15.3 Seed Quality, Germination Testing & Seed Treatment Chemistry

Ensuring high seed quality and protecting the vulnerable germinating seedling are paramount for establishing a healthy crop. Environmental stress, soil-borne pathogens, and insect pests pose severe threats during the critical period between planting and emergence. Certified Crop Advisers must understand how to interpret seed testing data and how to deploy chemical seed treatments effectively.

Germination and Vigor Testing

Seed quality is assessed through various standardized laboratory tests, each providing different insights into how the seed will perform in the field.

Warm Germination Tests

The standard warm germination test is required by law for agricultural seed sold in the United States and is the number printed on the seed tag. This test provides the percentage of seeds that will successfully germinate and produce normal seedlings under optimal, non-stressful conditions (typically warm and perfectly moist).

While the warm germination test is useful for determining baseline viability, it often overestimates field emergence because field conditions are rarely "optimal." It may not accurately predict how the seed will perform if planted into cold, wet, or crusted soils.

Cold Germination Tests

To better simulate the stressful early spring planting conditions commonly encountered in the Corn Belt, cold germination tests are utilized. These are tests of seed vigor rather than just viability. In a standard cold test, seeds are placed in soil (often field soil containing natural populations of pathogens) and exposed to cold, wet conditions (typically around 50°F / 10°C) for a period of roughly 7 days. After this stress period, they are moved to warmer temperatures to complete germination.

The resulting germination percentage from a cold test is usually lower than the warm test, but it is a far more accurate predictor of how the seed lot will perform when planted in cold, wet soils. Seed lots with high warm germination but poor cold germination have low vigor and should not be planted first in the spring; they should be saved for later planting when soils have warmed.

Tetrazolium (TZ) Test

The Tetrazolium (TZ) test is a rapid biochemical test used to assess seed viability, often providing results in just 24 to 48 hours compared to the 7 to 14 days required for standard grow-out tests. In a TZ test, seeds are longitudinally bisected and soaked in a colorless solution of 2,3,5-triphenyl tetrazolium chloride.

Living, respiring tissue in the seed embryo contains dehydrogenase enzymes that reduce the colorless TZ solution into a highly visible, stable red compound called formazan. Dead tissue does not respire and therefore remains unstained (white or pale). Seed analysts examine the pattern and intensity of the red staining across the embryonic axis and cotyledons to determine not only if the seed is alive, but also to estimate its overall vigor and detect mechanical damage (like freeze damage during seed maturation).

Seed Treatment Chemistry

Modern seed is a significant financial investment. To protect this investment, almost all commercial corn, cotton, and increasingly soybean seed is coated with a complex slurry of chemical seed treatments. These treatments provide a critical first line of defense, protecting the seed and seedling during the highly vulnerable period between germination and the establishment of a robust root system.

Fungicides

Fungicide seed treatments are designed to protect against a complex of soil-borne and seed-borne pathogenic fungi. These pathogens thrive in the cold, wet, poorly drained soils typical of early planting and are responsible for seed rot, pre-emergence damping-off, and post-emergence seedling blight.

Because different pathogens belong to completely different biological kingdoms, multiple fungicide active ingredients with different modes of action must be combined on the seed:

• Oomycete protection: Pathogens like Pythium and Phytophthora are water molds (oomycetes). Active ingredients like mefenoxam or metalaxyl (Group 4) and ethaboxam (Group 22) are highly effective against these specific organisms but have no effect on true fungi.
• True Fungi protection: Pathogens like Rhizoctonia, Fusarium, and Penicillium are true fungi. To control these, active ingredients like fludioxonil (Group 12), prothioconazole (Group 3), and various strobilurins (Group 11) are utilized in the seed treatment package.

Insecticides (Neonicotinoids)

Insecticide seed treatments provide protection against both soil-dwelling insects that feed on the seed and roots, and early-season foliage-feeding insects. The most widely used class of chemistry for this purpose is the neonicotinoids.

Common neonicotinoid active ingredients include imidacloprid, thiamethoxam, and clothianidin. These compounds act on the insect's central nervous system as nicotinic acetylcholine receptor agonists.

A key feature of neonicotinoids is that they are highly systemic. When the seed germinates, the chemical is taken up by the developing roots and translocated throughout the growing plant tissue. This provides systemic protection against soil pests like wireworms, seedcorn maggots, and white grubs, as well as providing 3 to 4 weeks of protection against above-ground, piercing-sucking insects like aphids, leafhoppers, and bean leaf beetles.

While highly effective, neonicotinoid seed treatments have faced intense environmental scrutiny due to concerns about their persistence, their presence in fugitive dust exhausted from planters, and their potential off-target impact on pollinators, particularly honey bees and native bee species.