Soybean Seed & Genetic Management

By Dr. Harold Willis

The soybean is a truly amazing and versatile crop plant. It is one of the oldest food plants, domesticated by 1100 BC in northeastern China. Its ancestor is a wild vine-like plant which produces tiny, hard seeds that are useless for food unless properly prepared.


In order to best manage soybean production, one needs an understanding of how the plant grows and develops.

Soybean seed development

Germination

After being planted in the soil, the seed absorbs moisture, changing from less than 13% moisture to about 50% in several hours. After one or two days the first root (called the radicle) emerges through the seed coat and begins growing downward to establish the root system.

soybean seed management
Soybean seeds after harvest, with the pods.

The upper part of the young plant (the hypocotyl) begins to lengthen, pulling the remainder of the seed upward. About five to fifteen days after planting, the new plant arches through the soil, and the oval seed leaves (cotyledons) open up. The cotyledons provide the seedling with food (that was stored in them) for about a week, plus they soon turn green and begin making a little additional food by photosynthesis. Later they drop off.

Seed germination and emergence is a critical period in the life of a soybean because poor emergence due to a soil crust, cold temperatures or seedling pests or diseases can drastically cut yield.

Pod Development

One or two weeks after the first flowers, the first seed pods appear, with most pods being set within the next three weeks. Inside the pod, three (or sometimes four) tiny seeds begin to grow and develop.

For the next 30 to 40 days, the seeds rapidly fill with food produced in the leaves. The seed-filling period is the most critical in the life of the soybean plant with regard to yield. If weather conditions are adverse, such as drought stress or leaf loss from hail, yields will be cut severely. At this time, the plant takes 30-40% of its total mineral needs from the soil, so soil fertility should be at a peak.

After most seeds have filled, the growth activities of the plant slow down rather suddenly (called senescence). The leaves slow down their photosynthesis and begin to turn yellow, eventually dropping off. Root nodules cease producing nitrogen.

Maturity

The newly formed seeds contain about 90% moisture. As the seeds fill with food, moisture content decreases to about 60-65%. When seeds are mature (filled), the moisture content is 45-55% and the pods and stems of the plant are yellow or brown. The mature seed itself will also be completely yellow when mature (if it is a yellow-seeded variety).

In warm, dry weather, seed moisture will continue to drop to about 13-14%, when the crop can be harvested. In some varieties especially, the dying plants tend to lodge, making harvesting difficult, and in some varieties, pods tend to split open (shatter), dropping the seed and reducing harvestable yield.

As soybean seeds lose moisture they change from large, kidney bean shaped to smaller and nearly round. When dry, the seed contains about 40% protein, 21% oil, 34% carbohydrates and 5% ash.

Soybean Varieties

There is an amazing number of soybean varieties. Just about every valley in China, Japan and Korea grows its own variety, adapted to local conditions. A collection of over 10,000 strains of soybean seeds is maintained by the USDA. A glance of an assortment of these seeds reveals seeds of every color and description — some red, some green, some black, some brown, some speckled or streaked, some large and some tiny.

The great majority of soybean varieties grown commercially today is for animal feed and oil production (for food processing and industrial uses). Most are yellow-seeded field varieties. Other varieties can be obtained for special uses: forage and hay (with an abundance of stems and leaves; small-seeded black and brown late varieties) and human food (large-seeded, various-colored varieties).

Hybrids

Commercial hybrid soybean seed is very difficult to produce. This is because of the way the soybean reproduces: it is self-pollinating. Hybrids are made by soybean seed breeders, but it is a laborious, expensive process. From various ancestral and hybrid varieties, the commercial varieties are developed, both by agricultural experiment stations and private seed companies.

Seed quality

Varieties are developed to produce high yields of good quality seed, to mature properly for the geographic area, to be resistant to lodging and shattering, to be cold and drought tolerant, and to resist diseases and pests.

Factors of seed quality may include low numbers of defective or shriveled seeds, high germination rate, high oil and/or protein content and human food value.
Soybean seeds sold by reliable seed dealers should come with certain important information: the variety, the Maturity Group number, percent
inert matter, percent weed seed, percent other crop seed, germination rate and resistance to diseases and/or pests. The U.S. Plant Variety Protection Act of 1970 and the earlier Federal Seed Act, as well as state seed laws, provide standards and protection to dealers, but some private growers may not adhere to these standards. Anyone can save some seed to grow the next year, but this is no assurance of quality.

Selecting a Variety

In selecting which variety you wish to plant, assuming you are growing field soybeans, you need to consider several things. First, buy the best quality seed you can find. Certified tested seed is usually worth the cost. You can test for germination rate by counting out 25 whole seeds and roll them up in a damp cloth. Keep in a warm (70 to 80 degrees F.) place. Sprinkle with water if necessary to keep the cloth moist. After five or six days, unroll the cloth and count the seeds that have germinated out of 25. Multiply by 4 and divide by 100 to get the percentage germination.

Be sure to get seed of a Maturity Group adapted to your area. You may want to vary slightly the maturity group depending on soil type (an early variety for cool, wet, fine-textured soils and a later variety on coarse, well-drained soils). Avoid early varieties in fields where tall broadleaf weeds may get out of hand. If you want to follow the soybeans with fall-seeded small grains, use an early-maturing soybean.

One way to allow for uncertain weather conditions is to plant more than one maturity, either in different fields or as a seed blend, a mixture of varieties. That way at least one variety should give a reasonably good yield. If you save your own seed to replant, you will not get the same proportion as what was in the blend.

Select a variety that is shatter and lodging resistant, especially if you intend to plant high populations, since the plants will grow taller, more slender stems.

Disease and insect resistance may be important if these have been a problem in your area; however, by improving your soil’s fertility and structure, most such problems should disappear.

Indeterminate varieties should be used in the North, and determinate varieties do not do well in soils that crust. For wide rows, bushy varieties are best, to fill in the space quickly.

If you use a grain drill for planting, avoid seed lots with many large seeds, which do not flow well through the drill. Use seed lots with 2,400 seeds per pound or less. Small-seeded varieties have some advantages: the seedlings emerge better through crusted soil, fewer pounds of seed are needed to establish a certain plant population, and it is often easier to produce high quality grain (because smaller seeds suffer less damage during harvesting and handling).

You can often get valuable advice on selecting varieties from your agricultural research and extension personnel or from seed dealers. They may have performance test results which can be a rough guide of what to expect from a variety.

Saving Seeds

If you want to replant your own seed, save out the needed amount. Seed should be dried to about 13% moisture and kept in ventilated containers (cloth bags, cardboard boxes, or glass jars with cheesecloth covers). It should be stored in a dry, ventilated area at cool temperatures (not higher than 70 degrees F. or lower than 32 degrees F.). Keep away from mice and rabbits.

Seed should maintain a good germination rate for the first year (80 to 85%), but after the second year of storage, germination may drop to 65%. Test the germination rate before planting.

Source: How to Grow Super Soybeans