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IX. Water Requirements and Utilization
JACKSON L. CARTER AND EDGAR E. HARTWIG
does not extend so deep and branch roots are more developed. Most of
the soybean root system is in the upper 2 feet of soil and in many cases
the upper foot (Borst and Thatcher, 1931; Williams, 1950).
AKD SOIL MANAGEMENT
The objective in irrigating is to supply sufficient water to keep the
plants growing normally. This is usually accomplished by keeping the
soil moisture within the root zones somewhere between the wilting point
and field capacity. If too much water is applied during the period of
early vegetative growth, lodging may occur, adversely affecting yields.
The pod-filling period is the most critical from the standpoint of yield
and frequently moisture from mid-August to mid-September is inadequate
for maximum seed production (Odell, 1959).
Irrigation is of advantage occasionally in the more humid areas of
the North Central and Southern States. At Stoneville, Mississippi, yield
increases from irrigation of sufficient magnitude to offer a profit over
the cost of irrigation have been obtained in only two of the past eight
seasons. In a dry season at McCredie, Missouri, on a claypan soil, Whitt
(1954) obtained a yield of 31 bushels per acre following one irrigation
of 4.7 inches on August 21, whereas, the nonirrigated plots yielded only
17 bushels. Irrigation increased seed size 50 per cent and raised oil
content of the seed from 20.3 per cent to 22.5 per cent. At Urbana,
Illinois, during the first week in August of 1959 when conditions were
dry and unfavorable for growth, 1.5 inches of water was applied to
soybeans, resulting in a yield of 45 bushels per acre compared to the
unirrigated plot which yielded 34 bushels (D. B. Peters, personal communication). In a dry season at Stoneville, Mississippi, one irrigation
during the period of fruit development gave as large yield increases as
several irrigations during the season.
At Conesville, Iowa, water applied during two dry seasons, 1953 and
1954, resulted in a 50 per cent yield increase for genetically early strains
but only 19 per cent increase for late strains (Schwab et uZ., 1958).
During the first season of this test, supplemental water was applied in
June, July, and August which stimulated vegetative growth. Lack of
moisture during the pod-filling period, especially on the later varieties,
reduced seed size on the irrigated plots when compared to the nonirrigated. Oil content of the seed from the irrigated plots was also lower,
showing the effect of the more unfavorable conditions during pod
filling in the plots that had been irrigated early but had exhausted their
moisture ahead of the pod-filling period.
Thus, it is evident that proper timing of supplemental water additions
is very important to soybean production.
THE MANAGEMENT OF SOYBEANS
Irrigation is essential for satisfactory soybean production in westcentral Nebraska, and proper timing of water applications have been
shown to be very important in securing high yields. During 1958 and
1959, on plots that were preirrigated to fill the profile to capacity, one
irrigation at early bloom stage gave a 2-bushel increase over the nonirrigated check, but one irrigation at late bloom stage gave the highest
yield, a 10-bushel per acre increase over the check (Somerhalder and
Schleusener, 1960). In addition to west-central Nebraska, soybeans are
grown on a commercial scale with supplementary irrigation on the high
plains of Texas.
In July and August, the average daily use of water by soybeans may
run as high as 0.3 inch under some conditions (Whitt and van Bavel,
1955). The amount of water that soils can hold for the use of plants
ranges from 0.5 inch per foot of depth in sand to 2.5 inches in clay. The
need of the plant for water under equal fertility levels in a given locality
is essentially the same regardless of the soil. Thus, soybeans grown on
a sandy soil will need irrigation or rainfall more frequently than those
produced on heavier soils.
A good crop of soybeans requires about 20 to 30 inches of water.
While most areas where soybeans are extensively grown have sufficient
rainfall to supply that amount, there are times when a drought tension
occurs, especially in late July, August, or early September-the critical
period for maximum soybean yields. If a farmer is equipped to apply
supplemental water to his cropping area, there may be times, especially
during the late bloom or pod-developing stages, when an irrigation will
substantially increase yield and seed quality. Irrigation, when moisture
supply is low, may increase soybean yields from a subnormal level to
a more nearly normal level, but irrigation has not resulted in abovenormal yields.
X. Growth-Regulating Chemicals
Much interest has been indicated in growth-regulating chemicals, both
from the standpoint of learning more about the behavior of the soybean
plant and of controlling or modifying growth in the field. None have
produced any beneficial effect on yield.
Application of either 2 g. or 8 g. of potassium gibberellate per bushel
of seed has reduced stands and yields of soybeans at eight locations from
Winnipeg, Manitoba, to Coahoma, Mississippi. Although treated plants
were taller in the seedling stage, at maturity the controls were more than
5 inches taller than treated plants at most locations. Reductions in stand
were attributed to hypocotyl breakage and other injury during germination and emergence (Howell et d.,1960). One of the objectives, that
JACKSOS L. CARTTER ASm EDGAR E. HARTWIG
of decreasing combine loss through increasing height of the first pods
from the ground, was not achieved, owing to plant damage, severe
lodging, and increased shattering.
As chemicals to increase pod set during blooming, van Schaik and
Probst (1959) tried six growth regulators, applied weekly during the
entire blooming period, but in no case did any of the treatments increase
A. WHENTO HARVEST
1 . Moisture Content of Seed
As plants reach maturity, the leaves yellow and drop and there
is rapid loss of moisture from the seed. Seed can be threshed at 15 per
cent moisture, but at t h s level it is too high in moisture for storage without drying. Threshing is most efficient when the moisture content of the
seed is below 14 per cent.
2. Chemical DefoEiation
The widespread use of defoliants and desiccants on cotton has probably contributed to the interest of their usage on soybeans both for
hastening maturity of soybeans and for killing weeds. Defoliation studies
conducted at Stoneville, hlississippi, showed that soybeans hand defoliated 3 weeks prior to normal maturity were reduced 30 per cent in
seed yield, but were ready for combining only 3 days earlier than those
maturing normally. These studies showed that any defoliation prior to
yellowing of the leaves reduced yield. Studies at Urbana, Illinois (unpublished data ) , showed that any treatment applied sufficiently early
to appreciably hasten the drying of the seed to the point where the
crop could be combined ahead of normal maturity resulted in serious
reductions in seed yield. When treatment was delayed until 50 per cent
of the leaves had dropped, no serious yield reductions occurred but
maturity was not hastened.
Three-year studies in Arkansas (P. E. Smith, 1956) showed an
advantage in using earlier-maturing varieties over attempting to hasten
maturity of later-maturing varieties.
Under conditions where weed-infested fields of soybeans matured
before frost, desiccants such as pentachlorophenol mixed in diesel fuel
have been used to kill weeds. Carlyle (1951) reports satisfactory results
in killing weeds in soybean plantings in Illinois with rates of 2, 4, and
6 gallons of pentachlorophenol in oil. In the delta area of Mississippi,
these materials have been effective in killing morning glories, but although effective in killing the leaves of large pigweeds and cockleburs,
THE MANAGEMENT OF SOYBEANS
they have not hastened the drying of the large stems. Pentachlorophenol
does not presently have label clearance for use on soybeans.
3. Losses from Respiration after Maturity
Respiration of ripening soybean seed is closely correlated with moisture content of the seed. Temperature also affects the respiration rate
but is of less importance than moisture. When soybean seed had 55 per
cent moisture, the respiration at three temperatures, 70°, 84", and 90"F.,
consumed enough hexose to cause weight losses of 0.03, 0.04, and 0.05
per cent per hour ( Howell et al., 1959). Thus, when periods of wet, humid
weather occur prior to harvest, weight losses can be considerable.
There was little evidence of leaching of sugars from seeds in pods.
The earliest harvester designed specifically for soybeans was a twowheeled, horse-drawn machine which straddled the bean row (Piper
and Morse, 1923). This special harvester was common in Virginia and
North Carolina, but was never commonly used in the North Central
States. Harvesting losses ranged from 20 per cent under favorable conditions to as high as 60 per cent under unfavorable conditions (Sjogren,
1939). In small-grain growing areas, the binder and thresher were
adapted for soybean harvest. Harvest losses from using the binder or
mower for cutting and then threshing ranged from 16 to 35 per cent of
the total yield, with an average loss of 24 per cent ( Sjogren, 1939).
The combine harvester was first used for soybeans in the mid-twenties. The combine harvester has been a major factor in the expansion
of soybean production. This machine required less labor than earlier
methods and was more efficient. Sjogren reports average harvest losses
of 12.36 per cent in Virginia, 8.34 per cent in Indiana, and 8.99 per cent
in Illinois. Observations of 62 combines operating in the coastal plain
of South Carolina gave an average total harvest loss of 9.7 per cent.
Harvest loss was associated with total yield. When only fields producing over 20 bushels per acre were considered, the average loss was
5.9 per cent. Several fields averaging 40 to 50 bushels per acre had a
harvest loss of less than 4 per cent (Park and Webb, 1959).
2. Combine Haruesting
a. Losses from machine adjustments. Harvest losses are principally
either at the cutter bar or from failure to thresh or clean the seed
properly. Losses at the cutter bar can result if ( 1 ) plants are cut too
JACKSON L. CAR'ITER AhX EDGAR E. HARTWIG
high and pods left on stalk; ( 2 ) plants are cut too high and beans
shattered from cutting through pods; or ( 3 ) beans are beaten out by
improper reel adjustment. Cutter bar losses may run as high as 20 per
cent of the total yield in Iowa (Barger and Weber, 1949). Ridges caused
by cultivation are a serious problem in cutter bar adjustment (Heitshu,
1956). South Carolina studies showed that an average of 2.2 per cent
of the seed were left undisturbed below the cutter bar and 3.0 per cent
were shattered to the ground by action of the cutter bar and reel (Park
and Webb, 1959). Under extremely dry conditions, the reel is removed
to avoid losses from its beating action on the plants.
Cylinder losses are of two types-unthreshed pods or split beans.
Unthreshed pods are usually the result of attempting to harvest too
early in the morning after a heavy dew, too soon after a rain, or during
damp weather. This loss is reported to be usually less than 1 per cent
in Iowa (Barger and Weber, 1949), but was found to be as high as 16.5
per cent in South Carolina (Park and Webb, 1959).
Split or cracked beans are the result of ( 1) too high a cylinder
speed; ( 2 ) i n s a c i e n t clearance between cylinder and concave bars;
and ( 3 ) too many bars in cylinder or concave. Excessive cylinder speed
is damaging to seed viability (Moore, 1957).
Separating losses result from beans being carried over the straw
rack. This results from improper adjustment of the straw rack or from
overloading the straw rack. Losses are frequently higher when the rack
is overloaded with grass and weeds, especially so when these weeds
b. Losses from lodging. Soybean plants erect or nearly erect are
considered best suited for efficient harvesting. Studies conducted in
lowa comparing a variety which was nearly erect with a variety leaning
considerably showed an increased harvest loss from lodging of 0.9 per
cent in a 3-year average. There was a 2.3 per cent loss for each inch
of cutter bar height above 3.5 inches up to 6.5 inches (C. R. Weber,
personal communication). In a field survey, Park and Webb (1959)
attributed a 1.1 per cent loss to lodging. They suggest that pick-up
guards and tined reels were of considerable value in lodged fields.
XU. Seed Storage
Soybeans are harvested over a short period of time and much of
the crop is marketed directly from the field. Soybeans held on the farm
should be stored in clean, dry bins, and at a moisture content not to
exceed 13 per cent. At this moisture level, soybeans will keep for a year
or more without deterioration and with substantially no insect damage.
THE MANAGEMENT OF SOYBEANS
If the moisture content is around 14 to 15 per cent, soybeans will keep
through the winter with little loss in quality, but serious deterioration
begins when the weather warms up. If the moisture content is below
12 per cent, germination will stay good into the second year (Holman
and Carter, 1952).
During cold weather, because of air circulation moisture tends to
accumulate in the surface layer of the beans near the center of the bin.
This condition should be watched and the beans in the surface layer
stirred or the bin ventilated to minimize losses from this cause. Forced
ventilation also tends to equalize temperatures in the bin, reducing
moisture movement. Adequate ventilation can be provided with relatively small volumes of air (Holman, 1960).
Forced-air drying of soybeans allows one to combine earlier than
could otherwise be done. Drying soybeans with heated air has the
advantage that it can be done at any time, regardless of weather conditions. Maximum air temperature for drying soybeans should not
exceed 110°F. for seed and 130" to 140°F. for market beans (Holman,
1951), and should be held lower than this during the initial stages of
drying if moisture content is high (Brandenburg et al., 1961).
The rapid expansion in soybean acreage in the United States has
pointed out new problems for which research has not fully provided
answers. To produce maximum yields, soybeans must have an adequate
supply of water and nutrients throughout the season and must be free
from injury by diseases, insects, and nematodes.
The possibility of raising the nitrogen supply by the development
of a specific rhizobium strain to inoculate each soybean variety should
be further explored. Studies of nitrogen metabolism, especially during
seed development, and the effect of soil structure and humus content
on root growth and nodulation may tell us how to provide an additional
yield increase. Further studies appear to be needed on the balance
between nutrients such as phosphorus and potassium and many other
elements, especially at high yield levels where all factors of the environment must be at their optimum.
Design of better planting equipment, including improved press
wheels would insure more even soybean stands with a lower seeding
rate. More specific weed control chemicals would reduce cost of production and loss from weed competition. As chemicals for weed control
become more widely used on all crops, greater attention must be given
to the effect of residual chemicals on the crop following in the rotation.
JACKSON L. CAR’ITER AND EDGAR E. HARTWIG
Improved combines and better harvesting would materially reduce
harvesting loss and raise the quality of the crop that is marketed.
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THE MANAGEMENT OF SOYBEANS
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THE MANAGEMENT OF SOYBEANS
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