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IX. Water Requirements and Utilization

IX. Water Requirements and Utilization

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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).



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.



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



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

pod set.

XI. Harvesting


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,



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.



1. Historical

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



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

are green.

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.



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).

XIII. Discussion

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.



Improved combines and better harvesting would materially reduce

harvesting loss and raise the quality of the crop that is marketed.


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