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The warm germination test is often used to judge soybean seed quality. However this test does not take seed vigor or stress tolerance into account. Seed lots with a high germination percentage, but low stress tolerance, may perform poorly in the field when planting or seedbed conditions are poor.
Seeds with high stress tolerance (more vigorous seeds) are more successful at establishing under stress conditions than are seeds with low stress tolerance.
Seedbed stress includes cold soils, wet soils, herbicide carryover, crusting, dry soils, deep planting, and other unfavorable conditions. All of these stresses can affect seedling emergence, stand establishment, and final population.
The relative vigor index (RVI) is calculated by adding the warm germination percentage to the accelerated aging percentage. This provides a numerical value that helps estimate the relative performance of a particular seed lot under seedbed stress conditions.
Table 1 shows the stress tolerance rankings for the relative vigor index. Producers can use the relative vigor index to evaluate various seed lots and reject those that may perform poorly under field conditions. It can also cue the producer to use extra care while planting because of the potential inconsistency with the seed lot.
¶ Table 1. Stress Tolerance Ratings Based on the Relative Vigor Index. |
|
| Index value | Stess tolerance ranking |
| 180 to 200 | Very high |
| 160 to 180 | High |
| 140 to 160 | Medium |
| 120 to 140 | Low |
| Less than 120 | Very low |
Explanations and management suggestions for each ranking follow. The suggestions are geared toward soybean seeds, but the same ideas apply to other seeds.
A high RVI result does not guarantee that a seed lot will successfully establish under adverse field conditions nor does a low RVI mean that a seed lot will fail. Rather, the RVI is a "warning flag", that tells a producer that there is potential for problems if conditions are not favorable.
Very high (180 to 200): Seeds with an RVI of 180 or more should have few problems emerging or establishing, even if seedbed stress is present. However, normal care should be taken at planting.
High (160 to180): Emergence should be good under a wide range of conditions. Some problems may occur if severe stress occurs or several stresses combine at one time.
Medium (140 to 160): Emergence should be fair to good under most conditions. Emergence can be reduced when moderate to severe seedbed stress occurs. Planters should be set to insure good seed placement and depth. Seeds in this range should be planted in warm soils (60°F or above) with adequate moisture. A fungicide can be beneficial, especially if these seeds are to be planted into cooler soils where germination may be slow.
Low (120 to 140): Emergence may be reduced severely when seedbed stress or a combination of stresses occurs. These seeds can be particularly vulnerable to herbicide carryover, crusting, or deep planting. Another seed lot, with higher stress tolerance, would be a better choice for planting. Planters should be set to insure good seed placement and depth. Seeds in this range should be planted in warm soils with adequate moisture, preferably 70°F or above. Emergence of these seeds will be somewhat irregular and stands may be somewhat erratic under stress conditions. A fungicide can be beneficial, but planting time management will be more crucial.
Very low (less than 120): Seeds in this category should not be used for planting. Even under ideal planting conditions, emergence will be irregular. Complete emergence may take 10 to 14 days and stands will probably be erratic and uneven. If these seeds are to be used for planting, they must be planted in an "ideal" seed bed with adequate soil moisture and a soil temperature of at least 70°F. A fungicide may be beneficial, but may not be economical.