It is fitting that soybean seeds are light yellow in color. Seeds of high-performing varieties are as precious as gold. Farmers may have trouble obtaining the seed of the varieties they want if something goes wrong this spring and they need to replant.
Seed companies sell high quality seed. But, this year they may need to lower their standards, slightly, to meet demand. Farmers and their advisers should remember that all seed sold in Missouri must possess a tag that contains important information about the quality of the seed. However, the methods used to calculate germination percentage are not very helpful for determining what might happen to the seeds under field conditions.
Planting high quality seed does not guarantee obtaining a good stand. Bad things happen to good seeds. Seed germination begins with water absorption. Water absorption changes the seed from a nearly dormant organism into a living, functioning seedling. As seed tissues imbibe water, enzymes necessary for growth are activated, stored reserves break down, and cell division and expansion occur. During germination, damage to seeds and seedlings may result from rapid water imbibition, chilling injury, low oxygen availability, and attacks from pests.
Soybean seeds are often planted into soils with less than optimal characteristics for rapid germination and emergence. The seed environment in early spring is often characterized by temperatures below 50 degrees Fahrenheit, soil moisture contents so high that oxygen is excluded, and the presence of opportunistic pests.
Although water absorption is required for germination, under some circumstances water can be absorbed too quickly. As tissues and cell components absorb water they greatly increase in size. If too much water is absorbed too quickly, cell walls can rupture causing cell death. Soybean seed coats moderate water absorption. They rapidly absorb water and then slowly release water to the other seed tissues. Seed coats that contain cracks cannot function properly, and injury from rapid water absorption is more likely. Seeds not handled carefully during harvest, storage, and transport are likely to contain cracks that might be invisible to the human eye, but can cause problems during germination.
Low oxygen availability is common in wet soils. Water in soil pores excludes oxygen needed for seedling growth. Initially, water absorption by seeds is not dependent on oxygen. In fact, both dead and live seeds absorb water. But, once water content of soybean seeds exceeds 50 percent continued water absorption depends on energy released by seed respiration. Oxygen demand increases rapidly and that oxygen must come from air within soil pores. Seeds with low vigor are less likely to withstand short exposures to low oxygen availability.
As seeds germinate, cell contents, rich in sugars, leak into the surrounding soil. Pathogens use this leakage as an energy source, multiply, and invade the seed. Cell content leakage is much greater from poor quality seeds or from seeds damaged by less than gentle handling. Germinating seeds and young seedlings are also attractive food sources for soil borne insects.
Protecting seeds and seedlings with seed-applied fungicides and insecticides might be appropriate this year. Although soybean seed treatment tests do not often result in yield increases, emergence percentages are almost always increased. Because supplies of many elite soybean varieties are extremely tight this year, the opportunity to replant poor stands with high yielding varieties is limited. Selecting high quality seeds and protecting the seeds with seed treatments will help ensure a good stand.
REVISED: August 3, 2012