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Germination is the most widely used indicator of seed quality. Seeds with high germination rates are generally considered more desirable for planting.
The germination rate alone can be misleading because the normal germination test is carried out under "ideal" conditions. Seeds have every chance to germinate and develop, regardless of their relative vigor.
Low vigor seeds may germinate and grow well in the laboratory, but their relative stress tolerance can be low. They can fail to emerge and establish a stand when planted under field conditions, especially when seedbed stress is present. Seedbed stress includes cold soils, wet soils, herbicide carryover, crusting, dry soils, deep planting, etc.
Producers can manage particular lots of seeds differently at planting if they know the relative stress tolerance of each lot. This information may or may not be furnished by the seed supplier. The "accelerated aging" test to be a fairly reliable indicator of soybean seed stress tolerance.
All seeds deteriorate during storage, even if the storage period is only a month or two. This deterioration commonly results in a loss of germination percentage. For example, the germination rates of typical stored soybean seeds can be 5% to 15% lower in the spring, following a winter of storage. The germination percentage of a seed lot with a large number of poor quality, low vigor seeds can drop by even more during the same storage period.
The accelerated aging test was originally developed by Mississippi State University as a way of estimating how rapidly soybeans would lose germination during storage. The test was intended to help seed producers compare seed lots and decide which lots would deteriorate too rapidly in storage. The producers could then avoid the processing and storage costs of the undesirable seed lots.
The accelerated aging test has been developed for use in a number of different crops other than soybeans. This includes cotton, corn, sorghum, and wheat.
The accelerated aging test is simply another type of germination test. The difference is that seeds are germinated after being exposed to a severe stress.
Soybeans analyzed with the standard or "warm" germination test are first placed on moist germination towels. The seeds spend eight days in the germination chamber with alternating temperatures of 68° and 86°F. These conditions help break
seed dormancy by providing an environment as close to perfect as possible.
Accelerated aging is analyzed by placing soybeans seeds in a stress chamber at 106°F and 100% humidity for 64 hours. Seeds are then processed using the warm germination procedures.
The accelerated aging procedures for other crops is specified in the Association of Official Seed Analysts rules for seed testing.
| Table 1. General Seed Quality as Related to Difference Between Standard Germination and Accelerated Aging Percentages. |
|
| Difference | Seed Quality |
| Less than 15% | Good |
| 15 to 30% | Moderate |
| More than 30% | Weak |
High vigor seeds can tolerate conditions in the stress chamber and maintain relatively high final germination rates compared to unstressed seeds. Low vigor, less stress tolerant seeds have final germination rates much lower than unstressed seeds.
Neither the normal germination nor the accelerated aging test can predict the exact emergence percentage of a particular lot of soybean seed, but they can give an estimate of the relative stress tolerance.
The warm germination results can be thought of as the percentage of total live seeds, regardless of high or low stress tolerance. Accelerated aging can be thought of as the percentage of highly stress tolerant (high vigor) seeds.
Under ideal conditions, seeds of both high and low stress tolerance can emerge and establish. However, seeds with low stress tolerance may take several extra days to reach full emergence, so the stand establishment will be somewhat uneven.
Under stress conditions, even highly stress tolerant seeds can have a difficult time emerging. Low stress tolerant seeds will emerge slowly, if at all. The result can be an extremely uneven stand or a partial stand that could require replanting.
Narrow ranges between the warm germination and accelerated aging percentages indicate more consistency within the lot of seed. For example, 95% warm germination and 85% accelerated aging, a range of 10%. A narrow range in the two percentages means the seed lot has a high number of stress tolerant seeds. This suggests the seeds should perform relatively well under either good or stressful planting conditions.
A large difference between the two percentages means the sample represents a mixture of seeds ranging from those with relatively high stress tolerance to those with relatively low stress tolerance. For example, 95% warm germination and 40% accelerated aging, a range of 55%. Under conditions of planting or emergence stress, seeds from this type of seed lot will perform erratically and inconsistently in the field.
The differences between the two tests are a general indicator of seed quality (see Table 1). Field emergence under favorable conditions is likely more related to the standard germination percentage. Emergence is likely to be more related to the accelerated aging percentage when field conditions are unfavorable.
Adding the percent warm germination to the percent accelerated aging provides a better index of seed vigor than using either value alone. This is called the “relative vigor index”. Refer to Crop File 3.01.022, "Using the Relative Vigor Index for Soybeans" for more detail.