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THESlS

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3 1293 011012923

This is to certify that the

thesis entitled

AN ATIALYSIS OF DIFFERENTIAL YIELDS IN ALFALFA
(MEDICAGO SATIVA L.) WITH SPECIAL REFERENCE TO FACTORS
AFFECTING NET PRODUCTION A’ID PHOTOSYNTHETIC ACTIVITY

 

presented by

Frederick w. Fuess

has been accepted towards fulfillment
of the requirements for

._PhD_degtee in CI’O c'ence 1..

méé’flw

Major professor

 

Datefibdfi‘wt qr /qéi

0-169

     
    

LIBRAR

Michigan St:
University

 

 

ABSTRACT

AN ANALYSIS OF DIFFERENTIAL YIELDS IN ALFALFA
(MEDICAGO SATIVA L.) WITH SPECIAL REFERENCE TO FACTORS

AFFECTING NET PRODUCTION AND PHOTOSYNTHETIC ACTIVITY

by Frederick W. Fuess

Field and controlled-environment studies were conducted to deter~

 

mine the causes of differential production of alfalfa (Medicago sativa

L.) under four cutting systems to determine Specifically why alfalfa

cut three times yields more than when out twice per season. Net pro-

duction, leaf area, leaf distribution and light interception were
measured weekly and leaf area indexes and net assimilation rates were

calculated. Net photosynthetic rates of excised alfalfa leaves from

both the controlled-environment chamber and the field were measured in

a Warburg respirometer.

Alfalfa cut three times per season yielded 0.77 ton more for an

average of two years than when cut twice primarily due to a measured

net leaf loss of 0.53 ton more in the two- than in the three-cutting

treatment, accounting for two-thirds of the difference in total yield.
The remaining one-third of the difference appears to have been due to

increased rates of net photosynthesis in the leaves of the three-

cutting treatment which were younger on the average than the leaves in

the two-cutting treatment.

A: “MW: rat‘s—2':—

 

Frederick W. Fuess
Leaf area indexes (LAI) reached an average ceiling value of 5.16
during the first cutting. LAI did not serve as an adequate predictor
of seasonal production but showed a positive relationship to yield for
individual cuttings in the frequently-cut alfalfa.
Net photosynthetic activity in alfalfa decreased with age.
Leaves more than three weeks old were less than one-seventh as active
photosynthetically as five-day old leaves.
The Warburg respirometer appears precise enough to support the
use of this method in determining comparative rates of photosynthetic

activity in alfalfa leaves.

 

AN ANALYSIS OF DIFFERENTIAL YIELDS IN ALFALFA

(MEDICACO SATIVA L.) WITH SPECIAL REFERENCE TO FACTORS

 

AFFECTING NET PRODUCTION AND PHOTOSYNTHETIC ACTIVITY

By

Frederick W. Fuess

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Crop Science

1963

 

‘3

at ‘TD 0 3
c/a/w

 

ACKNOWLEDGEMENT

The author wishes to express his appreciation to Dr. M, B. Tesar
for his helpful guidance, encouragement, and constructive criticism
during the course of these studies and for his valued help in the prepa-
ration of this manuscript. Appreciation is also expressed to Dr. C. R.
Olien for his invaluable assistance in the photosynthetic determinations
and to Professor H. M. Brown for his advice in the statistical analyses.

A special note of gratitude is expressed to my wife, Marilyn,
without whose moral, mental and financial support this study would not
have been possible and to Frederick IV and Cynthia, our children, for

their cooperation and devotion during the past three years.

ii

 

 

TABLE OF CONTENTS

Page
LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . iv
LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . v
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . 1
REVIEW OF LITERATURE . . . . . . . . . . . . . . . . . . . . . . 3
Yields and Carbohydrate Reserves . . 3
Leaf Area, Distribution and Light Interception . 4
Net Assimilation Rate 6

PART I. EFFECT OF CUTTING ON YIELD, NET PRODUCTION, LEAF

AREA, LEAF DISTRIBUTION, LIGHT INTERCEPTION AND
NET ASSIMILATION RATE . . . . . . . . . . . . . . . . . 8
Material and Methods . . . . . . . . . . . . . . . . . . . . 8
Experimental Results . . . . . . . . . . . . . . . . . . . . 18
Yield . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Net Production . . . . . . . . . . . . . . . . . . . . . 20
Leaf Area Index . . . . . . . . . . . . . . . . . . . . . 23
Leaf Distribution . . . . . . . . . . . . . . . . . . . . 29
Light Interception . . . . . . . . . . . . . . . . . . . 34
Net Assimilation Rates . . . . . . . . . . . . . . . . . 37

PART II. PHOTOSYNTHETIC STUDIES OF EXCISED LEAVES

IN A WARBURG RESPIROMETER . . . . . . . . . . . . . . 41
Material and Methods . . . . - . - - 41
Results and Discussion of Net Photosynthetic Studies . . . . 46
Exploratory Investigations 46
Principal Investigations 48
DISCUSSION OF RESULTS 54
SUMMARY 62
65

LITERATURE CITED .

iii

Table

LIST OF TABLES

Yield in tons of hay per acre and percent produced
by each cutting of alfalfa harvested one, two,
three, and four times per season in different
locations

Average leaf area indexes of alfalfa cut one,
two, three, or four times per season .

Leaf area indexes and percentages of total leaf
area located in 6-inch segments of alfalfa
stems 1961

Leaf area indexes and percentages of total leaf
area located in 6-inch segments of alfalfa
stems 1962 .

Net assimilation rates in grams of dry forage per
square meter of leaf area per day of alfalfa
under four cutting treatments

Net photosynthetic rates of leaves of alfalfa
located in different sections of the stem

Net photosynthetic rates of leaves of different

ages of alfalfa grown in a controlled-
environment chamber

iv

Page

19

28

30

32

38

SO

53

LIST OF FIGURES

Figure Page

1. Ozalid print method used to determine leaf area in
1961 showing (left to right) the green leaves and
stems, the printing frame, the developed print and
the cut-out leaf prints (lower right) . . . . . . . . ll

2. Leaf punch used to determine leaf area in 1962
showing (left to right) the green leaves and
stems, the punch with leaflets in place, the
leaf disks (upper right) and the sampled leaflets . . 13

3. Aluminum U-bar probe used to measure light intensities
in alfalfa showing the leveling bubble on the U-bar
and the Weston 765 illumination meter . . . . . . . . 15

4. Aluminum frame used to support the U—bar in a
horizontal position at six-inch increments from
the ground level . . . . . . . . . . . . . . . . . . . l6

5. Tons of hay produced per acre by alfalfa cut one,
two, three or four times per season. Weekly
determinations, 1961. LSD and R at 5% level . . . . . 21

6. Tons of hay produced per acre by alfalfa cut one,
two, three and four times per season. Weekly
determinations, 1962. LSD and R at 5% level . . . . . 22

7. Leaf area indexes (LAI) of alfalfa cut one, two,
three and four times per season. Weekly deter—
minations, 1961. LSD and R at 5% level . . . . . . . 24

8. Leaf area indexes (LAI) of alfalfa cut one, two,
three and four times per season. Weekly deter-
minations, 1962. LSD and R at 5% level . . . . . . . 25

9. Percentage light interception O, 6 and 12 inches
from ground level in alfalfa cut one, two, three

and four times per season. Weekly determinations,
1962 . . . . . . . . . . . . . . . . . . . . . . . . . 35

10. Modified Warburg flasks showing the method used to
attach the leaflets (upper photograph) and the
assembled flask attached to the manometers
(lower photograph) . . . . . . . . . . . . . . . . . . 43

Figure

11.

12.

13.

14.

LIST OF FIGURES (Continued)

Net photosynthetic rates (pl OZ/hr/cmz) of leaves
from different locations on stems grown in the

controlled-environment chamber for six weeks.
1962 .

Net photosynthetic rates (ul Oz/hr/cmz) of leaves
from different locations on stems from a cutting
stand of alfalfa, June 12, 1961

Percentage light interception at ground level (LI%),
leaf area indexes (LAI), tons of hay per acre (T/A)
and net assimilation rates (NAR) of alfalfa cut

one or two times per season. Weekly determinations,

1962 .

Percentage light interception at ground level (LI%)
leaf area indexes (LAI), tons of hay per acre (T/A)
and net assimilation rates (NAR) of alfalfa cut
three or four times per season. Weekly determines
tions, 1962

vi

Page

49

52

58

59

mrsopucnon

The importance of alfalfa as a hay crop in Michigan and in the)
northeastern half of the United States has been well established. In
Mflchigan, approximately 15% of the 10 million acres of cropland are
utilized for the production of either alfalfa hay or an alfalfa-grass
' hay. In the United States, alfalfa hay occupies 8% of the total crop-
land. The amount of alfalfa hay produced per acre in Michigan varies
from well over five tons per acre on the fertile, well-drained soils to
less than one ton in the marginal agricultural areas.

Alfalfa management practices have undergone several changes in
the past sixty years. 'During the early 1900's, it was believed that
frequent cutting stimulated growth and increased yields. After the re-
sults of long-term studies reviewed by Graber ES 3;, (10) showed in
the 1920Is that frequent cutting resulted in lowered yields in sub-
sequent years, agronomists in the.Great Lakes states recommended that
alfalfa be cut twice during the season at the full bloom stage. With
the advent of practical leafhopper control, more wilt-resistant and
winter-hardy varieties, increased emphasis on quality, and shorter
rotations, three harvests before September 1 have been recommended in
the northern part of the Great Lakes area. Since the late 1950's, a
few studies in the North Central states have indicated that a three-
cutting system produces more hay than either a two- or four-cutting
system for as long as three and possibly four years. Few attempts,

I

2
however, have been made to determine the factors which are responsible
for the yield differences except as the cutting treatment effects the
yield in subsequent years.

Consequently the purpose of the studies reported herein has been
to obtain fundamental information concerning the leaf production, light
interception, net production, and photosynthetic efficiency of alfalfa
(Mgdicago gatiyg L.) cut one, two, three, or four times per season in
an attempt to answer the specific question: Why does a three-cutting
system yield more hay than a two-cutting system over a period of three

to four years?

REVIEW OF LITERATURE
Yields and Carbohydrate Reserves

During the first two decades of this century, alfalfa management
systems were based primarily on empirical observations rather than
scientific investigations. These early recommendations have been re-

viewed by Graber t 21, (10). Since the late 1920's, many studies have

been conducted to measure the effects of time and frequency of cutting
on the yield and longevity of alfalfa stands with special emphasis on
the effects of cutting treatments on the carbohydrate reserves and
winter survival. An excellent review of these investigations from the
early 1930’s until 1950 has been presented by Willard (27).

Studies in the last 5 years have indicated that some of the
recently-released varieties which combine wilt resistance and winter
hardiness will withstand more frequent cutting than the less hardy,
wilt-susceptible varieties in common use in the 1930's. Folkins gt _1.
(9), working in Ontario, Canada, reported no significant decrease in
yield in the third year after cutting a Vernal alfalfa-timothy stand
three times before August 16. Over the three-year period, the three-
cutting system produced more dry matter than either a two- or a four-
cutting system.

A three-year study in Michigan (20) showed that Vernal alfalfa
when top-dressed annually with 83 lbs. K.yie1ded one ton more hay per

acre per year when cut three times instead of twice a year with the

3

4
last cutting being on September 1; the yield increase was only 0.4 ton
when not top-dressed.

A study in Wisconsin (17) using two alfalfa varieties, two
fertility levels, two levels of insect control, and five cutting treat-
ments over a three-year period has shown that three harvests before
September 1 produced 22% more dry matter and 49% more protein than a
two-cutting system. The only treatment which favored the two-cutting
system was the low fertility system with no insect control and dif-
ferences were only evident in the third harvest year.

The carbohydrate reserves of roots of Vernal alfalfa not cut or
subjected to a two- or three-cutting system were measured by Smith (16).
A noticeable drop in the carbohydrate content of the roots was evident
after each defoliation. By mid-November, however, the percentage of
total available carbohydrates in the roots was approximately the same

in all cutting treatments.

Leaf Area, Distribution and Light Interception

Watson (23) has suggested that the "measure of leaf area which
is relevant to the comparison of agricultural yields, that is, of weights
of different crops produced per acre of land, is the leaf area per unit
of land, which it is prOpOSed to be called the Leaf Area Index (LAI)".
It has recently been pointed out by Watson (26) that total dry matter
production varies with Size, activity, and duration of the photo-
synthetic system. His data indicate that the size of the photo-
synthetic system is the greatest variable as indicated by the fact
that varieties of potatoes and sugar beets with the highest mean leaf

area produced the highest total yield.

5

Many investigators have reported leaf area indexes for various
crops but no reports of leaf area measurements in alfalfa were found.
Davison and Donald (5) measured the leaf area of subterranean clover
planted at densities of from 1 to 50 plants per square link (63 square
inches). Their results show that all awards tended toward a common
.leaf area index of 8.7 at the end of the growing season. Maximum dry
matter production of the subterranean clover was reached when the leaf
area index was between 4 and 5. In studies with white clover, Brougham
(4) found that the LAI never exceeded 5.5. Donald and Black (7) re-
viewed the leaf area indexes of other crops.

An analysis of the apparent importance of leaves at different
locations on the stem has shown that the total LAI may be an over-
estimation of the total photosynthetic area. 0f the maximum LAI of
5.5 in a white clover sward, Brougham (4) found that only 60 percent
of the leaves were fully-expanded, active leaves; the remaining 40
percent of the leaves were either young and folded or dead and dying.
Senescense of lower leaves became evident approximately 35 days after
defoliation when the leaf area index reached 4.

Studies with corn (13) show that the production of dry matter
per unit of leaf area by the top, middle, and bottom leaves was of the
ratio of 4 to 2.2 to l. The study suggests that the low production of
the lower leaves was due to a reduction in the light intensity from
shading by the upper leaves. Similar results have been reported by
Donald (8) who has stated that under a dense leaf canopy, the lower
leaves may become parasitic.

The relationship between light interception and LAI as reported

by Brougham (3) has shown that the growth rate of a legume-grass

6
pasture increases until complete light interception at ground level is
approached and thereafter growth remains at almost a constant rate.
The leaf area index of 4 in clover and 7 in timothy, at which 95% light
interception occurred, has been defined as the "critical LAI". This
value appears to be similar to the "optimum LAI" as defined by Donald
(8). Leaf areas tend to increase beyond this "optimum" level until
the rate of death of old leaves equals the rate of appearance of new
.leaves which Donald has called the "ceiling LAI". He notes that plants
continue to increase in size after the ceiling LAI is reached as the
stems continue to elongate. Maximum yield is not reached until the net

photosynthesis of the whole plant reaches zero.
Net Assimilation Rate

Studies of net assimilation rates of many crops have been pre-
sented by many investigators and the techniques used to measure the
assimilation rates have varied. Early studies which attempted to cor-
relate yield with morphological measurements were difficult to interpret
because of variations in weather conditions during the growing season.
A complete review of the literature concerning the early attempts to
measure net assimilation rates is presented by Watson (25). Talling
(19) reviewed the more recent studies with special emphasis on photo-
synthetic studies in the field. Only a few of the more pertinent
investigations are presented herein.

In one of the earliest studies of assimilation, Thomas and Hill
(21) measured the changes in CO2 concentration above an alfalfa plot
enclosed in a small glass chamber. Their data show that the assimila-

tion rate of an alfalfa crop 48 inches tall was only twice as great as

7
that of plants 6 to 8 inches tall despite the fact that the taller
alfalfa had 3.6 times the leaf weight.

A mathematical approach to assimilation rates, originally sug-
gested by Briggs gt 21. (2), was refined and expanded by Watson (25).
He defined net assimilation rate (NAR) as the change in dry weight of
the plants per unit area of leaf per day. In reality, the NAR as
defined by watson is a measure of photosynthetic efficiency rather than
of growth.

One of the major errors cited in the calculation of NAR has
been the incomplete recovery of the root system. However, differences
in the NAR of wheat calculated from the total dry weight including the
recovered roots and calculated from the dry weight of the shoots alone
were small (24).

Studies of the assimilation rates of individual leaves have been
conducted to determine the optimum conditions for maximum photosynthesis
under the artificial conditions present in such a test. The temperature
and light intensity at which isolated leaves have the greatest rate of
apparent photosynthesis appears to vary with the species tested. Light
intensities between 1,200 and 2,000 foot candles and temperatures be-
tween 20 and 300 C. have been recommended (1) (6).

The age of the leaf appears to have a greater effect on the
photosynthetic rate than the chlorophyll content of the leaf. Moss (14)
has reported that the yellowish-green corn leaves are as effective
photosynthetically as dark green leaves of the same age. Pea leaves
have been reported to reach their maximum photosynthetic rate when they

are still folded and have not turned dark green (15).

PART I

EFFECTS OF CUTTING FREQUENCY ON YIELD, NET PRODUCTION, LEAF AREA,
LEAF DISTRIBUTION, LIGHT INTERCEPTION AND

NET ASSIMILATION RATE

Material and Methods

The experiments were conducted on a tile—drained Conover silt
loam of high fertility. The sites used in 1961 and 1962 were located
within 500 feet of each other and were part of the experimental fields

of Michigan State University.

1961

 

A series of Vernal alfalfa plots in their third harvest year was
used in 1961. These plots were seeded in 7-inch rows in April, 1959,
with a grain drill equipped with band seeding tubes and were sprayed
to control weeds. Two harvests were taken from the plots in both 1959
and 1960. The area was top-dressed with l3# of P and 72# of K per
acre in April, 1961.

The following four cutting treatments were imposed with a
self—propelled mower with a 3-foot cutting bar set to cut the forage

to a 2-inch height:

W

 

Number of
cuttings
per season Dates of cutting
One August 30
Two June 21, August 30
Three May 31, July 12, August 30
Four May 17, June 21, August 2,3 August 30

 

aout July 26, 1962

A Split-plot design with five replications was used in 1961.
The four cutting treatments which occurred at random in each replication
occupied the whole plots which were 6 by 68 feet in size. Each whole
plot was divided into seventeen 4- by 6-foot sub-plots.

The sub-plots were not randomized but were harvested in con-
secutive order to facilitate the sampling and cutting of the experi-
mental area. Each sub-plot was harvested only once during the season.
Only those sub-plots which had not been previously sampled were cut
with the mower.

At weekly intervals from May 10 to August 30, 1961, an area
five rows wide (35 inches) by 36 inches was cut to ground level by
hand. This sample was removed from the subvplot leaving a border of
one and one-half feet on each side and one foot on the harvested end
of the plot. A sub-sample of from 12 to 20 stems was selected at
random from the harvested material of two of the five replications.
It was found that the leaf percentage and leaf area of the plots ex-
hibited the greatest variability during early periods of regrowth.

Therefore, the larger sub-samples were taken immediately after the

10
cutting treatments were imposed.

The stems of the sub-sample were cut into 6-inch segments
immediately after collection and placed in plastic bags in an insulated
box. Stems were cut individually and the point of attachment of the
petiole determined the 6-inch segment into which the leaf was placed.
The plastic bags were stored in a cooler at 40° F for not more than
three days. The remainder of the forage from the 35- by 36-inch sub-
plots was placed in a paper bag and dried to a constant weight at
1400 F. Yields are reported on an acre basis as tons of hay containing
12% moisture.

Leaf area and leaf distribution were determined by removing the
leaflets from the petiolules and petioles with a pair of tweezers.

The petiolules, petioles, and stems were grouped together and will
hereafter be referred to as stems. An "Ozalid" print was made of the
leaflets from one replication (Figure l). The leaves and stems from
each 6-inch segment were dried, weighed, and the dry weight ratio of
leaves to stems determined.

The "Ozalid" prints of the leaflets were cut out, dried, and
weighed. The weight of a known area of Ozalid paper was determined
and the areazweight ratios of the leaves in each 6-inch segment was
calculated. The leaf area index and leaf distribution of each sub-
plot was determined for each of the five replications from the leaf:

stem ratio and the areazweight ratio.

1962
The experimental area used in 1962 was band seeded with Vernal

alfalfa on summer-fallowed ground on August 7, 1961, at a rate of

11

 

 

Fig. l.--Ozalid print method used to determine leaf area in 1961

showing (left to right) the green leaves and stems, the printing frame,
the developed print and the cut-out leaf prints (lower right).

12
11-1/2 pounds of seed per acre. A total of 140 pounds of phosphorus
and 260 pounds of potassium per acre was applied in three applications
before and at the time of seeding. The field was clipped with a
tractor-drawn rotary mower on April 21, 1962, to remove the large weeds.

A split-plot design with four replications was used in 1962.

The four cutting treatments comprised the whole plots and occurred

at random in each replication. The whole plots, 12 by 20 feet in size,
were each divided into 18 sub-plots, 12 by 6 feet in size. The larger
size of both the whole plots and the sub-plots in 1962 afforded a
border of 4-1/2 feet on each side and 3 feet on the harvested end of

the 35- by 36-inch sampling area. The borders helped to prevent shading
of the newly-cut whole plots by the uncut plots.

The sampling procedure in 1962 remained the same as in 1961.
However, the number of stems selected as a sub-sample was decreased
to 10 in those plots with growth of over 24 inches. The leaf area:
weight ratio was determined for two replications rather than one as in
1961.

In 1962, the leaf areazweight ratio was determined by weighing
leaf disks of known area. Forty-eight leaflets were selected at random
from the leaves present in each 6-inch segment. The leaflets were
stacked on a rubber stopper in groups of six and a disk 0.23 square
centimeters in area was obtained with a sharpened, hollow steel rod
from as near the center of the leaflets as possible (Figure 2) since a
comparison of the "Ozalid" print and disk methods indicated that the
disk method gave results which were consistently 2% below the print
method if the disks were removed from the center of the leaflets.

However, if the disks did not include the mid-vein of the leaf, the

13

 

 

Fig. 2.--Leaf punch used to determine leaf area in 1962 showing
(left to right) the green leaves and stems, the punch with leaflets in
place, the leaf disks (upper right) and the sampled leaflets.

14

area tended to be overestimated by as much as 20% as compared with the
print method. .

weekly samples were taken from May 3 to August 30, 1962. The
cutting treatments imposed on the whole plots followed the same schedule
as in 1961 with the exception that the date of the third harvest of the
4-cutting treatments was advanced to July 26, 1962. The plots were
sprayed weekly with one pound of DDT per acre to control leafhoppers.

Light interception was measured using a Weston Model 756 illumina-
tion meter fitted with a quartz window. The photosensitive cell of the
light meter was fitted into a small aluminum U-bar (Figure 3). The end
of the U-bar was fitted with a small hardwood point to facilitate pas-
sage through the forage. A small, round bubble-type level was attached
to the aluminum bar to insure horizontal placement of the photosensitive
cell in the forage. An aluminum frame was constructed to hold the bar
and photosensitive cell at 6-inch increments from the surface of the
ground (Figure 4). The aluminum bar with the photosensitive cell in
place was pushed between the rows of forage a distance of 2, 2-1/2, and
3 feet and light readings were taken at each distance. Readings were
made between 11 a.m. and l p.m. on clear days.

The net assimilation rate (NAR) for each weekly interval was

calculated using the following formula:

 

W — W
2 1
NAR =
(t2 - t1) L1 + L2

2

where W1 and W2

total leaf area at times t1 and t2 respectively. The NAR is expressed

equal the dry weight of the forage and L1 and L2 the

as grams of dry forage per square meter of leaf area per day (g/mZ/day).

15

 

 

Fig. 3.--A1uminum U-bar probe used to measure light intensities
in alfalfa showing the leveling bubble on the U-bar and the Weston 76S
illumination meter.

16

 

 

Fig. 4.--Aluminum frame used to support the U-bar in a horizontal
position at six-inch increments from the ground level.

17
Statistical significance was determined using_Duncan's new multiple-
range test (18). Where applicable the least significant ranges at the
5% level are reported for p - 2 (LSD) and p a the maximum numbers of

entries (R).

Weather Conditions
The monthly precipitation in inches and deviation from normal
recorded at the United States Weather Bureau station located approxi-

mately one mile from the experimental areas were as follows:

L

 

 

__ - _‘—;

 

 

 

 

 

19;? 1967
Month Normala
Total Deviation Total Deviation

April 3.55 .73 1.23 -l.59 2.82
May 1.04 -2.72 1.82 -l.94 3.76
June 2.17 -1.21 3.42 .04 3.38
July 2.71 .01 2.41 - .29 2.70
August 3.97 1.13 1.85 - .99 2.84

Total 13.44 -2.06 10.73 -4.77 15.50

 

a47-year average.

Rainfall from the beginning of the growing season to the time
of the last harvest date in 1961 and 1962 was 2.06 and 4.77 inches be-
low normal, reSpectively. In 1961, below normal rainfall in May and
June was partially compensated for by above normal rates in July and
August. The 1962 season was very dry with below normal rainfall in
every month except in June when precipitation was only 0.04 inches

above normal. Near drought conditions were experienced in the latter

18
part of July since more than one-half of the total monthly rainfall

fell on July 3.
Experimental Results

The average yields from each harvest and the total yields of
alfalfa managed under the four cutting treatments are shown in Table 1.
Alfalfa cut three times was significantly higher in yield for the two
years than any of the other cutting treatments. The yield of 5.37 tons
of hay under the 3-cutting treatment was 0.77 tons greater than under
the 2-cutting treatment and 0.63 tons greater than under the 4-cutting
treatment. The 3-cutting system yielded much more--2.47 tons--than
the plots cut only once on August 30.

There was no difference in yield between 1961 and 1962 in the
2- or the 3-cutting treatments. The l-cutting treatment, however,
produced 0.57 ton more hay in 1961 than in 1962. The difference in
yield may have been due to the very dry season in 1962. Visual obser-
vations indicated that regrowth from crown buds was noticeably decreased
and the limited amount of regrowth produced was not vigorous enough to
penetrate the dense cover of mature forage in 1962.

The difference of 1.28 tons between the 1961 and 1962 yields in
the 4-cutting treatment appears to have been caused by several factors.
A severe infestation of leafhoppers in July and August of 1961 reduced
the number of leaves and stunted the growth, especially during the last
period of regrowth. The very warm spring of 1962 was responsible for
at least a part of the increased yield in that year as approximately

40% of the total difference in yield of the 4-cutting treatment can be

19

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20
accounted for in the first harvest on May 17.
The percentages of the total yield harvested at each cutting
date were similar in 1962 and 1961. In all cases, the first harvest
was responsible for the largest part of the total yield--73, 47, and
33% in the 2-, 3-, and 4-cutting treatments, reapectively. Under the
4-cutting system, 87% of the total yield was in the first three cuttings

and only 13% was produced in the last cutting.

Net Production

 

The net production curves (based on harvestable material) shown
in Figures 5 and 6 were constructed from weekly production data col-
lected from the 35- by 36-inch sub-plots. Since the data representing
the period prior to the first defoliation of each cutting treatment
were combined and averaged, all curves representing net production
from the beginning of the season until the time of the first harvest
are identical.

Except for minor differences in net production attributable to
leafhopper damage in 1961 or dry weather in July and August in 1962,
the general pattern was similar in the two years on the two different
areas.

Accumulated net production per acre increased steadily under
the l-cutting treatments from the beginning of the growing season.
Maximum yields of 3.92 tons on July 5, 1961 (Figure 5) and 3.80 tons
on July 12, 1962 (Figure 6) were obtained. After these maximum levels
were reached the tons of harvestable material produced per acre de-
creased. The more severe decline in harvestable material under the

l-cutting system during August of 1962 than in August of 1961 appears

21

One Cutting

B a 1/10 Bloom

 

 

 

 

 

0 1 1 1 1 1 1 .1 1 1 1 1 L 1 1 1. 1 1
Two Cuttings
.4F-
B
31.
2..
m 1‘-
H
o
4
H0 lllJLL
<11
m a
2 Three Cuttings
E3
31-
B
2— I I

 

1L.—
oLlJ LL1111/14—T—1/1

l

Four Cuttings

 

 

 

3L—
21—
I: II
. .1LJLM
10 25 7 21 5 19 2 16 30
May June July August

Fig. 5.--Tons of hay produced per acre by alfalfa cut one, two,
three or four times per season. Weekly determinations, 1961. LSD and
R at 5% level.

22

One Cutting

 

 

s = 1/10 Bloom

 

 

 

 

 

 

 

 

 

O- Li1111LL1L1111111
4 _. Two Cuttings
L
3 B
21:—
c111"
u
:3
l l 1 l L, l
:30
9.1
U)
a
[3 Three Cuttings
3 L
B
2* :1: I
1..—
0 l L, 1 1 .1 J 1 1‘11 1
Four Cuttings
3—
2*—/ I I
0 1 1 1 1 1 1 1 1 Liz/Till”1——I”1
10 25 7 21 5 l9 2 16 30
May June July August

Fig. 6.--Tons of hay produced per acre by alfalfa cut one, two,
three and four times per season. Weekly determinations, 1962. LSD and
R at 5% level.

23
to have been due to the drouth conditions which existed. Since the
first cutting in the 2-, 3-, and 4-cutting systems was made before
maximum production was obtained, decreases in weekly yield were not
evident during the period prior to the first defoliation.

In the 2-cutting treatment, production rates during the first
six weeks of regrowth in 1961 and the first five weeks in 1962 closely
paralleled the production curves representing the early part of the
season. The amount of harvestable material produced under the 2-
cutting system decreased in late August of both years. A severe leaf-
hOpper infestation in 1961 appears to have been partially reSponsible
for the decrease in the production. In 1962, weekly applications of
an insecticide were successful in controlling the leafhoppers yet there
was an actual decrease in harvestable material near the end of the
season because of drouth.

As the number of cuttings increased from two to four, the pro-
duction rate in each successive regrowth period decreased. In con-
trast to the l- and 2-cutting treatments, however, there was a gradual
increase in production and at no time was there a decrease in production
between one week and the next in the 3- and 4-cutting treatments. The
dry weather in 1962 did not seriously affect the growth in the 3- or

4-cutting treatments.
Leaf Area Index

The ratio of leaf area (one surface only) to land area is ex-
pressed as the leaf area index (LAI). Weekly determinations of the
LAI of alfalfa grown under four different cutting treatments are shown

for 1961 in Figure 7 and for 1962 in Figure 8.

24

    

 

  

 

6 r_ One Cutting
L. B
4 L—
21—
__ B a 1/10 Bloom
011111111111111LL1

 

Two Cuttings
Gr B
1...

 

 

Three Cuttings

Leaf Area Index (LAI)
ox

 

 

 

 

F.

L.

4 a.

t. B II
2 F-

1__ /———/
0111? ltLLl 111111
6 F_ Four Cuttings
41-

.- II
2 r- //// I

F- 4’/,,a””"fl_- ‘_’/’,/””
0 *1 I ii I 11 ii L1_L L111 L. 1 L 11

10 25 7 21 5 l9 2 16 30
May June July August

Fig. 7.--Leaf area indexes (LAI) of alfalfa cut one, two, three
and four times per season. Weekly determinations, 1961. LSD and R at

5% level.

25

One Cutting

     
  

- B a 1/10 Bloom

 

 

 

 

olillllLllLlllJJJJ
6 [_ Two Cuttings

4. :;’//////A\\“‘*\\\\\\

2 ._

O I. L 1 11 1 1 L

Three Cuttings

F.
4/
I I

F. L,.///fi
111 1111.11LLL1

Four Cuttings

Leaf Area Index (LAI)

 

 

 

6r

ll.-
4 ::;////

.. II
21.—

L...-
011111-111L11L1

10 25 7 21 5 l9 2 16 30
May June July August

Fig. 8.--Leaf area indexes (LAI) of alfalfa cut one, two, three
and four times per season. Weekly determinations, 1962. LSD and R'at

5% level.

26

Leaf area, in the l-cutting treatment, increased from the be-
ginning of the season until June 7, 1961, when a ceiling LAI of 5.44
was reached. In 1962, unseasonably warm weather in early May hastened
leaf development and the ceiling LAI of 4.89 was reached on May 25.
After reaching the ceiling level in both years, the LAI of the 1-
cutting treatment decreased gradually until the first harvest was re-
moved on-August 30. At this time, the LAI was similar for the two
years--0.80 in 1961 and 0.62 in 1962. Abscission of leaves and a
decreased rate of leaf formation as the plants approached maturity
appear to have been responsible for these declines. The loss of dry
weight which accompanied the decrease in LAI resulted in a decrease in
net production during the latter portion of the season (Figures 5 and
6). Since the rates at which LAI decreased in 1961 and 1962 were similar,
at least a part of the more pronounced decrease in harvestable material
in 1962 than in 1961 was probably due to the dry weather conditions.

In the 2-cutting treatment, the LAI reached a ceiling level 2
and 4 weeks before the first cutting date in 1961 and 1962. The leaf
area declined steadily from these ceiling levels until the first harvest
on June 21. Since the ceiling LAI was reached two weeks earlier in 1962
than in 1961, a more noticeable decrease in leaf area occurred before
the June 21 cutting date in 1962 than in 1961. After cutting, leaf
area in the 2-cutting treatment increased in both years and reached
fairly similar ceiling levels of 2.27 on July 26, 1961, and 2.60 on
July 12, 1962. An adequate insect control program in 1962 tended to
stabilize the LAI during July. Leaf area did decrease, however, during
August of 1962 indicating that only a part of the decrease in August of

1961 was due to leafhopper damage. By August 30 when the 2-cutting

.27
treatment was harvested for the second time, the LAI had dropped to
0.68 in 1961 and 1.10 in 1962.

Except during the first harvest period of the 3-cutting system
in 1962 when a ceiling LAI was reached on May 25, leaf area increased
after each cutting and a significant decline in leaf area was not
obtained under either the 3- or 4-cutting treatments in either year.
The decreased rate at which LAI changed during the latter portion of
each regrowth period indicated, however, that the ceiling LAI was being
approached. As in the case of production, leaf area did not increase
as rapidly and the maximum leaf areas indexes during each harvest de-
creased with each successive defoliation. In the 3-cutting treatment,
the LAI curves were very similar in 1961 and 1962 except for the last
two weeks in August when favorable precipitation in 1961 caused a
greater increase in the LAI than in 1962.

Seasonal average leaf area indexes for the two years decreased
from 2.60 to 2.12 to 1.62 as the number of defoliations per year in-
creased from two to three to four (Table 2). The reduction of the LAI
to zero after each defoliation and the relatively slow recovery during
the first two weeks of regrowth were responsible for these low aver-
ages. The average leaf areas of the l- and 2-cutting treatments were
not significantly different despite the fact that the LAI of the 2-
cutting system was reduced to zero after the first defoliation.

There appears to be a positive relationship between the average
LAI for a single cutting period and the amount of forage produced
during the same period. When two years are compared, the year with
the cutting period having the highest LAI likewise had the highest

yield.

28

TABLE 2.--Average leaf area indexes of alfalfa cut one, two, three,
or four times per season

 

Number of Date
cuttings of 1961 1962 Average
per season cutting
one 7‘ August 30 2.84 a1 2.34 b 2.59 a
two June 21 4.07 3.92 4.00
August 30 1.32 1.79, 1.55
Average 2.45. .27 m a
three May 31 3.25 4.07 3.66
July 12 1.91 2.14 2.03
August 30 1.24 1.17 1.20
Average I795. 273;. 2TI2'b
four May 17 2.56 3.66 3.11
June 21 1.29 1.97 1.63
August 22 1.01 1.81 1.41
August 30 .82 1.23 1.02
Average I723 d 2700 c I762 C

 

1
Areas in the same column with same letter are not different
at 5% level.

2Harvested July 26, 1962.

29

Leaf Distribution

 

Leaf distribution as expressed by the percentage of total leaf
area and the LAI in each 6-inch segment of the alfalfa stems is shown
in Tables 3 and 4 for the two years. Only total leaf area was measured
on the first two sampling dates in 1961 and after August 2 in the 1-
cutting treatment in both years. In both years abscission of lower
leaves started about three weeks before maximum LAI was reached. During
these early periods, however, the rate of formation of new leaves ex-
ceeded the rate of leaf loss which resulted in increased total leaf
areas from one week to the next. It is recognized that the changes in
leaf distribution were in part due to stem elongation. Visual observa-
tions, however, indicated that the greatest cause of the decreasing
leaf area in the lower portion of the stems was the abscission of lower
leaves.

In both years, the percent of the leaf area and the LAI in each
6-inch segment of the lower portion of the stems drOpped during May
until on May 31 only 19 and 9% of the total leaf area was located in
the lower 12 inches of the stems in 1961 and 1962, reapectively. From
May 31 until June 21 when the 2-cutting treatments were harvested for
the first time, there was a continual loss of leaves in this area until
only 2% in 1961 and 1% in 1962 of the total leaf area was in the lower
12 inches of the forage. Leaf area indexes in this lower area were
correspondingly low and near zero. Decreases in the LAI of the 6-inch
segments were less pronounced after June 21 due to the more uniform
abscission of leaves from the entire stem.

A LAI of from 1.00 to over 2.00 was present in at least one of

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34
the 6-inch segments of the l-cutting treatments from the beginning of
the growing season until July 5. In general, these larger leaf area
indexes were located between 18 and 30 inches from the base of the
stems and represented over 30% of the total leaf area.

Rapid decreases in the percent leaf area and the LAI in the
lower 6 inches of the stem were evident in the 2-cutting system from
June 21 to August 30. By August 2 only 5% of the total leaf area in
1961 and 1% in 1962 was located in the lower 6 inches of the forage.
The higher total leaf area indexes in 1962 resulted in a complete
absence of leaves in the lower 6 inches of forage which was heavily
shaded.

The relatively short regrowth periods in the 3- and 4-cutting
treatments and the relatively low leaf area indexes accounted for a
higher percent of the leaves in the lower portion of the stems. In
these two treatments, eSpecially in the 4-cutting system, cuttings
were made before most of the lower leaves became senescent. Further-
more, higher light intensities reaching the lower leaves as shown in

Figure 9 favored increased leaf retention as cutting frequency increased.

Light Interception

 

The average percent light interception at O, 6, and 12 inches
from ground level is shown in Figure 9 for the 1962 year. Since light
readings were taken only on clear days, data obtained during the period
three days prior to and three days after the weekly sampling dates were
averaged and recorded as the average percent light interception for
sampling date. Light readings 6 and 12 inches above ground level are

not presented for the l-cutting treatment after JUne 21 since lodging

35

One Cutting

 

100F- ...coooOooo’oo .

8O 44......6" ‘ .0" .
A

60L A‘

’ A

4o~ «2.12" A

20L

oJJJLlIJlLJVJLJfiJfiIJ

 

100
80
6O
4O

20

 

 

 

 

100
80
60

40

Percent Light Interception

20

 

   

 

 

Four Cuttings
100

80
60

40

   

20

 

 

 

 

 

o IJL 4!:4
5 19 2 16 30
June July August

Fig. 9.--Percentage light interception O, 6 and 12 inches from
ground level in alfalfa cut one, two, three and four times per season.
Weekly determinations, 1962.

36
and dense growth made passage of the photosensitive cell of the light
meter through the forage impossible without disturbing the natural
arrangement of the foliage.

The percentage of light interception at ground level in the 1—
cutting treatment was 95% on May 17 and remained high during the re-
mainder of the growing season. There were practically no leaves (12%
of total) in the 0-6 inch segment on May 17 and only 4 and 1% of the'
total leaves were in this lower area on May 24 and 31. Under the 2-,
3-, and 4-cutting systems, light interception at ground level was
negligible during the first week of regrowth but increased rapidly
during the second and third week of regrowth. Increases in the percent
of light interception after the first five weeks of regrowth were
generally slight as evidenced by the tendency of the percent light in-
terception to remain relatively stable during the latter portion of
each regrowth period in the 2- and 3-cutting treatments. Except for
the last regrowth period of the 3- and 4-cutting treatments, light
interception at ground level exceeded 50% during some part of each of
the regrowth period. Light interception never exceeded 90% after the
first cutting was removed in the 2-, 3- and 4-cutting treatments.
Decreases in the percent light interception at ground level in the
2-cutting treatments after August 2 were due to decreasing leaf areas
which allowed more light to penetrate.

Light interception 6 inches above ground level closely followed
the pattern of the ground level readings during the early part of the
season. In the 2- and 3-cutting treatments, only 3% of the ambient
light reached the 6-inch level from May 24 to the time the first cutting

treatment was imposed on May 31 and June 21, respectively. Apparent

37

decreases in the light interception with advancing age appear to have
been due primarily to decreasing leaf areas and secondarily to high
leaf densities in the area where light readings were made. This latter
situation resulted in the shading of the photosensitive cell which
caused an overestimation of the percent light interception in some cases.

During the early part of the season, light interception at the
12-inch level fluctuated greatly. At least a part of this fluctuation
was due to lodging during the middle of May. As the alfalfa regained
its normal upright habit of growth, light interception increased. After
the removal of the first hay crop, light interception at the lZ-inch

level never exceeded 60% in the 2-cutting treatments.
Net Assimilation Rates

Net assimilation rates are presented as a relative measure of
the leaf efficiency and are expressed as the grams of dry forage pro-
duced per square meter of leaf area per day-(Table 5). Data are pre-
sented only for those periods when yield and leaf area measurements
were available for two consecutive weeks. Since the leaf area remaining
after each defoliation was not measured (because of inaccuracies in
measuring small amounts of leaves) until the foliage had been allowed
to grow for one week, net assimilation rates are not presented for the
week following defoliation.

Net assimilation rates remained relatively high during the month
of May in the uncut plots but by mid-June there was a noticeable de-
crease in the NAR. This decrease continued until in early July when

the NAR of the l-cutting treatment dropped below zero. The loss of

lower leaves and small branches during this period accounted for a

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39
considerable decrease in the dry weight from week to week. These
losses combined with lower average leaf areas resulted in high negative
net assimilation rates. These negative rates were resPonsible for the
low average NAR of 0.38 in 1961 and -0.78 in 1962 in the l-cutting
treatments.

The NAR of the 2-cutting treatment showed some decline before
the first harvest on June 21 but it was high for approximately five
weeks after defoliation. During the month of August, net assimilation
rates dropped below zero. Despite the negative NAR in the latter part
of the season and an assumed rate of zero during the week immediately .
after cutting, the average NAR under the 2—cutting system was 1.98
in 1961 and 1.77 in 1962.

Net assimilation rates under the 3- and 4-cutting treatments re-
mained high during most of the growing season. There was a noticeable
decline in net assimilation rate during the last two weeks of the last
cutting of the 3-cutting treatment in both 1961 and 1962. The average
NAR of the 3-cutting system differed considerably in the two years
(3.87 and 2.93) and the rates were higher than in the 2-cutting treat-
ments in both years. Considerably higher average rates are reported
for the 4-cutting treatments (4.76 and 3.86) and again the differences
between the average rates in 1961 and 1962 were large.

Two apparent deviations in the 1962 results should be mentioned.
The very low NAR in the l-, 2-, and 3-cutting treatments reported
during the weeks of May 31 appear to have been due to very dry condi-
tions during which the increase in weight was slight. 0n the other
hand, the NAR of 7.33 in the l-cutting treatment reported during the

week of July 5 may have been due to increased growth rates caused by

40
very heavy rains in early July.

The very high net assimilation rates during the second week
after cutting in all treatments probably were the result of new growth
being produced at the expense of stored carbohydrates and also to high
photosynthetic rates of young leaves (Part II). Since only the above-
ground portion of the plants was harvested, depletion in root weight
was not measured. It is assumed that by the end of the third week
after defoliation when the alfalfa had reached a height of approxi-

mately 12 inches, production of forage based on leaf area was valid.

PART II

PHOTOSYNTHETIC STUDIES OF EXCISED LEAVES

IN A WARBURG RESPIROMETER

Materials and Methods

The net photosynthetic rates of leaves both from the field and
a controlled-environment chamber were determined. Attempts to compare
leaves located near the tOp and near the bottom of individual alfalfa
stems growing in the field proved extremely difficult. Diseases and
insects usually damaged the lower leaves before sufficient plant height
was attained. A limited number of healthy leaves from field plots was
tested on June 12, 1962. Most of the material used, however, was
grown in.a controlled-environment chamber.

During the winter of 1961-62, approximately 200 clonal alfalfa
cuttings were rooted. The parent plant was selected at random from a
field of Vernal alfalfa. Three or four rooted cuttings were placed
in each of thirty-six 7-inch pots filled with field soil obtained from
an old alfalfa field. The pots were divided into six groups of six
pots each and each group was cut every six or seven weeks during the
summer and fall of 1962. The plants were fertilized regularly with a
complete nutrient solution to maintain normal growth.

In December, 1962, five pots of the most productive plants were
selected from each group, fertilized immediately after cutting with

41

42
granular fertilizer at a rate of 22 pounds P and 123 pounds K per acre
and placed in the controlled-environment chamber. During the period in
the chamber, the plants in each group of five pots were cut at 6-week
intervals. Immediately after cutting, the plants were re—fertilized.
The pots were watered and rotated daily. Temperature of 50° F. at night
and 700 F. during the day were maintained during the growing period.
Light was supplied for the 14-hour photoperiod by a bank of both incan-
descent and fluorescent bulbs which delivered a light intensity of approx-
imately 3,000 foot candles at the surface of the pots. The pots were
grouped as closely as possible so that only 10% of the ambient light
reached the surface of the pots when maximum growth was attained.

Stems containing leaves to be sampled were cut at ground level
and placed in a vial of water in an insulated box for tranSportation to
the laboratory. Each stem was measured and the location of the leaflets
to be tested recorded. Leaflets were separated by making a cut as
close as possible to the junction of the leaflet and the petiolule or
petiole. The age of the leaves was determined by tagging the petiole
of the growing leaf as soon as the leaf was completely unfolded.

Net photosynthetic rates were determined in a modified Warburg
respirometer. A two-piece flask, approximately 50 milliliters in size,
was designed to accommodate large leaflets (Figure 10). Two small
plastic bars were attached with small bolts to the top of the flask.
The bars were fitted with U-shaped wires which were held in place by
small notches at the ends of the bars. A small square of acetate was
placed between the wires to support small leaflets and to hold the
larger leaflets parallel to the light source. Flasks were calibrated

with Brodie solution.

43

Fig. 10.--Modified Warburg flasks showing the method used to
attach the leaflets (upper photograph) and the assembled flask attached
to the manometers (lower photograph).

44

 

45

Light was supplied by a series of 300-watt incandescent bulbs
located below the plastic bottom of the water bath. A 0- to 75-ohm
slide-wire resistor was inserted into the electrical circuit at the
base of each light socket. These resistors made it possible to adjust
the current to each bulb so that the light intensity at the position of
the leaflet was 1,800 foot candles as measured with a Weston Model 765
illumination meter equipped with a quartz window. A variable voltage
transformer was inserted in the main circuit to permit reduction of the
light intensity of the bulbs on one-half of the apparatus.

To insure an adequate supply of carbon dioxide, a buffer solution
was used to provide a carbon dioxide concentration of approximately 15%
,(calculated) in the leaf environment. The buffer solution consisted of
4 grams of potassium bicarbonate (KHCO3) dissolved in 45 milliliters of
a saturated solution of sodium borate (Na2B4O7-10H20) and 5 milliliters
of distilled water. Three milliliters of the buffer were placed in
the bottom of each flask. After coating the ground glass surfaces with
lanolin to insure a perfect seal, the bottoms of the flasks were attached
to the tops with rubber bands.

After allowing the flasks to equilibrate in the water bath with
the lights on for a period of 15 minutes, manometric readings were re-
corded every 10 minutes for a period of one hour. At the end of each
run, the leaflets were printed on "Ozalid" paper, dried and weighed.
The net photosynthetic rates were determined on both an area and a dry
weight basis using standard methods of calculation (22).

All three leaflets of a trifoliate leaf were used whenever pos-
sible, but only the two lateral leaflets were used when the size of

the individual leaflets prohibited the use of the entire trifoliate

46
leaf. Sample size varied from 10 to 20 milligrams of dry weight which
was equivalent to 3 and 5 square centimeters of leaf area.

Net photosynthetic rates are reported as the net microliters of
oxygen evolved per hour per square centimeter of leaf area. Rates were
calculated initially on both an area and a weight basis. Some varia-
tions in the relative rates were evident when rates were calculated on
a weight basis. An increase in the area:weight ratio as the leaves
were shaded for longer periods appears to have been responsible for
some of these variations. Since leaf area rather than leaf weight had
been used as a basis for all other calculations, it appeared that net
assimilation rates based on leaf area would be more consistent.

Attempts to measure reapiration rates proved futile as changes
in manometer readings were slight during the one hour test period.
Respiration rates, which are normally considered to be low (12) were,
therefore, ignored and net photosynthetic rates (photosynthesis minus

respiration), are reported.

Results and Discussion of Net Photosynthetic Studies

Exploratory Investigations

 

Preliminary investigations were conducted to determine the pre-
cision of the Warburg respirometer as a means of determining net photo-
synthetic rates of alfalfa leaves. Since the two lateral leaflets of
the trifoliate leaf of alfalfa are similar in size, shape and age, it
was assumed that the net photosynthetic rates of these paired lateral
leaflets would be more nearly identical than of a lateral leaflet and
the terminal leaflet. A comparison of the net photosynthetic rates

of paired lateral leaflets showed that the rates did not differ

47

significantly at a temperature of 20° C. and a light intensity of 1800
foot candles. Measurements of the net photosynthetic rates of paired
lateral leaflets at light intensities of 1800, 1350, 900, and 600 foot
candles showed a reduction in the average rate of Spproximately 20%
when the light intensity was reduced to 1350 foot candles. Light in-
tensities of 900 and 600 foot candles caused reduction of 38% and
60%, respectively, as compared to 1800 foot candles. These investiga-
tions indicated that the leaves were not light saturated at 1350 foot
candles when incandescent lights were used. Lack of data at light
intensities higher than 1800 foot candles made it impossible to deter-
mine if light saturation occurred at 1800 foot candles. Since, however,
all reported data were obtained at 1800 foot candles, the net photo-
synthetic rates of different leaves are assumed to be comparable.

Initial studies in August 1962 using leaves collected from the
field indicated that the net photosynthetic rate of individual leaves
of the same age was inversely correlated with the distance of the leaf
from the crown. To test this apparent relationship, the net photo-
synthetic rates of a series of leaves grown in the controlled-environment
chamber were measured. Stems, ranging from 3 to 36 inches in length,
were selected from material which had been defoliated 2 to 6 weeks
previously. A statistical analysis of the net photosynthetic rates of
the youngest fully unfolded leaf (as near the same age as it was pos-
sible to determine), from each stem showed no correlation between the'
distance from the crown and the net photosynthetic rate. This was con-
tradictory to the results obtained from the field. The dry conditions
in the field in 1962 at the time of sampling may have been reaponsible

for these contradictory results. The short stems obtained from the

48

field in August 1962 had a smaller total leaf area and may not have
been subjected to as great a water stress as the leaves on the longer
stems with larger leaf areas.

Further analysis of these same data showed that leaf size per se
did not influence photosynthetic rates. The net photosynthetic rates
of the small, ovate leaflets on the 3-inch stems from the controlled-
environment chamber were not significantly different from the larger,
more elongated leaflets of the same age from the stems 36 inches long.
These studies of leaves grown under the closely-controlled environment,
then, showed that in alfalfa (1) net photosynthetic rates of leaves
are not a function of distance from the ground level and (2) photo—

synthetic activity is independent of leaf size.

Principal Investigations

 

In an attempt to determine the photosynthetic contribution of
leaves from different locations on the stems, the net photosynthetic
rates of leaves from different locations on a series of 17 stems were
determined. Stems ranging from 19 to 40 inches in length were selected
from material in the controlled-environment chamber. The stems were
selected at random from plants which had been allowed to grow under
conditions which shaded the lower leaves for a period of from 37 to
40 days in order to simulate an alfalfa stand under field conditions.

The net photosynthetic rates and the relative position of the
leaves on the stems are shown in Figure 11. In all stems, with the
exception of numbers 1, 2, 3, and 14, where the rates of the two upper-
most leaves were approximately equal, net photosynthetic rates decreased

as the distance from the apex of the stem increased. Stem number 13

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49

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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50
showed the greatest range of net photosynthetic rates with the upper
leaf (139) nine times as active photosynthetically as the lower leaf
(15). Similar results were found in stems number 6 and 14 where the
net photosynthetic rates of the upper leaves were six and seven times
greater than that of the lower leaves.

The results from Figure 11 were summarized by averaging the net
photosynthetic rates of the leaves from each 1/4 of the total length
of the stem (Table 6). The number of leaves from each section of the
stem varied due to errors in technique or abscission of the lower
leaves. It is very striking that the average net photosynthetic rates
decreased progressively from the top (125) to the bottom (29) of the
stems. This difference of the magnitude of 4 to 1 indicates that the
youngest leaves had the highest net photosynthetic rates. Although
the lowest leaves appeared healthy and turgid, their pale-green color
indicated that the chlorophyll content may have been lower than that
of the dark-green leaves in the upper part of the stems.

TABLE 6.--Net photosynthetic rates of leaves of alfalfa located
in different sections of the stem1

‘

 

fir

Section of stem,

 

v—v—fi. fi.‘

Average net

 

 

 

Source of % distance Number ?f photosynthetic
stms observations

from bottom rates

Controlled- 76-100 17 125 52
environment 51-75 15 99 b
chamber 26-50 7 71 c
0-25 4 29 d

Field, 51-100 5 137 a
June 12, 1962 0-50 5 113 b

l

19 to 40 inches long from controlled-environment chamber
21 to 24 inches long from field.

2Rates from the same source and in the same column with the
same letter are not different at 5% level.

51

Although the determination of the net photosynthetic rates of
upper and lower leaves grown under field conditions was hindered by
early abscission of the lower leaves in August, 1962, five stems 21 to
24 inches long were selected from a field of first cutting Vernal alfalfa
on June 12, 1963. Because of an abnormally late spring, leaf retention
was greater at lower levels of the alfalfa plants than at a similar date
in other years. The stems were selected on the basis of the presence
of healthy, disease-free leaves in the lower half of the stems. The
net photosynthetic rates and the location of the individual leaves in
the stems are shown in Figure 12. The younger leaves from the upper
portion of the 21- to 24-inch stems had a significantly higher net
photosynthetic rate (137) than the older leaves from the lower half of
the stem (113) as shown in Table 6. These data indicate that dramatic
reductions of the net photosynthetic rate of alfalfa leaves with in-
creasing age, as shown in the case of the alfalfa grown in the controlled-
environment chamber, also occur under field conditions.

To test further this apparent relationship between the age of
the leaf and the net photosynthetic rate, determinations were made
using leaves of precisely-known ages (Table 7) since the leaves were
tagged when unfolding. Although there was no significant difference
in the average net photosynthetic rates measured 5 and 12 days after
unfolding (119 and 121), significant and marked decreases in the net
photosynthetic rates occurred as the days since unfolding increased
from 12 to 28. The leaves which had been unfolded for 12 days had an
average net photosynthetic rate which was more than seven times greater
than that of the leaves which had been unfolded 28 days (121 to 17).

The sharp decrease in the average net photosynthetic rates between 22

52

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53

and 28 days (80 to 17) appears to have been due to a rapidly accelerated

aging of the leaves.

TABLE 7.--Net photosynthetic rates of leaves of
different ages of alfalfa grown in a controlled-
environment chamber

 

 

 

Days Number Net
since of photosynthetic
unfolding leaves rate
5 4 119 a1
12 7 121 a
17 6 112 b
22 6 80 c

28 2 17 d

 

1Rates in same column with the same letter
are not different at 5% level.

DISCUSSION OF RESULTS

The objectives of the studies reported herein were to determine
the effects of one, two, three and four cuttings on the production and
photosynthetic area and efficiency of alfalfa in an attempt to answer
specifically why alfalfa out three times per season (9) (17) (20) yields
more than when out twice for a period of three or four years. }The
effects of frequent defoliation on root reserves and the longevity of
alfalfa stand were not studied. Effects of cutting treatments were
intentionally measured for only one season at each of the two locations
to prevent the residual effect of previous treatments from influencing
yields and other data. Carbohydrate reserves were not measured since
it has been well established from previous studies (10) (11) that in-
creased frequency of cutting decreases carbohydrate reserves and causes
stand depletion even after the first harvest year. The stand may be
vigorous enough for three or four years for near maximum yields under
a high fertility program, however, even though it has been partially
depleted by frequent cutting.

Since alfalfa yield is a product of photosynthesis, it appeared
likely that the average size of the photosynthetic area would be one
of the most important factors which determined crop yield. Experimental
results presented herein have shown that the average photosynthetic sur-
face (LAI) of alfalfa during the growing season is not, in itself, a
suitable index of crop production. The highest Z-year average yields

54

55
of alfalfa were produced under a 3-cutting system which had an average
LAI of 2.12. Significantly lower yields were produced under both the
l- and 2-cutting systems in which the average leaf area indexes were
higher--2.59 and 2.60, reapectively. There did, however, appear to be
a positive relationship between the average leaf area values for shorter
periods of time-~e.g.,the five to seven week period between harvests
in the 3- and 4-cutting treatments-~and the tons of hay produced.
Higher average leaf area indexes during harvest periods were associated
with higher yields both within treatments and between years.

Leaf abscission was evident after the forage had reached a
height of about 12 inches. The rapid rate of leaf formation during the
early part of each regrowth period, however, resulted in an increase in
the LAI from one week to the next. Approximately one week before the
1/10 bloom stage was reached maximum leaf area indexes were attained
and from that time until the end of the growing period, the decreased
rate of leaf formation together with an increased rate of leaf abscission
resulted in a decrease in the LAI from one week to the next. In order
to get an estimation of leaf loss, determinations were made based on
the average weight:area ratio of the leaves in the 6- to 24-inch
segments. Loss of leaves resulted in losses of harvestable material
equivalent to 409 pounds of hay per acre per unit of leaf area lost.
Under the 2-cutting treatment in 1962, a decrease of 1.98 units of leaf
area during May and June resulted in a loss equivalent to 810 pounds
of hay. The decrease of 1.50 units in the same treatment during August
was equivalent to 614 pounds of hay. Thus, under the 2-cutting treat-
ment, 1424 pounds of high quality hay was lost during the growing season

due to reductions in LAI alone. Decreases in LAI in the 3-cutting

56
treatments during the 1962 growing season of 0.45 units amounted to a
loss of harvestable material equivalent to 184 pounds of hay--1240
pounds less than the 2-cutting treatments. Similar computations for
1961 showed a difference in leaf loss of 875 pounds, averaging 1057
pounds or 0.53 ton more of leaf loss per year under the 2- than under
the 3-cutting system. Much of the yield difference favoring the 3-
over the 2~cutting treatment is attributable to these decreases in
LAI due to leaf loss.

Newly-released insect- and disease-resistant varieties may serve
as one method of reducing the rate of loss of older leaves. Natural
leaf loss, however, due to senescence presents an entirely different
problem. Controlled-environment studies under diseaseafree conditions
in this study indicated that natural abscission of lower leaves occurred
five or six weeks after defoliation. Maturation and reduced light
intensities appeared to be responsible for at least a part of this
natural leaf loss.

Percent light interception at ground level, as in the case of
leaf area, was not a good measure of productivity. In the l-cutting
treatment, 95% visible light was intercepted by the forage during all
but the first two weeks of the growing season. The yield of the 1-
cutting treatment in 1962, however, was less than one-half as large as
the 4-cutting treatment in which more than 50% of the ambient light
was not intercepted during 2/3 of the growing season. The lack of dry
matter production under conditions of high light interception may have
been due to the very small photosynthetic area of the plants in the 1-
cutting treatments during the latter part of the growing season. Stems

which were brown and lignified and infected with black stem disease

57
(Ascochyta imperfecta) had little photosynthetic capacity but were re-
sponsible for much of the light interception.

The net production curves, leaf area indexes, percent light
interception at ground level and net assimilation rates for the 1962
season are summarized in Figures 13 and 14. Under field conditions,
peak light interception occurred early in the spring. Approximately
one week after 95% light interception at ground level was attained on
May 17, leaf area reached the ceiling level. Production per acre con-
tinued to increase for several weeks after the leaf area attained its
maximum even though considerable production was lost due to leaf loss.
Increases in production after the ceiling LAI was reached appear to
have been due to stem elongation, new leaf formation, lignin and
cellulose deposition, and seed formation. During the periods of
initial regrowth in the 2-, 3- and 4- cutting treatments, leaf area
indexes rose sharply but tended to stabilize or even decrease before
maximum production levels were reached. After the first defoliation
of the 2-, 3- and 4-cutting treatments, light interception never
reached 95% and, during most of the periods of regrowth, was below
50%.

Net assimilation rates were generally higher in the 3- than in
the 2- cutting treatment which partially explained the higher total
yield under the 3-cutting system. Marked decreases in the net assimila-
tion rates during the latter part of each of the cutting periods of
the 2~cutting system appear to have been the result of the loss of
harvestable material and decreased net photosynthetic rates of the
older leaves.

Net assimilation rates could not be related to production

58 4
One Cutting .110

 

 

Tons/Acre (T/A)

 

 

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Two Cuttings
10

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Fig. 13.--Percentage light interception at ground level (LI%),
leaf area indexes (LAI), tons of hay per acre (T/A) and net assimilation
rates (NAR) of alfalfa cut one or two times per season. 'Weekly deter-

minations, 1962.

59

Three Cuttings

 

 

 

 

 

 

 

    

 

 

 

 

 

 

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s4 1 ,
0
Four Cuttings
10
I l J J 15
LJ lfiL JJL LII Jog
- -5
3 LI
F. JV 100'l
:3 80‘
\
5.2-
m 604 6
3 ‘E/A
<
\
2
o 1 J (LAI 4
5' #4
25
0- 4LL i—L-l as #1 O
10 24 7 21 5 19 2 16 30

May June July August

Fig. l4.--Percentage light interception at ground level (LI%),
leaf area indexes (LAI), tons of hay per acre (T/A) and net assimilation
rates (NAR) of alfalfa cut three or four times per season. Weekly

determinations, 1962.

60
especially in the l- and 4-cutting treatments. Losses of harvestable
material and low average leaf areas during the latter portion of the
growing season resulted in a negative average NAR in the l-cutting
treatment. The very much higher net assimilation rates under the 4-
cutting treatment were probably due to a depletion of the carbohydrate
root reserves rather than higher photosynthetic rates alone, eSpecially
during the second week after each defoliation.

Some of the inaccuracies in determining NAR might have been
eliminated if root weights had been determined and total leaf production
recovered. The practicality, however, of recovering even a major por-
tion of the roots and dead leaves of a perennial crop such as alfalfa
seems questionable under field conditions.

The data throw considerable light on the question: "Why does
alfalfa yield more under a 3- than a 2-cutting system for a three or
four year period?". When alfalfa was defoliated every six or seven
weeks under a 3-cutting system LAI seldom reached the ceiling level,
leaf abscission occurred at a low rate, and net production was high.
Under the 3-cutting system leaf abscission seldom exceeded the rate of
formation of new leaves and the amount of harvestable material never
decreased from one week to the next. Since net production always con-
tinued to increase under the 3-cutting system, higher yields were
obtained than in the 2-cutting treatments in which rates were reduced
during the latter part of the first harvest period and gradually de-
creased during the latter part of the second harvest period.

The reduced net production which was obtained during the latter
part of each of the two harvest periods in the 2-cutting system was

due not only to excessive leaf loss in the lower 12 inches but probably

61
to a decrease in the photosynthetic efficiency of the aging lower
leaves. The data in Part II of this study show that the net photo-
synthetic rate of young leaves of alfalfa grown in a controlled-
environment chamber is similar to that of leaves two weeks old but
after the third week, the decline in the net photosynthetic activity
is marked, being only about one-seventh as great as that of leaves
five days old. Limited field data which indicated a decrease in photo-
synthetic efficiency with increasing age support the data from the
controlled-environment chamber. Has the leaf of alfalfa fulfilled its
function of food manufacture one month after emergence, then to become
a storage organ for a short period of a week or two before abscission?
If it is to be harvested after it has completed the major portion of
its function as a photosynthetic organ, the harvest schedule would have
to be one in which forage is not cut so often as to seriously reduce
carbohydrate reserves and reduce longevity of stand but frequently
enough, say every six or seven weeks as in the 3-cutting system, to
prevent leaf loss and maintain a high level of photosynthetic activity
in the leaves rather than infrequently as in the 2-cutting system.
This information supports the recommendations to farmers that a 3-
cutting system is sound as based on recent data (9) (17) (20) and
data in this study which show that under a 3-cutting system it is
possible to produce more hay of better quality than under a 2-cutting

system.

 

SUMMARY

Investigations were conducted to determine the causes of differ-
ential production of alfalfa under l-, 2-, 3-, and 4-cutting systems in
order to answer Specifically why alfalfa cut three times per season
yields more than when out twice for a period of three or four years.
Field experiments were conducted at two locations in successive years.
Net production, leaf area, leaf distribution and light interception
were measured weekly; leaf area indexes and net assimilation rates were
calculated. Net photosynthetic rates of excised alfalfa leaves from
both a controlled-environment chamber and the field were measured in
a Warburg respirometer.

These studies produced the following conclusions:

1. Higher yields of alfalfa were obtained from the 3-cutting than
from either the l- or 2-cutting systems in both years. The two-
year average yields were 2.90, 4.60, 5.37 and 4.74 tons per acre
when harvested one, two, three or four times, respectively.

2. Leaf losses were evident and light interception at ground level
was 50% four weeks before 1/10 bloom in the first cutting. LAI
was approximately 3.16 and the plants were 12 to 18 inches tall at
this time. Leaf area increased until an average ceiling LAI of.
5.16 was obtained one week before 1/10 bloom when the forage was
approximately 24 inches tall. The LAI of the l-cutting treatment
decreased to a low of 0.71 on August 30. Under the 2-, 3- and

62

 

63
4-cutting treatments the average LAI decreased with each succes-
sive cutting.

Decreases in LAI during the growing season were responsible for
average losses of 4.46, 2.81, 0.25, and 0.00 units of leaf area
under the l-, 2-, 3- and 4-cutting treatments, respectively.
Gradual decreases in harvestable material from mid-July until
August 30 resulted in much lower yields in the l-cutting treat-
ment than in any of the other treatments.

Net assimilation rates were higher in the 3- than in the 2-cutting
systems. Relative differences in average net assimilation rates
among the four cutting treatments indicated that net assimilation
rates were not closely related to production in an erect perennial
forage crop such as alfalfa where leaf losses and changes in root
weights can not be readily measured under field conditions.

There were practically no leaves present in areas of the stem where
less than 5% of the ambient light penetrated. Interception of 95%
of the light by the low amount of leaves and high amount of woody
stems during most of the growing season in the l-cutting treatment
indicated high light utilization but production was low under these
treatments. As the number of defoliations increased, average per-
cent light interception decreased.

Leaves from both the field and the controlled-environment chamber
showed a marked reduction in net photosynthetic rates with age.
Leaves more than three weeks old were less than one-seventh as
active photosynthetically as five—day old leaves from the controlled-

environment chamber.

Measurements of net photosynthetic rates with the Warburg

-1“

64
respirometer appear to be precise enough to support the use of
this method in determining comparative rates of photosynthetic
activity in individual leaves of alfalfa.
Alfalfa cut three times per season yielded 0.77 ton more for an
average of two years than when cut twice primarily due to a measured
net leaf loss of 0.53 ton more in the 2- than in the 3-cutting
treatment, accounting for two-thirds of the difference in total
yield. The remaining one-third of the difference in production
appears to have been due to increased rates of net photosynthesis
in the leaves of the 3-cutting treatment which were younger on the

average than the leaves in the 2-cutting treatment.

10.

ll.

LITERATURE CITED

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