf enclosing the first true leaf, emerges through the soil and
stops growing, and the first true leaf grows through the coleoptile (98).

Growth and development. Several strategies have been developed to characterize the
growth stages of wheat after emergence (54, 111, 124). Of these methods, Feeke’s scale

which was later modified and illustrated by Large, is the most commonly used (113).

3

These methods are of importance in identifying the phases in wheat development that are
appropriate for particular management practices. An ability to identify the developmental
stages of cereals can aid in the selection of effective and safe times for the application of
fungicides and herbicides (124), and it can also assist in selecting the proper time to
make rust, mildew, and other pest assessments (77).

Upon germination, when the wheat plant has one or two leaves, it is at F eeke’s stage 1.

Later in the fall, if conditions are mild, the wheat plant begins to put out branches
known as culms or tillers (98). Tillers arise from buds that exist on the nodes of the stem.
These nodes are positioned at or just below the surface of the soil (7 7). The number of
tillers a plant develops is dependent on several environmental factors. The number of
tillers is positively correlated with soil fertility, moisture (119), and plant spacing (107),
and is negatively correlated with plant density (117) and lateness of planting (44). Only
tillers that are formed early in the development of the plant are likely to mature and
produce seed (45). At the inception of tillering the plant is at Feeke’s stage 2.

During the coldest periods of the year winter wheat goes through a phase(s) of slowed
and/or arrested development. These cold periods act to vernalize winter wheat.
Vernalization is necessary for timely flowering (75).

With the warmer temperatures of spring winter cereals resume rapid grth and begin
actively tillering (98). When winter wheat is fully tillered it is at Feeke’s stage 3. When
the tillers are formed the leaf sheaths begin to lengthen and form a pseudostem. As the
leaf sheaths are lengthening the plant is at Feeke’s stage 4.

Later, when the leaf sheaths become strongly erect and form a pseudostem, the plant is

4

at Feeke’s stage 5. Following the formation of a pseudostem, the wheat plant undergoes
a period of rapid development called stem extension or jointing (7 7). During stem
extension the stem of the plant along with the developing head grows up through the
pseudostem as one of the intemodes begins to elongate. The node above this elongated
intemode becomes detectable as a swollen nob on the stem (56, 17). When this node
becomes visible the plant is at Feeke’s stage 6. At the same time that jointing is
occurring, the head is differentiating and growing.

When a second intemode elongates and a second node becomes visible on the stem,
the plant is at Feeke’s stage 7. This process of intemode elongation and node formation
continues until the last stem section, the peduncle, expands. The peduncle is the section
of the stern that is directly connected to the head (98). Each expanding intemode is
longer than the one proceeding it, and the peduncle can account for as much as 50% of
the stem length (32).

When the last leaf (flag leaf) of the plant first becomes visible, the plant is at Feeke’s
stage 8. When the ligule of the last leaf emerges the plant is at Feeke’s stage 9. This
stage is easily identified, because unlike earlier stages, no subsequent leaf tips are visible
in the whorl of the plant. When the seed head is enshrouded by the sheath of the flag leaf
the wheat plant has reached Feeke’s stage 10. At F eeke’s stage 10.1 the head is just
escaping through the split in the sheath of the flag leaf, and by Feeke’s stage 10.5 the
head is fully emerged.

The head (spike, ear) of the wheat plant is made up of a notched main axis (rachis)

which is an extension of the stem (17). The Spikelets, consisting of two protective

5

glumes and several florets, are arranged alternately along the rachis and are attached via
small branches (rachilla). As many as nine florets can develop within each spikelet, but
typically only two or three florets are fertile within any spikelet (17).

Each floret has two glumes (lemma and palea). The lemma and palea surround three
stamens, one pistil, and two highly branched styles (56). Flowering (anthesis) typically
begins a few days after the completion of heading. The initiation of flowering is signified
by the swelling of two primary leaves (lodicules) situated at the base of the ovaries (l 7).
When the lodicules swell they force a separation of the lemma and palea and a spreading
of the styles (77). While the lemma and palea are separated the filaments of the anthers
elongate rapidly, reaching three times their original length in about three minutes. In this
manner the anthers are puShed out of the florets. As the anthers extrude they split open
on the sides and shed their pollen. Some of the pollen is shed inside the floret and some
external to the floret where it can be carried by wind. Some of the pollen grains that have
been shed become wedged in the stigmas. On occasion, pollen from one floret reaches
the stigma of a floret on another head, but generally wheat is self-pollinated. Out-
crossing has been estimated to occur in only 1 to 4 percent of florets under field
conditions. Under conditions conducive to male sterility, out-crossing can occur with
much greater frequency (15). About 20 minutes after lodicule swelling, the lemma and
palea close and anthesis is complete (98). The initiation of flowering is called Feeke’s
stage 10.5.1. Flowering is complete at Feeke’s stage 10.5.4.

Roughly two hours after a pollen grain is distributed it gerrninates and forms a pollen

tube. This pollen tube, containing two reproductive nuclei, grows into the opening of the

6

ovule (micropyle) and discharges its nuclei into the embryo sac. One nuclei fuses with
the egg nucleus to form the zygote, while the other unites with two polar nuclei forming a
single nucleus. This nucleus belongs to the first cell of the endosperm (77).

Seed ripening is a gradual process that consists of several stages. In the watery-ripe
stage the seed is growing quickly. This increase in size is due to rapid cell division in
both the endosperm and the embryo. During the milky-ripe phase a white milk-like
liquid can be squeezed out of the seed. By completion of the milky-ripe phase the
embryo is fully developed. When the seeds of a plant are in the milky stage the plant is at
Feeke’s stage 11.1. Seed is at the mealy-ripe stage when the contents of the seed take on
a meal-like or doughy consistence. When the contents of the seed have attained this soft,
dry stage, the plant is at Feeke’s stage 11.2. By the fiilly-ripe stage the plant is
completely yellow and the seed is firm with the endosperm having a starchy texture. The
wheat plant is now at F eeke’s stage 11.3. The final stage of the wheat plant is referred to
as the dead-ripe stage. The plant is dry and brittle and the seed is very hard. When the
seed is ripe for harvest the plant is at Feeke’s stage 11.4 (77).

Winter wheat is preferred in the Midwest because it matures earlier and generally
yields better than spring wheat. Early maturity is considered advantageous because
wheat is exposed to damaging diseases for a shorter period of time (76). Early maturity
also prevents conflicts between wheat harvest and the harvest of other crops such as corn
and soybeans.

Planting. Winter wheat is sown from the middle of September to the middle of

7

October in the Midwest with recommended seeding dates falling earlier in the northern
parts and later in the southern parts of the region (84). Planting is delayed to avoid
feeding damage by the Hessian fly maggot (mayetiola destructor), which is one of the
most damaging of the numerous arthropod pests of wheat (23). Some pests and cultural
problems are favored by early planting while others are advantaged by later sowing.
Selection of a seeding date can be viewed as a balancing of several perils, but it is
nonetheless a valuable management tool (74). Wheat often follows soybeans as part of a
rotation (87). Soils, following soybean, harvest are typically loose and require minimal
tillage to prepare them for wheat planting. Seed drills that can operate in untilled soil
have provided an opportunity for the planting of wheat directly into soybean stubble.
Herbicides may be deemed necessary to kill existing weeds at the time of planting (76).
Wheat is typically planted at a rate of 60 to 120 kg/ha and a depth of 2 to 4 inches with
higher rates recommended at later planting dates (75).

Fertilization. The fertilization practices utilized in wheat production are numerous in
the Midwest. The quantities and types of fertilizers applied are based largely on soil tests
and yield goals (76). Nutrients are typically applied in the spring due to concerns of
nitrogen loss. Occasionally applications are split between fall and spring. Nitrogen is
usually the limiting nutrient in wheat production with applications of phosphorus,
potassium, and other nutrients occasionally deemed necessary.

Harvest. Winter wheat is usually harvested with a combine in mid to late summer.
Harvest is the most critical operation in the growing of cereals. By the time a wheat stand

matures it has received many inputs, and return on investment hinges on a successful

8

harvest (102). Weather conditions at harvest should be dry, and the crop should be ripe.
Combines can only operate efficiently when the crop is ripe and dry. Successfirl storage
of grain is also favored by a dry crop (102).
Competition. Weeds compete with cereals for nutrients, moisture, light, and space
(77). Low yields can result from this competition or from allelopathy (10). Yield is often
not directly proportional to weed numbers. Relative crop and weed emergence is of
primary importance in determining the impacts of a weed infestation on yield (91).

Crops and weeds have similar nutrient requirements and differ only in the specific
amounts of nutrients required for optimum growth (126). Because they are needed in
large amounts and because they are often the limiting factor for plant growth,
phosphorous and nitrogen are the two plant nutrients most often considered in
competition studies. However, it is believed that competition for any vital plant nutrient
can occur (127). Whether fertilizer favors the crop or the weed is a question of some
debate (126). At least one case has been documented in which fertilizer has increased
weed growth to such an extent that the crop experienced increased competition (64).

Like crop plants, weeds use large amounts of water. On a plant per plant basis some
weed species require much more water than the crop species in which they grow. It has
been estimated that one mustard plant removes four times as much water fiom the soil as
one oat plant (2).

Light intensity can be a major factor affecting plant growth. Shading of soil and
weeds by a crop can reduce weed competition (36). A competitive advantage is given to

plants that have their leaves positioned in a way that maximizes light interception (2).

9

The physical positioning of leaves is more critical than total leaf area in predicting the
outcome of competition (126).

Effects of weeds on yield components. Weeds can affect many yield components of
cereals. Scragg et a1. (91) observed that competition from wild mustard (Sinapis
arvensis) reduced head number but had no effect on the head or seed weight of barley.
Similarly, Farahbakhsh et al. (34) found that the main effect of wild-oat (Avenafatua),
black-grass (Alopecurus myosuroides), and common chickweed (Stellaria media) on
wheat yield was on the number of fertile tillers per plant. Teer (67) reported that the yield
component of spring barley that was most affected by weed competition was the number
of seeds per head. It has been shown that the weight of 1000 seeds of barley can be
reduced by shading (67).

Effect of weeds on seed quality. Weeds can have many undesirable effects on cereals
in addition to causing yield reduction. Grain that contains a large amount of weed seed is
worth less money when sold. Weeds like wild garlic (Allium vineal) can impart an off-
flavor to grain products. Weeds can interfere with harvesting and can raise the moisture
content of grain (10).

H' | I E I' [D . I

History. Zimmerman and Hitchcock (128) first reported that the substituted benzoic
acids had growth-regulating properties in 1942. The herbicidal properties of 2,3,6-TBA
(2,3,6-trichlorobenzoic acid) were first evaluated in field experiments in England and the
United States in 1948 (50). Dicamba (3,6-dichloro-o-anisic acid) differs from 2,3,6-TBA
only in that it has a methoxy group at the number 2 position instead of a chlorine.

Dicamba was developed by Velsicol® and first appears in a North Central Weed Control

10

Conference publication in 1959 under the name Velsicol B (9). In 1961 it was referred to
as Banvel D, and by 1963 it assumed its current name in the North Central Weed Control
Conference literature (3, 6). Later, Sandoz Agro® attained the rights to dicamba before
selling them to BASF®. Dicamba is registered under the trade name Banvel® for the
control of broadleaf weeds in corn, sorghum, small grains, and other crops. The
maximum rate of Banvel® recommended in winter cereals is 0.14 kg ai/ha. Another
formulation containing dicamba is registered as Clarity® for use only in corn (4, 5).

Site and mode of action and broadleaf weed symptoms. The mode of action of
substituted benzoic acids such as dicamba is similar to that of 2,4-D (2,4-
Dichlorophenoxyacetic acid) (63). These herbicides alter hormone balance and protein
synthesis which results in abnormal growth of plants. Stem twisting (epinasty), callus
tissue development, leaf malformations (cupping, crinkling, parallel veins, leaf
strapping), and plant death often occur in broadleaf plants as a result of exposure to these
herbicides (40).

Selectivity of dicamba. Chen et a1. (20) studied the response of cucumber (Cucumus
sativus L.) , a susceptible species, and wheat to four auxin-like herbicides. All of the
herbicides increased the DNA and protein content of the roots of cucumber and wheat
with the greatest increase occurring in cucumber. The greatest difference in the response
of the two species was in the effect on RNA levels. Increasing concentrations of all four
herbicides decreased RNA levels in wheat and increased the levels in cucumber. It
appears that auxin-like herbicides make more of the DNA template available for
transcription and thus increase RNA production in cucumber. Two studies have

suggested that the increase in RNA levels of susceptible species is due to the uncoiling of

ll

nucleohistones. The uncoiling of DNA nucleohistones facilitates enhanced transcription
(19, 11). The herbicides increased the protein per unit RNA of wheat. Conversely, the
protein per unit RNA of cucumber was 50% lower after treatment. This suggests that the
extra RNA produced in susceptible plants may be faulty and incapable of coding for
proteins. It is also possible that the aberrant RNA competes with normal RNA with a
resultant synthesis of abnormal proteins. These abnormal proteins may be responsible for
the distorted growth associated with dicamba. A later study showed that the
incorporation of l“C-leucine into protein in cucumber roots was decreased 70% 10 hr
after treatment with dicamba (19).

Chang and Vanden Born (18) found that dicamba was absorbed more rapidly and
completely by Tarany buckwheat (F agopyrum tataricum L.) and wild mustard (Sinapis
arvensis L.), two susceptible species, than by wheat and barley. More dicamba was
translocated out of treated leaves of Tarany buckwheat and wild mustard than wheat and
barley one or more days after treatment. Metabolism of dicamba occurred in all four
species but not at the same rate. Dicamba was metabolized more slowly in Tarany
buckwheat and wild mustard than in wheat and barley. A major metabolite (5 -hydroxy—
3,6-dichloro-o-anisic acid) was common to all four species. A minor metabolite (3,6-
dichlorosalicylic acid) was present in wheat and barley but not in the other two species.
These results suggest that differences in the sensitivity of these species can be explained
by a combination of uptake, translocation, and metabolism.

Quimby and Nalewaja (83) submerged leaf sections of wheat and wild buckwheat
(Polygonum convolvulus), a susceptible species, in a dicamba solution and found that

differences in the susceptibility of the two species could not be explained by differences

12

in uptake. l“C-dicamba accumulated in the meristems of wild buckwheat but not in
young tillers of wheat. The main culms of wheat conjugated and/or metabolized l“C-
dicarnba more rapidly than the meristems of wild buckwheat. These results suggest that
differences in tolerance between the two species can be attributed to translocation and
metabolism.
Metabolism. Broadhurst et al. (16) found that there was no free dicamba remaining in
wheat plants 18 days after treatment. A small amount of conjugated dicamba remained
after 29 days. 5-hydroxy-2-methoxy—3,6—dichlorobenzoic acid was the major metabolite
(90%) after hydrolysis. 3,6-dichlorosalicylic acid and the parent compound were minor
metabolites after hydrolysis.
e ° ' i f W 'v

The selectivity of a herbicide between a weed and a crop is never absolute. In
addition to environment and crop stage, the genetic makeup of the crop is also a factor
(14, 21). Differences in the response of cereal varieties to herbicides are common. The
first reports of varietal sensitivity did not occur with a herbicide but with the insecticide
DDT. Shortly thereafter, differences in the response of some barley varieties to one of
the first wild-oat (Avenafatua) herbicides (barban) and slight differences in the
sensitivity of cultivars of cereals to 2,4-D became evident. With the development of the
substituted urea herbicides (chlortoluron and metoxuron) for the control of black-grass in
Europe the number of reports of differential sensitivity increased. Some wheat varieties
tolerate four times the recommended dose of chlortoluron with only slight yield
suppression while other varieties suffer considerable injury from the standard rate (116).

Since the time of these early reports the differential sensitivity of cereal cultivars to

13

numerous classes of herbicides has been studied (114, 69, 101, 118, 115, 100, 68, 58, 71,
59). Particular attention has been paid to the growth regulator herbicides dicamba and
2,4-D (28, 86, 88, 106, 85, 35, 81, 43, 24, 52, 12, 51, 31, 46, 79, 95, 120).
Herbicide tolerance testing. As the number of cases of differential sensitivity
increased, widespread testing of cultivars for sensitivity to herbicides began. The
responsibility for testing crop tolerance is often unclear or unexcepted (110). It is the
responsibility of the plant breeder to develop new varieties of cereals that have high yield
potential and seed quality. It is important that these varieties produce acceptable yields
over a wide range of environmental conditions and show adequate resistance to insect,
disease, and other hazards to which they may be exposed. Whether herbicide tolerance is
a criteria with which plant breeders should be concerned is debatable. It can be claimed
that breeders should produce varieties that are tolerant to the herbicides to which they are
subjected, and conversely, it can be argued that it is the duty of agrochemical companies
to ensure that their herbicides do not injure the crops to which they are applied (58).

Agrochemical companies typically test their herbicides on currently used varieties and
sometimes a few that are nearing release. Plant breeders conduct limited testing of new
varieties with widely used herbicides (110). When contemplating the regularity of
occurrence of problems with particular combinations of herbicides and varieties, it
becomes evident that these screening programs are insufficient. It would be highly
impractical to evaluate every herbicide/variety combination, but a testing procedure that
examines the combinations that are considered most dangerous is advisable (115).

A wide margin of safety between the amount of herbicide required for acceptable

weed control and the amount necessary to cause crop injury is critical (118). Identifying

14

safe combinations and earmarking potentially dangerous ones can be difficult. Crop
injury is a notoriously variable phenomenon (108). Damage from herbicides is not easily
predicted, and it is often associated with atypical growing conditions (115).
Environmental factors such as temperature, precipitation, and soil type can have a
profound impact on the response of a crop to a herbicide. Often one year of testing at one
location is not enough to gauge the safety of a variety/herbicide combination (35).
Conversely, several seasons may pass before conditions arise that favor crop injury (116).
This is distressing, because a particular cultivar can become popular and widely
distributed before a problem arises.

Herbicide screens typically consist of cultivars planted in long narrow rows in one
direction and herbicides cross-applied in the other direction. Usually these studies are
not replicated and only visual injury is assessed. These simple screens expose major
varietal differences in herbicide tolerance, but a less dramatic effect is often better
measured in replicated yield trials (115). Herbicides are often applied at multiples of the
recommended dose to mimic the effect they would have in years when the environmental
conditions favor crop injury. The assumption that a high rate of herbicide simulates what
occurs under conditions of extreme herbicide activity is imperfect, but it is a practical
alternative to repeating experiments in several years and/or several locations (110). If a
variety tolerates a herbicide applied at three or four times its normal rate it is reasonable
to assume that the combination is safe. If injury occurs from the single dose then the
variety is classified as sensitive. Varieties that operate between these two extremes
require more critical testing to establish if crop safety is adequate (116).

The detection of cases of mild sensitivity to herbicides are of some importance. It has

15

been shown that many growers in parts of England are unlikely to lose yield by not
applying herbicides to their cereal crops for one year (33). Any benefit from the use of a
herbicide can be lost if the herbicide even slightly injures the crop.

The frequency and magnitude of differences in the response of cereal varieties to
herbicides varies with the particular herbicide. Fiddian (35) reported on the effects of
different herbicides applied to 17 winter wheats, 38 spring wheats, 30 spring oats, and 58
spring barleys. Differences in the response of cultivars to MCPA and 2,4-D were small.
Differences in the response to mecoprop and 2,3,6-TBA may be of practical importance,
and there were great differences in the response of varieties to barban. Sarpe et al. (86)
found that 2,4—D and dicamba caused injury to some varieties of wheat when applied late,
while bromofenoxim was tolerated by all varieties regardless of the growth stage at the
time of application. Snape et al. (101) reported that wild populations of emmer wheat
were polymorphic for their response to chlortoluron and metoxuron, while all of the
populations that were studied were resistant to difenzoquat.

It has been suggested that strong varietal differences in tolerance are not likely to stem
from differences in plant structure or differences in the uptake and translocation of the
herbicide in the plant (116). It is believed that the tolerance mechanisms by which
cereals avoid injury include deactivation of the herbicide, inability to change the
herbicide to its active form, and differences in the site of action.

Genetic studies have revealed that many differences in the tolerance of varieties to
herbicides are controlled by one or two major genes, and therefore, in these cases the
characteristics are simply inherited (116). Herbicide sensitivity that is simply inherited

can be tracked in breeding programs. This allows breeders to avoid susceptible lines if

l6

materials that are otherwise similar are available. If plant breeders are aware that they
have released a sensitive line they are able to issue warnings about the possibility of
herbicide damage (59). When assessing the value of potential varieties, it is important
that breeders are aware that susceptibility to a herbicide may have been bred into a
particular line. Many potentially valuable varieties may have been disregarded because
they were sensitive to the herbicide used in the selection plot (110).

Testing of cereal varieties for tolerance to dicamba. Edwards and Miller (28)
applied dicamba + MCPA (0.07 + 0.14 kg ai/ha) at the 3.5 to 5-leaf stage and the 6 to 7-
leaf stage to 10 varieties of spring wheat. The yield reductions ranged from 6% to 24%
when averaged across application timings. With yield reductions of 6% at the 3.5 to 5-
leaf growth stage and 41% at the 6 to 7-leaf stage, ‘Waldron’ proved to be the most
sensitive of the varieties studied. The most tolerant variety was ‘Coteau’. Its yield was
reduced 6% at the 3.5 - 5-leaf growth stage and 5% at the 6 to 7-leaf stage.

Schroeder and Banks (88) evaluated the sensitivity of several soft red winter wheat
varieties to dicamba. Dicamba was applied at 0.14 and 0.21 kg ai/ha at F eeke’s stage 3
and 4. ‘Coker 916' treated at F eeke’s stage 4 with 0.21 kg ai/ha of dicamba yielded 14%
lower than its control, while the yield of ‘Coker 983' and ‘Stacy’ was not significantly
affected by any treatment. At another location, dicamba applied at Feeke’s stage 4
reduced the yield of ‘Florida 302' 16% below its untreated control when applied at a rate
of 0.14 kg ai/ha. The yield of ‘Florida 302' was reduced 43% below its control with the
0.21 kg ai/ha rate at this same growth stage and location. ‘Florida 301' and ‘Coker 983'
were not significantly affected by these treatments.

Tottrnan (106) evaluated the safety of a mixture of dicamba + 2,3 6-TBA + MCPA +

17

mecoprop (0.10 + 0.14 + 0.56 + 0.84 kg ai/ha) on three varieties of wheat. Ear dry
weight was calculated as a mean that resulted from 10 treatments made 3 to 4 days apart
between F eeke’s stage 4 and 10. The treatment applied at Feeke’s stage 7 reduced the
yield of ‘Maris Ranger’ and ‘Maris Huntsman’ 20% and 40%, respectively, while having
no effect on the yield of ‘Capelle’. The injury in this study appeared to be caused by
dicamba. In a second study, dicamba (0.28 kg ai/ha) significantly reduced the ear dry
weight of ‘Maris Ranger’ (33%) and ‘Maris Huntsman’ (17%) . The ear dry weight of
‘Cappelle’ was not significantly affected by any treatment.

Robison and F enster (85) evaluated 12 herbicide treatments applied to five cultivars of
winter wheat. Dicamba was applied at 0.14 and 0.21 kg ai/ha to wheat that was at
Feeke’s stages 1,2, 9, and 10.5. The yield of ‘Guide’ was reduced 14% at the lower rate
and 24% at the higher rate. The yield of ‘Scout’ was not significantly affected by the
0.14 kg ai/ha rate but was reduced 20% by the 0.28 kg ai/ha rate. The yield of ‘Trapper’
was also not affected by the lower rate, but the higher dose reduced the yield of ‘Trapper’
26%. The yield of ‘Lancer’ and ‘Gage’ was not significantly suppressed by either rate.
Restrictions. The use of several cereal varieties and some herbicides has been
restricted due to problems with herbicide tolerance. The application of difenzoquat,
barban, metoxuron, and chlortoluron is only recommended on certain wheat varieties
(115, 100). The commercial development of the spring oat variety ‘Margarn’ was
abandoned after it was discovered that it was sensitive to several herbicides (116).
Genetic basis of differential sensitivity. Lupton and Oliver (59) discovered that
metoxuron tolerance is simply inherited by the progeny of crosses between varieties of

resistant and susceptible winter wheat. In some crosses tolerance appeared to be

l8

determined by a single recessive gene while in other crosses two genes seemed to be
involved. Lallukka (52) studied the tolerance of eight spring barley cultivars to a mixture
of dicamba, MCPA, and mecoprop. The yield decreases that occurred in the study
seemed to be caused by dicamba. A genetic aspect of the sensitivity seemed apparent,
with the two most susceptible varieties sharing the same parent. Snape et al. (100) were
the first to locate a gene conferring herbicide resistance to wheat. It has been suggested
that this gene may prevent difenzoquat from inhibiting DNA synthesis at the site of
action of the herbicide (73). Other chromosomes carrying genes that rrrildly alter the
degree of tolerance of wheat to difenzoquat were also found in this study. It was
speculated that these genes may affect the retention and translocation of the herbicide.

The origin of genes conferring herbicide resistance to wheat has been explored (68,
101). Differences in the response of wild populations of emmer wheat (T riticum
dicoccoides), the progenitor of all cultivated wheats (129), to chlortoluron, metozuron,
and difenzoquat were evaluated. All populations of the wild species were resistant to
difenzoquat. This was unexpected, because cultivated wheats are polymorphic for their
response to difenzoquat. This suggests that susceptibility to difenzoquat is a result of
cultivation. Like the cultivated species, wild emmer wheat was polymorphic for its
response to chlortoluron and metozuron. Individual families responded similarly to
chlortoluron and metozuron, suggesting that resistance to these herbicides is determined
by the same genetic control. These results show that genes conferring resistance to
chlortoluron, metozuron, and difenzoquat evolved before the domestication of wheat and
not as a result of the use of these chemicals.

.0
_i"i',‘i'iur 1"“ ',ii_i_"i'i I afigr _l'| g'

19

The tolerance of cereals to growth regulator herbicides is highly dependent on the
developmental status of the apical meristem at the time of treatment (113). There appear
to be two developmental phases of cereals that are particularly sensitive to ‘hormone’
herbicides (105). The first phase occurs during spikelet initiation. Applications of
herbicides made when Spikelets are differentiating can alter cell division and
development leading to malformations in the head (62). Early applications of MCPA and
2,4-D can cause abnormalities by altering the arrangement of new leaf and spikelet
primordia (62, l, 39, 53, 90). Typically this period of sensitivity ends when the spikelet
primordia are clearly differentiated (111). Later applications of ‘hormone’ herbicides are
believed to cause injury by affecting meiotic divisions in the pollen and egg mother cells
(25, 57, 70, 80). These divisions usually take place when the head is approximately 2 cm
in length (105). It has also been suggested that later applications of dicamba may affect
the assimilate supply to the developing seed (112).

Relation of external stages of cereals to stages of apical development. When
attempting to properly time the application of herbicides, it is impractical to ask growers
to dissect plants and determine the developmental phase of the developing meristem. For
this reason, application recommendations are based on external features that are hopefully
correlated with the apical status of the crop (108). It is widely held that some stages of
external development roughly correspond with stages of apical development. Usually the
spikelet primordia are developed by Feeke’s stage 5 (112). For this reason, applications
of 2,4—D and dicamba are often made at this stage. The cell divisions that give rise to egg
and pollen cells occur when the flag leaf begins to emerge and the 3rd node of the plant is

detectable (Feeke’s stage 8) (105). For this reason applications of growth regulator

20

herbicides should be avoided at this stage.

The application of 2,4—D and dicamba is recommended from the end of tillering until
the beginning of jointing (40). Although this guideline has some practical merit, it
remains very crude. The fully tillered stage cannot be easily defined and tillering does
not accurately predict apical development (105, 111). Tottrnan (111) tried to correlate
tiller number, leaf number, stem extension, and leaf sheath length with apical
development. Studies were conducted using several varieties at three locations during
three years to insure that any observed correlations would be consistent with different
environments and varieties. Tiller number did not correlate well with the stage of apical
development. The number of leaves on the main stem correlated only slightly better than
tiller number. A reasonably clear prediction of the stage of development of the head
could be made from the length of the stem. This would eliminate the need for a
microscope for timing herbicide applications, but it would still require growers to dig up
and dissect plants. Leaf sheath length correlated very highly with apex stage, and its
measurement is very practical.

Evaluation of the safety of dicamba application timings for cereals. Much research
has been conducted to identify safe growth stages for the application of herbicides to
cereals (29, 108, 114, 47, 81, 66, 43, 60, 28, 86, 61, 85, 37, 55, 41, 112, 107, 105, 8, 121,
30, 27, 31, 48, 92, 41, 89, 85). Following is a review of some of the research that lead to
the development of recommendations for timing applications of dicamba to cereals.
Martin et al. (60) applied 0.14 kg/ha of dicamba to spring wheat at Feeke’s stage 1, 3,
and 10. The experimental area was kept weed-free throughout the growing season. The

yield of wheat treated at the mid-boot (F eeke’s stage 10) growth stage was reduced 28%

21

below that of the control. The other two treatments did not significantly affect wheat
yield.

Martin et al. (61) conducted another study evaluating the effects of growth stage on
the fitness of herbicide treatments, but this time the focus was on winter wheat.
Applications of dicamba (0.14 kg ai/ha) were made at Feeke’s stage 1, 3, and 10. The
experimental areas were kept weed-free throughout the time the study was being
conducted. Dicamba treatments applied at Feeke’s stage 1 and 10 reduced wheat yield
12% and 13% respectively. The treatment applied at the firlly tillered stage (F eeke’s
stage 3) did not significantly affect yield.

Friesen and Baenziger (38) conducted a field study in which they applied dicamba at
0.28 and 0.56 kg ai/ha to wheat at Feeke’s stage 1, 2, 4, 5, 6, 10, and 10.5. Dicamba
decreased wheat yield when applied at the last five growth stages at a rate of 0.28 kg ai/ha
and at all stages when applied at a rate of 0.56 kg ai/ha at one location. Wheat yield was
decreased with the 0.28 kg ai/ha rate at the last four grth stages at another location.
The 0.56 kg ai/ha rate significantly decreased the yield below that of the untreated
controls at the last five growth stages at this same location.
WWW

Growth regulator herbicides can alter numerous morphological and chemical
properties of cereals. The effects of these herbicides vary with crop growth stage, rate,
and the specific herbicide. It is generally accepted that the parts of the plant that are
affected are those that are being formed at the shoot apex at the time of herbicide
exposure (108). It has been suggested that herbicide injury to plant structures formed

long after herbicide application is caused by herbicide that is not immediately

22

metabolized or removed from the plant (3 8).

Root effects. Chen et al. (20) germinated wheat seeds in petri dishes containing
various concentrations (0.01 to 100 ppmw) of 2,4-D, 2,4,5,-T, dicamba, and picloram. All
four herbicides decreased root length at concentrations greater than 0.1 ppmw. With
increasing herbicide concentration there was a corresponding decrease in the fresh weight
of wheat roots.

Leaf effects. Tottrnan (108) found that very early applications of dicamba could cause
the formation of leaves with fused edges (onion leafing) which impeded the emergence of
subsequent leaves and the head. Similarly, the Weed Research Organization (108)
observed that dicamba applied at the 7-leaf 8-tiller stage severely deformed the later
leaves which prevented the normal emergence of heads. Dial and Thill (26) observed that
the ester form of 2,4-D caused fused leaf margins when applied to oats at the 5-leaf stage.
Many culrns with abnormal leaf sheaths bore two or more panicles at harvest. F riesen
and Baenziger (3 8) reported that bent intemodes occurred when dicamba was applied at
the 3-leaf stage. The bending appeared to be caused by leaf sheaths that were constricted
at the nodes. With barley the constriction of the leaves was severe enough to impede the
emergence of heads. Malformations in the stems and leaves of the main culms of wheat
and barley occurred when dicamba was applied at 0.28 kg ai/ha at the 4-leaf stage. The
same treatment at the 6-leaf stage caused the same effects in the tillers but not in the main
culms.

Maturity effects. Keys (42) observed that a 0.14 kg ai/ha rate of dicamba applied at
the 4-leaf stage slightly delayed the maturation of wheat. Quirnby and Nalewaja (82)

reported that wheat maturity was delayed by 4 and 6 days by 0.14 and 0.42 kg ai/ha of

23

dicamba, respectively applied at the late-tiller stage. Scragg (90) reported that uneven
ripening of all cereals, and in oats, secondary growth after harvest sometimes occurred as
a result of the use of “hormone” herbicides. Shaw et al. (93) found that a 0.45 kg ai/ha
rate of the butyl ester of 2,4—D delayed the heading of barley by about 7 days when
applied at the 2-joint stage. Woestemeyer (122) found that a 0.45 kg ai/ha rate of 2,4-D
ester applied at the firlly tillered and late-boot stage delayed wheat maturity by about 4
days. Phillips (78 ) found that 2,4-D ester applied at 0.23 kg ai/ha at the 2-joint stage
delayed the development of wheat by 4 to 7 days.
Stem effects. Overland and Rasmussen (72) found that a 0.90 kg ai/ha rate of 2,4—D
ester caused weakening and bending of barley stems and bending of wheat stems when
applied at the early shooting stage. Scragg (90) observed that “hormone” herbicides often
caused straw weaknesses. Shaw et al. (93) observed that there was a tendency for wheat
to lodge when 2,4-D was applied at the fully tillered and late-boot stage. 2,4-D ester
applied at 0.45 kg ai/ha 10 days before harvest caused lodging in a study conducted by
Phillips (78). Tottrnan (52) found that a herbicide mixture containing dicamba, MCPA,
and mecoprop decreased lodging by shortening straw length and causing lighter heads.
Keys (42) observed that a 0 .14 kg ai/ha rate of dicamba applied at the 4-leaf stage
increased the tillers per plant 14% and decreased plant height. Martin et al. (60, 61)
applied standard rates of MCPA, 2,4-D, and dicamba to spring and winter wheat at the 3—
leaf, fully tillered, and mid-boot stage. Several treatments reduced plant height and the
number of heads per meter of row. Plant height was positively correlated with grain yield
(r=0.87), indicating that herbicides that reduce plant height can also reduce grain yield.

Quimby and Nalewaja (82) observed a decrease in the number of wheat heads per meter

24

of row when dicamba was applied at 0.84 kg ai/ha preemergence. Plant height was also
reduced by dicamba treatments applied at preemergence, 2 to 4-leaf, early tiller, late tiller,
and boot stages.

Head effects. Tottrnan (108) found that early applications of dicamba could cause the
formation of abnormal heads. Spikelets were arranged oppositely instead of alternately
on the head. Slightly later applications caused multiple (supernumerary) Spikelets to be
formed at some nodes, and in some cases the glumes of Spikelets became fused together.
Olson et a1. (70) applied the butyl ester of 2,4-D to wheat and barley at several early
growth stages and noted many head abnormalities such as supemumerary Spikelets,
elongated intemodes, heads with opposite spikelet arrangement, and branched heads. The
Weed Research Organization (108) observed the formation of opposite Spikelets when
dicamba was applied at a dose commonly used for weed control to wheat and barley with
5 leaves and 4 tillers and 7 leaves and 8 tillers. Edwards and Miller (28) reported that a
decrease in the number of Spikelets per head of wheat resulted from an early application
of dicamba + MCPA (0.07 + 0.14 kg ai/ha). Friesen and Baenziger (38) described
malformations in the heads of the main culms of wheat and barley when dicamba was
applied at 0.14 kg ai/ha at the 4—leaf stage. The same treatment at the 6-leaf stage caused
the same effects in the tillers but not in the main culms. A report from the Weed
Research Organization (41) described darkened heads and thin ears (rat-tailing) as a result
of dicamba (0.11 kg ai/ha) applied to spring wheat after the initiation of j ointing.

Tottrnan (109) applied a mixture of dicamba + 2,3,6-TBA + MCPA + mecoprop (0.20 +
0.28 + 1.12 + 1.68 kg ai/ha) to wheat with two or three nodes. Plants injured by this

treatment had dark-colored heads that appeared very thin (rat-tailed).

25

- Seed effects. Robison and F enster (85) reported that 2,4-D amine (0.56 kg ai/ha), 2,4-
D ester (0.28 kg ai/ha), and dicamba (0.14 kg ai/ha) reduced the number of seeds per
head of wheat while having no effect on seed weight when applied at the boot stage. A
report from the Weed Research Organization (41) described inflorescence sterility as a
result of dicamba applied to spring wheat after the initiation of jointing. Martin et al. (60)
found that dicamba (0.14 kg ai/ha) applied at the fully tillered stage reduced the number
of seeds per head of spring wheat 15%. The same treatment decreased the number of
seeds per head 27% when applied at the mid-boot stage. There was a positive correlation
(r=0.83) between the number of seeds per head and yield. Derscheid et al. (24) reported
that 0.45 kg/ha acid equivalent of 2,4-D increased the seed weight of oats when applied at
the 5-leaf, tillered, heading, and milk stage. In some instances the seed weight increase
was accompanied by a reduced number of seeds per head and yield. Yield was affected
in a manner similar to the number of seeds per head suggesting that the decrease in seed
number was of primary importance in causing the yield reduction.

Tottrnan (109) applied a mixture containing dicamba + 2,3,6-TBA + MCPA +
mecoprop (0.20 + 0.28 + 1.12 + 1.68 kg ai/ha) to wheat with two or three nodes. The
herbicide treatment reduced seed number but had its greatest impact on seed size.
Affected seeds were small and shrivelled which prevented there collection via mechanical
harvest. These results suggest that ovule fertilization occurred and endosperm grth
was initiated, but it ceased prematurely giving rise to the small shrivelled seeds. F riesen
and Baenzi ger (3 8) reported that floret sterility and abnormal seed development occurred
when dicamba (0.14 kg ai/ha) was applied to wheat and barley at the 4-leaf stage in a

greenhouse study. The ovaries of treated florets often collapsed, and cell size and

26

arrangement was abnormal. In many florets seed development had been initiated but not
completed. This resulted in shriveled seeds that were referred to as aborted. Quimby and
Nalewaja (82) observed an increase in seed weight when 0.14 and 0.42 kg ai/ha of
dicamba were applied at the boot and flower stage. Seed weight was decreased by
dicamba (0.42 kg ai/ha) applied at the early tiller stage in both years. Both the 0.14 and
0.42 kg ai/ha rate applied at the late tiller stage decreased seed weight in the first year of
the study and the 0.14 kg ai/ha rate increased seed weight in the second year. Edwards
and Miller (28) reported that dicamba + MCPA (0.07 + 0.28 kg ai/ha) affected two
cultivars of wheat differently. ‘Waldron’ showed a decrease in Spikelets per head and
seeds per head while ‘Coteau’ showed a decrease in seed size when they were treated
with an early application of the herbicide mixture. As a compensatory effect ‘Waldron’
showed an increase in seed size.

Quimby and Nalewaja (82) found that Dicamba (0.14 and 0.42 kg ai/ha) applied at the
late tiller stage, and dicamba (0.42 kg ai/ha) applied at the boot stage reduced the percent
germination of seeds harvested from plots when tested immediately after harvest. When
retested several months later, only seed fi'om plots treated with 0.42 kg ai/ha at the late
tiller stage had reduced germination percentages. This suggests that dicamba can cause
increased seed dormancy.

Protein, flour, mineral content, and baking quality effects. Shellenberger and
Phillips (97) reported that 2,4-D had no effect on the mineral content and milling and
baking qualities of wheat. Applications made during the early heading stage increased
seed protein. Bode et al. (13) found that a 1.82 kg ai/A rate of 2,4-D increased seed

protein content and decreased flour yield and baking quality of soft red winter wheat.

27

Klingrnan (49) reported that 2,4-D only caused increased seed protein content when it
decreased grain yield. Martin et al. (61) applied typical rates of several herbicides to
winter wheat at the 3-leaf, fully tillered, and mid-boot stage. 2,4-D ester (0.4 kg ai/ha)
decreased seed protein content 20% when applied at the 3-leaf stage in the first year.
Dicamba (0.14 kg ai/ha), MCPA (0.55 kg ai/ha), and dicamba + 2,4—D amine (0.14 + 0.4
kg ai/ha) increased seed protein 10% when applied at the fully tillered stage in the first
year. Dicamba, 2,4-D ester, dicamba + 2,4-D amine, and dicamba + MCPA (0.14 + 0.4
kg/ha) increased seed protein 10, 10, 15, and 14 percent respectively, when applied at the
mid-boot stage in the first year. Conversely, dicamba applied at the 3-leaf, fully tillered,
and mid-boot stage reduced seed protein content 10, 10, and 9 percent respectively in the
second year. Although seed protein content was not consistent with year, seed protein
was negatively correlated with yield (r = 0.65) suggesting that increased seed protein
content in winter wheat will be coupled with lower yield.

Cytological effects. Friesen and Baenziger (3 8) placed wheat and barley seeds in
various concentrations of dicamba and allowed them to germinate. Cytological
examinations revealed that prophase, metaphase, and anaphase proceeded normally once
initiated. However, abnormal chromosome configurations indicated that dicamba
interfered with normal spindle activity leading to slowed mitosis. A reduced number of
dividing cells in the meristem was further evidence that mitosis was being stalled. Often
polynucleate cells were found in florets that were sectioned. The ovaries of treated florets
often collapsed and cell size and arrangement was abnormal. In many florets, seed
development had been initiated but not completed. This resulted in seeds that were

referred to as aborted. Normal cell development resumed when the concentration of

28

dicamba dropped below a threshold value. Tottrnan (52) observed that a mixture of
dicamba + MCPA + mecoprop caused the formation of shrivelled seeds that contained an
unidentified sugary liquid instead of starch.

Morgun et al. (65) found that while dicamba had a high genetic activity, MCPB and
2,4-DB did not cause mutations when used at concentrations that are typical for the
control of weeds. For this reason it was suggested that dicamba may not be suited for use
in seed production.

Chen et al. (120) germinated wheat seeds in petri dishes containing various
concentrations (0.01 to 100 ppmw) of 2,4—D, 2,4,5,-T, dicamba, and piclorarn. All four
herbicides increased DNA and protein levels and decreased RNA levels. Arnold and
Nalewaja (11) applied dicamba (0.14 kg ai/ha) to wheat at the 3 to 4-leaf and late-boot
stage. The RNA and protein content of wheat treated at the late—boot stage increased
rapidly during the first 2 days after treatment and was further increased after 4 days.
Wheat protein and RNA content did not change as a result of the treatment at the earlier
stage.

‘Wakefield’ wheat. ‘Wakefield’ is a soft red winter wheat (T riticum aestivum L.)
variety that was developed at the Virginia Agricultural Experiment Station and released
in July of 1990. ‘Wakefield’ has several genes for disease resistance, lodging resistance,
and high yield potential. The performance of ‘Wakefield’ in nurseries suggests that it is
suited to areas outside the mid-Atlantic region were it was developed (103).

‘Harus’ wheat. ‘Harus’ is a soft white winter wheat (T riticum aestivum L.) that was
developed at the Agriculture Canada Research Station, Harrow, Ontario and released in

1985. ‘Harus’ is a short-strawed, early-maturing, lodging-resistant, high-yielding, variety

29

with good disease resistance. The performance of ‘Harus’ was good in south and central

Ontario (129).

10.

11.

12.

13.

30

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CHAPTER 2

RESPONSE OF ‘WAKEFIELD’ WINTER WHEAT (T riticum aestivum)
TO DICAMBA

ABSTRACT

Dicamba is a herbicide that is used for the control of broadleaf weeds in wheat.
Typically, when dicamba is applied at proper rates and growth stages, little or no crop
injury results. However, the grain yield of ‘Wakefield’ winter wheat (T riticum aestivum
L.) was depressed more than 95% in 1995 and more than 60% in 1997 by dicamba
applied at typical rates and stages, while the yields of ‘I-Iarus’, ‘Lowell’, and ‘Chelsea’
were not affected by these treatments. Two years of research have confirmed that
dicamba has minor effects on the number of Spikelets and the seed weight of ‘Harus’ and
‘Wakefield’. Dicamba drastically reduced the number of normal seeds per spikelet of
‘Wakefield’ at typical growth stages in both the field and greenhouse. The seed number
of ‘Harus’ was only decreased at late growth stages in the field. Decreases in grain yield
were of the same magnitude as decreases in normal seed number. When dicamba was
applied, small, shriveled seeds that were lost during mechanical harvest resulted.
Dicamba had little effect on the number of sterile florets. Greenhouse studies showed
that untreated ‘Wakefield’ plants cross-pollinated with treated ‘Wakefield’ pollen source
plants develop very few abnormal seeds, while treated plants cross-pollinated with treated
or untreated pollen source plants develop abnormal seeds. Hence, the development of

abnormal seeds when dicamba is applied is not the result of abnormal pollen.

4O

41

INTRODUCTION

Weeds compete with cereals for nutrients, moisture, light, and space (22). Yield
reductions can result from this competition and may also occur in response to allelopathy
(2). Dicamba (3,6-dichloro-o-anisic acid) is a grth regulator type herbicide that
controls some annual and perennial broadleaf weeds in wheat (l, 2, 14, 27 ). Often
dicamba is used when weed species are present that are tolerant to 2,4-D (26). The mode
of action of dicamba is similar to that of 2,4-D (19). Dicamba alters the hormone balance
and protein synthesis of broadleaf plants which results in abnormal growth. The exact
sites of action of dicamba are not known and are believed to be multiple (11). Stem
twisting (epinasty), callus tissue development, leaf malformations (cupping, crinkling,
parallel veins, and leaf strapping), and plant death often occur in broadleaf weeds as a

result of exposure to dicamba (l 1).

 

 

{I untreated Idlcambn (0.” kg IVhI)IppII¢d II Fuke's rule 5

 

5000
4500
4000
3500
3000
2500
2000
1500
1000

500

Grain yield
(kg/ha)

 

Chelsea Lowell Wakefield

Figure 1. 1995 herbicide sensitivity study

 

 

 

42

Typically wheat is tolerant to dicamba when it is applied between the end of tillering
(F eeke’s stage 3) and just prior to jointing (F eeke’s stage 5) (11). However, the yield of
‘Wakefield’, a soft red winter wheat (T riticum aestivum L.), was reduced over 95% by a
standard application (0.14 Kg ai/ha applied at Feeke’s stage 5) of dicamba in a 1995
herbicide sensitivity study at Michigan State University (Figure 1). Affected ‘Wakefield’
heads had a dark color and a thin (rat-tailed) appearance (Figure 2). There are reports of
rat-tailed heads in the literature, but they occurred when dicamba was applied after the

initiation of jointing (12, 31).

 

unheated dicamba (0.14 kg ai/ha)

 

Figure 2. ‘Wakefield’ wheat treated Feeke’s stage 5.

 

 

 

‘Wakefield’ was not the first variety to show sensitivity to dicamba. Edwards and
Miller (9) showed that the grain yield of ‘Waldron’ was decreased more than that of nine

other spring wheat varieties when dicamba + MCPA was applied at the 3.5 to 5 and the 6

43

to 7-leaf stage. Schroeder and Banks (27) reported on two varieties of soft red winter
wheat that were moderately sensitive to dicamba applied at F eeke’s stage 4. Tottrnan
(29) evaluated the safety of a mixture of Dicamba + 2,36-TBA + MCPA + mecoprop on
three varieties of wheat. Two varieties had reduced yields when the dicamba mixture was
applied at F eeke’s stage 7, while a third did not. The dicamba component in the herbicide
mixture appeared to be responsible for the observed injury symptoms. Robison and
Fenster (25) applied dicamba at F eeke’s stage 1, 2, 9, and 10.5 to five cultivars of winter
wheat and found that the grain yield of only one variety was affected by a typical use rate.
The injury reported in the these studies was not as severe as the injury that occurred with
‘Wakefield’, and many of the treatments were made at later stages of growth.

Growth regulator herbicides have been reported to affect cereals in many different
ways. Injury symptoms vary with the particular herbicide, rate of application, cereal
species and application timing. Root (5), and leaf (8, 10) abnormalities, delayed maturity
(24, 34), plant height reductions (24), straw weaknesses (21, 28), and seed protein
increases (18) have been described. Head abnormalities such as the development of
oppositely arranged Spikelets instead of placement in the usual alternate configuration,
elongated head intemodes, branched heads, multiple (supernumerary) Spikelets at one
node, and decreases in spikelet number have occurred (9, 20, 30). Increases (6, 24, 27)
and decreases (9, 24) in seed weight and decreases in seed number due to floret sterility
(10, 12, 31) have been reported. There have been reports of shriveled seeds as a result of
exposure to dicamba and dicamba mixtures (10, 31). F riesen and Baenziger (10) referred

to this condition as seed abortion.

44

The objectives of this research were to: 1) compare the sensitivity of ‘Wakefield’ and
‘Harus’ wheat to dicamba, 2) determine the per head grain yield component(s)
responsible for the yield reductions in ‘Harus’ and ‘Wakefield’ wheat, 3) determine if
dicamba causes floret sterility and/or seed abnormality in ‘Harus’ and ‘Wakefield’ wheat,
and 4) determine if dicamba causes the development of abnormal pollen in ‘Wakefield’

wheat.

45

MATERIALS AND METHODS

Eieldmma

A field experiment was conducted at the East Lansing campus of Michigan State
University in 1995-96 and a duplicate field experiment was conducted in 1996-97 to
evaluate the sensitivity of ‘Wakefield’ and ‘Harus’ winter wheat (T riticum aestivum L.)
to dicamba applied at two rates and five growth stages. The soil was a capac loam (Aerie
Endoaguals, fine-loamy, mixed, mesic) with an organic matter content of 2.9% and a pH
of 6.9. The soil was tilled with a field cultivator prior to planting. Nitrogen,
phosphorous, and potassium were applied as recommended by soil test results. The
experiments were planted on October 12, 1995 and September 27 , 1996 with a seeding
rate of 4.4 million seeds per hectare and a planting depth of 1.9 to 2.5 cm. Plot size was
1.5 by 3.4 m, and plots consisted of 7 rows spaced 15 cm apart. Experimental plots were
bordered by plots planted with ‘Harus’.

Dicamba was applied to wheat with a tractor-mounted compressed air sprayer. Flat

fan nozzles (8003) and a spray pressure of 206 kPa were used to deliver 187 L/ha of a
herbicide solution.

Dicamba was applied at Feeke’s stage 2, 3, 5, 9, and 10 (F2, F3, F5, F9, and F10) at
0.14 and 0.28 kg ai/ha (Figure 3). The treatments were applied on April 24, May 13 and

22, and May 5 and 11 in 1996. The treatments were applied on April 15 and 26 and May

 

46

   
  
   

 

 

 

 

 

 

 

 

 

 

Ripening
Headingi
Seedling Tillering Jointing Boot “mm
in
“boot’ head
emergen -
S .
ligule? >
of last I
leaf
visible
4
\
recommended last leaf ‘
dicamba just visible 0 .
application
interval
I I second ‘
node ‘
f t vi'
leaf 111:1: srble p
Shem“ visible
strongly
_ , leaf t ‘
"1’:an sheaths
one begms , lengthen 3
shoot 2 21:; N
I l
l
w 2* I.
w. ______ __ . t. 1 ‘k .‘u’ J!
F1 F2 F3 F4 F5 F6 F7 F8 F9 F10 F10.1-.5F11

‘Dicamba was applied at each numbered arrow in the field and and at arrows 2, 3, 4, and 5 in the greenhouse.

Figure 3. Feeke’s scale and growth stages for field and greenhouse studies.

47

2, 20, and 31 in 1997. When wheat first begins to develop tillers it is at F2. F3
corresponds with the end of tiller development. F5 occurs just prior to jointing when the
leaf sheaths become strongly erect. F9 is marked by the emergence of the ligule of the
highest leaf (flag leaf) of the plant. At F10 (boot) the spike is enshrouded by the sheath
of the flag leaf and is nearing emergence.

Ten heads were collected from each plot prior to harvest in 1996 and 1997. The
nmnber of Spikelets per head was counted. The heads were mechanically threshed in the
first year and hand-threshed in the second year, and the number of seeds per spikelet was
determined. Wheat Spikelets are composed of florets arranged alternately along a tiny
branch or rachilla (22). Three additional heads were collected in 1997, and the lowest
two florets of each spikelet were dissected. These are the florets that produce most of the
yield in wheat. The number of normal, abnormal, and absent seeds was calculated from
these florets. Plots were mechanically harvested on July 28, 1996 and July 29, 1997, and
the weight of 1000 seeds and grain yield were determined. Yields were adjusted to 13%
moisture. ’

Treatments were comprised of a factorial combination of variety, application rate,
and time of application. Treatments were arranged in a split-plot design where individual
means were compared using a least significant difference test at the 5% level. There were
treatment by year interactions for the data, so data from each year were analyzed
. separately.

Greenheusesmdx.

Greenhouse experiments were conducted to evaluate the sensitivity of ‘Wakefield’ and

48

‘Harus’ wheat to dicamba applied at two rates and 4 growth stages. Approximately 36
seeds of ‘Wakefield’ and ‘Harus’ wheat were planted in 10-cm square pots every seven
days for several weeks. The wheat was allowed to germinate and grow for one week
under greenhouse conditions conducive to wheat growth. The wheat was then placed in a
cold room at 4 C to induce vemalization. After 56 days the plants were removed from the
vemalization chamber and transplanted into round pots with a diameter of 10 cm. Three
plants were transplanted into each pot.

Dicamba was applied to wheat with a greenhouse sprayer at a rate of 0. 14 and 0.28 kg
ai/ha. Treatments were made at F 3, F5, F9, and F10. Plants fiom different planting dates
were used in order to allow all treatments to be applied on the same day within each run.

Data were collected on the main culm of each plant in each pot. The number of
Spikelets per spike was calculated. The two basal florets from each spikelet were
dissected, and the number of normal, abnormal, and absent seeds per spikelet were
determined. Spikes were hand-threshed and total seed weight per spike were determined.

Treatments were comprised of a factorial combination of variety, application rate, and
time of application. Treatments were arranged in a completely randomized design were
individual means were compared using a least significant difference test at the 5% level.
There were treatment by run interactions for the data, so data from each run were
analyzed separately.

Hybridization study. Treated and untreated plants of ‘Wakefield’ were hybridized to
evaluate the response of seed bearing plants and pollen from treated pollen donor plants

to dicamba. ‘Wakefield’ plants were planted, vemalized, and transplanted as in the

49

previous study. Dicamba was applied at F9 at a rate of 0.14 and 0.28 kg ai/ha.

At the time of head emergence all anthers were removed (emasculated), and all but the
two basal florets were removed fiom each spikelet of several spikes to prevent self-
pollination. These spikes were covered with paper bags to prevent cross-pollination. At
the time of anthesis, spikes were cut from plants that were not emasculated, and their
stems were placed in plastic vials containing water in order to prolong spike longevity
and pollen dissemination. These excised spikes were placed inside the bags containing
the emasculated spikes in order to induce cross-pollination.

Treatments consisted of : 1) pollen from untreated plants crossed to plants that were
emasculated and untreated, 2) pollen from treated plants crossed to plants that were
emasculated and untreated, 3) pollen from untreated plants crossed to plants that were
emasculated and treated, 4) pollen from treated plants crossed to plants that were
emasculated and treated, 5) self-pollinated untreated plants, and 6) self-pollinated treated
plants. Heads were dissected about 30 days after pollination, and the number of abnormal
seeds per spikelet were counted.

Treatments were comprised of a factorial combination of application rate (untreated
and 0.14 kg ai/ha), hybridization (self-pollinated and cross-pollinated), and plant type
(seed bearing plant and pollen donor plant). Treatments were arranged in a completely
randomized design with each treatment being replicated four times. Data were subjected
to an analysis of variance. Treatment by run interactions were not significant for the two
experiments utilizing the 0.14 kg ai/ha rate of dicamba, so these data were combined.

Data from experiments utilizing the 0.28 kg ai/ha rate were also combined.

50

RESULTS AND DISCUSSION

Wrist.

The fall and winter of 1995-96 were very severe from a wheat production standpoint.
Early onset of cold temperatures resulted in poor vegetative growth in the fall. Prolonged
periods with no snow cover and extremely cold temperatures lead to low plant density
and vigor. The fall and winter of 1996-97 were more mild which resulted in good plant
growth and winter survival.

Grain yield in field. Due to the similarities in the response of wheat at the two rates,
only results obtained fi'om the untreated controls and the 0.14 kg ai/ha rate of dicamba are
presented in this chapter. The yield of ‘Wakefield’ was reduced more by exposure to
dicamba than that of ‘Harus’ in both years, and later treatments were more injurious to
both wheat varieties (Figure 5 and 6). Dicamba reduced the grain yield of ‘Wakefield’
50% when applied at F10 in 1996. No treatment reduced the grain yield of ‘Harus’ in
1996 (Figure 5). In 1997, the grain yield of ‘Wakefield’ was reduced by dicamba applied
at F3, F5, F9, and F10 by 57, 63, 99, and 100 percent, respectively (Figure 6). No other
wheat varieties have been reported to respond this severely to a typical use rate (0.14 kg
ai/ha) of dicamba (9, 25, 27, 29). The grain yield of ‘Harus’ treated at F 10 was lower
than the yield of ‘Harus’ treated at F 3, but no treatment reduced the grain yield of ‘Harus’

below that of the untreated control in 1997. The superior performance of ‘Harus’ treated

51

at F3 presumably resulted fi'om a combination of the timely removal of weeds by
dicamba and an absence of crop injury. Late applications of dicamba have been shown to
cause yield reductions in a number of studies (9, 25, 29). Due to the risk associated with
late applications of dicamba, it is not recommended for use beyond F5 (11). The reasons
for the differences in the yield response between years is not clear. There were
differences in the rate of development of wheat between the two years and this may have
been a factor. Additionally, the lower leaf area that was present in 1996 may have
resulted in a reduction in the amount of herbicide taken up by plants.

Seed weight per head in greenhouse. Due to the similarity in the response of wheat
to both rates, only results obtained from the untreated controls and the 0.14 kg ai/ha rate
of dicamba are presented in this chapter. The total seed weight per wheat head of
‘Wakefield’ was consistently more sensitive to dicamba than that of ‘Harus’ in both runs
of the greenhouse study (Figure 7 and 8). The seed weight per head of ‘Wakefield’ was
severely reduced by every treatment in both greenhouse experiments. The reductions
caused by dicamba applied at F3 and F5 were similar in the first run, as were the
reductions caused by the F9 and F 10 treatments. Unlike the field study, in which the
yield reduction of ‘Wakefield’ was a function of grth stage at the time of application,
the head seed weight reduction was severe and independent of growth stage in the second
run of the greenhouse study. Head seed weight of ‘Harus’ was reduced more by dicamba
exposure at F5 and F9 than F10 in the first run. This is inconsistent with results from
field studies in which wheat exhibited more injury from progressively later treatments. It

is somewhat surprising that a wheat variety that is tolerant to recommended applications

52

of dicamba in the field (‘Harus’) would be injured in the greenhouse. F reisen and
Baenziger (10) observed that wheat was more sensitive to dicamba under lush greenhouse
conditions. Head seed weight of ‘Harus’ was not affected by the F3 treatment, and the

F 5, F9, and F10 treatments caused increasingly more injury in the second run.

Seeds per spikelet in field. The number of seeds per spikelet of ‘Wakefield’ was
reduced when dicamba was applied at F9 and F10 in 1996 (Figure 5). The number of
seeds per spikelet of ‘Harus’ was reduced by dicamba applied at F10. The number of
seeds per spikelet of ‘Wakefield’ was reduced by dicamba applied at every growth stage
in 1997 (Figure 6). Dicamba exposure at F9 and F10 caused an almost complete loss of
seeds. No single treatment significantly reduced the number of seeds per spikelet of
‘Harus’ in 1997. Reductions in seed number have been previously reported, but this
response usually stems from high rates or late applications (9, 17, 31). The reductions in
the number of seeds per spikelet of both ‘Wakefield’ and ‘Harus’ corresponded very
closely to grain yield reductions.

Seeds per spikelet in greenhouse. The number of seeds per spikelet of ‘Wakefield’
was strongly decreased by every dicamba treatment in both greenhouse experiments
(Figure 7 and 8). Unlike the field study, exposure to dicamba at F5 was not as injurious
as exposure at F3 in the first run. Dicamba applied at F9 and F 10 caused abnormal
development of almost all seed in the first run. Every treatment resulted in a reduction in
the number of seeds per spikelet of ‘Wakefield’ by more than 90% in the second run.
The number of seeds per spikelet of ‘Harus’ was decreased by dicamba at F 5 in the first

run and F9 and F10 in both runs. Seed number was not decreased by dicamba at F5 and

53

F9 in the field studies. Seed reduction in ‘Harus’ was not as sharp as that observed with
‘Wakefield’. Head seed weight was highly dependent upon the number of seeds per head
with both cultivars.

Spikelets per head in field. The number of spikelets per head of ‘Wakefield’ was not
affected by any treatment in 1996 (Figure 5). There was a slight depression of the
number of spikelets per head of ‘Harus’ when dicamba was applied at F3, F5, and F9.
Spikelet primordium are typically formed by F5, so it may be that the observed reduction
in spikelet number resulted from spikelets that were formed but were highly
underdeveloped. The highest number of spikelets occurred when dicamba was applied to
‘Harus’ at F10, and this slightly compensated for the lower seed number that occurred at
this grth stage. There was no significant effect of the growth stage at the time of
application on spikelet number in 1997 (Figure 6). The number of spikelets per head of
‘Harus’ was not affected by any treatment in 1997. Reductions in spikelet number had a
negligible effect on grain yield.

Spikelets per head in greenhouse. The number of spikelets per head of ‘Wakefield’
was not affected by any treatment in the greenhouse study (Figure 7 and 8). The spikelet
number of ‘Harus’ was increased by dicamba applied at F3 in the first run. Multiple
spikelets at single nodes resulted from this treatment. Tottrnan (30) reported that early
applications of dicamba were capable of causing the development of supemumerary
spikelets. The F9 treatment caused a 14% reduction in the number of spikelets. This
decrease probably had an effect on total head seed weight. No treatment affected the

number of spikelets per head of ‘Harus’ in the second run.

54
Seed weight. The weight of 1000 seeds of ‘Wakefield’ was slightly increased by
dicamba applied at F9 and F 10 in 1996 (Figure 5). Quimby and Nalewaja (24) obtained
similar results when they applied dicamba at F10. These same two treatments caused
reductions in the seed weight of ‘Wakefield’ in 1997 (Figure 6). Reductions in seed
weight have been reported previously (9, 24). The seed weight of ‘Har'us’ was not
significantly affected by dicamba in either year. However, the weight of 1000 seeds of
‘Harus’ was highest when dicamba was applied at F10, and this may have been a

compensatory response to the reduction of seed number at this growth stage.

 

normal wheat grain

unfertilized ovary
(absent grain)

  

Figure 4. Observed structures in wheat heads

 

 

 

Effect of dicamba on seed development in field. Inspection of treated heads of
‘Wakefield’ and ‘Harus’ in the field revealed normal seeds, sterile florets (absent seeds),
and very small, shriveled (abnormal) seeds (Figure 4). These seeds, when dried,

contained little or no endosperm and did not contribute to grain yield. Abnormal seeds of

55

‘Wakefield’ occurred when dicamba was applied at all five grth stages (Figure 9). The
number of abnormal seeds increased with later applications. Dicamba caused a similar
number of abnormal seeds to develop in both ‘Wakefield’ and ‘Harus’ when applied at
F10, but no other application caused a large number of abnormal seeds in ‘Harus’ (Figure
9). The increases in the number of abnormal seeds corresponded closely with decreases
in yield and harvested seed number with ‘Wakefield’. This correlation was not as strong
for ‘Harus’. Dicamba caused 85% of the basal florets to produce abnormal seeds when
applied at F 10, but dicamba decreased the grain yield by only 46% at this growth stage.

It appears that dicamba affected the basal florets within a spikelet more severely than the
other florets. While dicamba caused an 85% decrease in the number of normal seeds in
the basal florets, it only caused a 68% decrease in seed number in the other florets.
Dicamba had a minimal effect on the number of sterile florets per spikelet of ‘Harus’ and
‘Wakefield’.

The abnormal seeds were too small and light to be collected by mechanical harvest.
From this it can be concluded that dicamba did not truly reduce seed number, but
instead, it reduced the number of seeds that could be mechanically harvested.

Shriveled seed as a result of late applications of dicamba has been previously reported
(10, 31). Friesen and Baenziger (10) referred to the condition as seed abortion. Efforts
were made with a light microscope to determine if an embryo was actually present in the
seeds. Embryos of the size that exist in mature wheat seeds were not present, but it was
not determined if smaller or underdeveloped embryos were present. Reports indicate that

growth regulator herbicides may affect meiotic cell divisions in the pollen and egg

56

mother cells (7, 16, 20, 23). One report suggests that dicamba that is present at the time
of seed development can affect assimilate supply (32). However, seed abnormalities do
not typically result from applications of dicamba made at F2, F3, and F5. It has been
established that wheat metabolizes dicamba (3, 4). It is possible that ‘Wakefield’ does not
metabolize dicamba rapidly, and shriveled seed may result from herbicide remaining in
the plant at the time of gamete formation, anthesis, and seed growth and development.
When late applications of dicamba are made to ‘Harus’, the plant does not have the
opportunity to metabolize the herbicide before it causes the formation of abnormal seed.
Applications of dicamba to ‘Wakefield’ at early growth stages may “simulate” dicamba
exposure to ‘Harus’ at late groth stages if an appreciable amount of active herbicide
remains in the plant for a long period of time. The fact that the injury symptoms are
similar for both ‘Harus’ and ‘Wakefield’ supports this hypothesis.

Effect of dicamba on seed development in greenhouse. Seed development of treated
plants was closely observed in the greenhouse. Pollen shed, ovary fertilization, and the
early enlargement of the caryopsis appeared to be normal. When the treated seeds
reached approximately one half the size of mature wheat seed, they began to appear
flaccid. Squeezing of the treated seed produced a sugar-water solution. Squeezing of
untreated seeds of the same size yielded a starchy substance. Tottman (15) described a
similar condition when a mixture of dicamba + MCPA + mecoprop was applied to wheat.
In time the affected seeds became dry and shriveled, appearing identical to the abnormal
seeds that were observed in the field.

Dicamba caused the development of abnormal seeds at each growth stage in both runs

57

of the greenhouse study (Figure 10). Dicamba had a stronger effect on the number of
abnormal seeds at the later two growth stages than at the earlier stages in the first run.
Dicamba caused the replacement of almost all normal ‘Wakefield’ seeds by abnormal
seeds at each growth stage in the second run. A large number of seeds became abnormal
when dicamba was applied to ‘Harus’ at F5 in the first run and at the last two growth
stages in both runs. An appreciable number of injured seeds developed only at F10 in the
field with ‘Harus’. Increases in the number of abnormal seeds coincided with decreases
in the number of normal seeds and total head seed weight for both varieties. The number
of sterile florets (absent seeds) was not affected by any treatment. A few abnormal seeds
occurred in some untreated heads indicating that the condition could be caused by factors
other than dicamba exposure.

Abnormal seeds develop in the hybridization study utilizing the 0.14 kg ai/ha rate only
when the seed bearing plants were treated. (Figure 11). Pollen from treated plants did not
cause the development of abnormal seeds. Self-pollinated treated plants responded
similarly to plants that were treated and cross-pollinated with a treated pollen source.
Surprisingly, the treatment consisting of treated plants hybridized with untreated pollen
donors caused significantly more abnormal seeds than treatments in which both the
pollen source plant and the seed bearing plant were treated. A clear explanation of this
phenomenon is not available. Often dicamba delays the maturity of wheat (24, 34). It is
possible that crosses involving untreated pollen source plants were made at different

times than crosses involving treated pollen source plant. Differences in the environment

58

at the time of pollination may have caused the increase in the number of abnormal seeds.

When seed bearing plants were treated with 0.28 kg ai/ha of dicamba, very high
numbers of abnormal seeds developed (Figure 11). Virtually no abnormal seeds
developed when dicamba was not applied to pollen or plants. When treated pollen source
plants were hybridized with untreated plants 13% of the seeds became abnormal. This
suggests that dicamba can affect pollen if it is applied at a high rate. However, male
sterility is not of primary importance in causing the development of abnormal seeds. The
fact that injured pollen can cause the condition may counter the assertion that dicamba
affects the assimilate supply to the developing seed. It seems unlikely that the pollen
source would have any effect on the amount of nutrients available for seed growth and
development. It is interesting to note that affected pollen elicits the same type of injury
as affected plants.

Grain yield and the various per head grain yield components of ‘Wakefield’ were
more sensitive to dicamba than were those of ‘Harus’. Dicamba has the potential to cause
drastic yield reductions in ‘Wakefield’ when it is applied at typical rates and growth
stages, and this herbicide/variety combination should be avoided. Genetic studies have
revealed that many differences in the tolerance of cereal varieties to herbicides are
controlled by one or two major genes, and therefore, in these cases the characteristics can
be simply inherited (33). In the future, any variety that is developed with ‘Wakefield’
parentage should be screened for tolerance to dicamba.

Dicamba has a very strong effect on seed morphology, but a minor effect on other

59

plant attributes. The injury caused by dicamba applied to ‘Wakefield’ at proper grth
stages is in every way analogous to the injury observed in less sensitive varieties when
dicamba is applied at later stages of development. Very simple screens in which wheat
heads are closely examined at the time of harvest would certainly reveal sensitivity
problems of this type.

‘Wakefield’ was not injured by dicamba applied at F5 in the field study in 1996 but
was severely injured by dicamba applied at this grth stage in 1997. Perhaps
‘Wakefield’ should be included in herbicide sensitivity screens that focus on dicamba
sensitivity. Data that are generated in these screens in years when ‘Wakefield’ is not

injured would be considered less reliable.

10.

11.

60

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Holroyd, J. 1962. New post-emergence herbicides in cereals. Part H Field
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Grain Yield

Wakefield

 

U F2 F3 F5 F9 F10

3°

Grains per Spikelet

U F2 F3 F5 F9 F10

 

 

 

u—ar—An—Ir—Ir—I
ONAQOOONA¢N
1 1

l l l

Spikelets per Head
I I I | | l

l

 

 

 

 

 

 

 

 

U F2 F3 F5 F9 F10

63

 

Harus

 

 

U F2 F3 F5 F9 F10

 

 

 

 

 

 

 

 

    

U F2 F3 F5 F9 F10

 

 

     

a
U F2 F3 F5 F9 F10

Means with the same letter are not significantly different at the 5% level

Figure 5. Effect of dicamba on grain yield and yield components in field 1996.

Grain Yield
(kg/ha)

Grains per S pikelet

Spikelets per Head

IJIIII

Weight of 1000 Grains

Wakefield

 

U F2 F3 F5 F9 F10

 

U F2 F3 F5 F9F10

 

 

 

 

 

 

 

U F2 F3 F5 F9 F10

 

 

 

 

 

'3“ _.
U F2 F3 F5 F9 F10

64

 

Harus

 

U F2 F3 F5 F9 F10

 

    
    

 

 

U F2 F3 F5 F9 F10

 

 

U F2 F3 F5 F9 F10

 

 

Illll

  

 

 

 

 

        

U F2F3 F5 F9 F10

Means with the same letter are not significantly different at the 5% level

Figure 6. Effect of dicamba on grain yield and yield components in field 1997.

65

Wakefield Harus

 
   

Grain Weight per Spike
(% of control)
eases 88§EEE

 

 

 

 

 

 

 

 

4)
30
Z)
10
0 0
13 F5 19 F10 F3 P9 F10
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._.

Grains per Spikelet
(% of control)
o '5‘ B 8 8 8 8 a 8 8 8

   

 

F3 F5 F) F10 F3 E3 f9 FlO

Means with * are significantly different than their respective untreated control at the 5% level
Means with the same letter are not significantly different at the 5% level

Figure 7. Response of yield components to dicamba in greenhouse nm 1.

66

 

 

 

 

 

 

 

 

 

 

Wakefield Harus
120
110
0 1(1)
$1 90
(I)

a a)
fig 70
a 8 6°
.5 ,H 50
3 3 40
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110 I 110

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10. 10-

0- 0-

100

90 90

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g :o‘ 70 70

ea 60 60

e 8 50 50

3‘8 40 40

'3 § 30
L:

O 20 20

10 10

o

   

 

F3 F5 F) F10 F3 F5 19 F10

Means with ‘ are significantly different than their respective untreated control at the 5% level
Means with the same letter are not significantly different at the 5% level

Figure 8. Response of yield components to dicamba in greenhouse run 2.

67

 

Wakefield

= normal

  
 
 

 

 

 

 

 

 

 

 

 

A 100%

o\° -

E 90% ,I‘ '

E 80% s

(D .

*5 70% 4

D .

g 60% i " '

g 50% E 'i

E 40% J’s _ i

o 30% i— t I

G ’ L ~,-

43 l

< 20% ;_. '

_. ..

a 10% 1L

0

Z 0% + .
untreated

A 100% s

sf, .,

a 90%, 1

a 30% 4. ,

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‘D ' .

.8 60% . ,,

<

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M

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o 30% s

a

.o 0

<2 20% .

u—i‘

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o

z 0%

 

 

 

 

 

 

 

 

 

 

 

 

untreated F2 F3 F5 F9 Fl 0

Figure 9. The effect of dicamba on seed quality in field 1997.

J‘y’ I l" ‘J le Harus

5
°.
.\

90% 90%

80% .. 80%
70%. 70% _
60% 60%
50% 50%
40%. A _. 40% . ‘1
30% i 30% .

20% . ' 20% '

 

10% ' L 10%

Normal, Abnormal, and Absent Grains (%)

 

 

 

 

 

 

 

0% 0%

 

 

 

 

100%
90%
80% 80%
70% 70% . ~
60%. 72* 60%
50% 50% »
40% ., 40%.

30% . 30%

 

20% . 20% . ’

10% 10%

 

Normal, Abnormal, and Absent Grains (%)

 

0%,' 0%“

 

 

 

 

 

 

 

 

 

 

 

   

r
F10

* = Significantly greater than other mean within the same growth stage at the 5% level ¥= 32:81ng
= e

Figure 10. The effect of dicamba on seed quality in greenhouse.

Abnormal Grains per

Abnormal Grains per

 

69

60% _ Experiments using 0.14 kg ai/ha of dicamba

 

 

 

 

 

 

 

Pollen donor plant ' + - + + -
Seed bearing plant - 4+ - + - +

Not hybridized Hybridized

 

 

 

 

 

 

 

100%
90%
80%
70%
60%
50%
40%
30%
20%
10%

0%
Pollen donor plant - + - + + -
Seed bearing plant - + - + — +

Not hybridized Hybridized

Floret (%)

 

 

 

 

 

 

 

 

 

+ = treated
' = untreated Means with the same letter are not significantly different at the 5% level

Figure 11. The effect of dicamba on seed bearing and pollen donating plants.

HICHIGRN STATE UN

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