While much of the soybean seed sold in Michigan will have warm germination rates listed at 90 percent, some seed lots will be tagged at 85 percent and some may even be tagged at 80 percent. The warm germination percentage listed on the seed tag is guaranteed and it is always conservative.
The extremely dry conditions occurring during the 2017 season and the record-setting heat in mid-to-late September contributed to the lower warm germination test scores. These conditions caused the seed to be too dry, increasing the amount of mechanical damage that occurred to the seed coat and the embryo during harvest and handling operations. Mechanical damage oftentimes is not visible.
The wet weather that occurred throughout much of October was also a factor as the repeated wetting and drying cycles and delayed harvest caused the seed coats to wrinkle. Wrinkled seed coats may be cracked or completely removed during seed treatment operations, significantly reducing germination rates. Seeds having wrinkled seed coats have also been shown to imbibe water quicker than seed having smooth seed coats, increasing the potential for imbibitional chilling injury.
If you are concerned about the moisture content of your seed, ask your seed supplier if they can provide this information for each of your seed lots or test them yourself. Soybean seed that is below 10 percent moisture is extremely fragile and is at risk of incurring additional mechanical damage during seed treatment and transport operations.
While it is always important to handle soybean seed carefully, this becomes even more important when handling dry or lower quality seed. Operate all equipment used to transport bulk seed slowly and keep augers as full as possible. Reduce the height that seed falls from augers and conveyors and never drop bagged seed from any height.
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Michigan State University Extension suggests the warm germination test as an excellent indicator of how the seed will germinate under ideal conditions. However, it does not predict how the seed will germinate and emerge in cool or stressful soil conditions.
Some type of vigor test such as the cold germination or the accelerated aging test is required to determine this. The vigor test results should not be used to estimate germination percentages of a given seed lot, but they can be used to compare and rank the ability of different seed lots to germinate and emerge under stressful soil conditions.
Vigor test results are not typically provided to producers with the seed. The main reason for this is there is not an industry-wide standard for testing protocols and reporting results.
Consider submitting samples from all seed lots having warm germination scores of 85 percent or less for vigor testing. This is important as seedling vigor deteriorates faster than warm germination. Samples can be submitted to the Michigan Crop Improvement Association in Okemos, Michigan. The cold germination test takes 14 days and will cost $14 per sample. The accelerated aging test takes at least 10 days and costs $15 per sample. Seeds having wrinkled seed coats have been shown to reduce vigor.
Use the vigor test scores to rank the seed lots and determine their planting order. Always plant your highest quality seed first and your poorest quality seed last. Avoid planting lower quality seed into cold, wet soils, poor seed beds or high residue conditions that may reduce seed-to-soil contact. Fungicide seed treatments will help protect the seed from soil-borne pathogens and preserve the existing quality of the seed. However, they cannot improve the quality of the seed.
The warm germination score listed on the tag should always be used to adjust planting rates. A simple method for accomplishing this is to divide your desired harvest population by the warm germination score and then divide again by the warm germination score to estimate actual emerged seedlings.
For example, if seed having a warm germination score of 85 percent will be planted and a harvest population of 100,000 plants per acre is desired, then 138,408 seeds per acre should be planted (100,000 ÷ 0.85 ÷ 0.85 = 138,408).
An excellent soybean Seeding Rate Calculator developed at the University of Illinois will help producers fine-tune their seeding rates to achieve their desired final plant stands.
This article was produced by the SMaRT project (Soybean Management and Research Technology). The SMaRT project was developed to help Michigan producers increase soybean yields and farm profitability. SMaRT is a partnership between MSU Extension and the Michigan Soybean Checkoff program.