Interpreting Warm and Cool Germ of Cotton Seed and Understanding How Each Should Be Used (Collins, Edmisten, & Foote)
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Currently there is quite a bit of seed quality research underway, both at NCSU and at other universities through collaboration with other states’ Extension Cotton Specialists. We are investigating how testing might need to be restructured, testing other parameters that might be better predictors of field performance, or potentially redefining how current tests are conducted. Despite all of these efforts underway at the moment, for now, we still rely on the standard warm germ and cool germ tests as the primary measures of seed quality, until something more predictive of field performance is developed.
The warm germ and cool germ tests are very different in how they are conducted, and they are also very different in regards to what they mean and how they should be interpreted for practical use.
The warm germ test is conducted using alternating temperatures of 86 degrees (8 hours) and 68 degrees (16 hours), for 12 days. Upon completion, the number of germinated seed are counted and the final count is reflected as a percentage (excluding abnormal or malformed seedlings per the standards and rules for this test). For a seed lot that tests 85 percent in the warm germ test, this, in effect, means that 85 percent of seed should germinate (not necessarily emerge) when planting under favorable or ideal conditions. Emergence is a product of germination and several environmental factors (crusting, soil moisture or saturation, pathogens, etc) over a period of time, therefore emergence of seedlings can not always be predicted by the warm germ test alone. The warm test is fairly repeatable, meaning that several samples of the same seed lot should test reasonably close to each other, within a small margin of error. This repeatability is why the warm germ test is the only test that has to meet a legal standard for sale of that seed.
The cool germ test is conducted at 64.4 degrees, and that temperature is held constantly for seven days. To be counted as germinated, seed must sprout, but the radicles of sprouted seed must reach 4 cm in order to be counted. The final count of all sprouted seed with a radicle length of at least 4 cm is also reflected as a percentage. However, THE RESULTS OF THE COOL GERM TEST SHOULD NOT BE VIEWED IN THE SAME LIGHT AS THE WARM GERM TEST. The results for cool germ test are not nearly as repeatable as those of the warm test, hence the reason there is no legal standard for cool germ. Given the 4-cm qualifier and the lower temperatures used in the cool test, repeatability of results becomes difficult. It is not surprising to observe a 20 to 30 percent range (maybe more on occasion) in cool test results from seed samples collected from the same lot, and tested simultaneously or consecutively. The results of the cool test CAN NOT be viewed as the expected germination rate of seed when planted in cool conditions. Rather, the results of the cool test should be considered in terms of relativity to that of other seed lots. For example, take a seed lot that results in a 30 percent for the cool test. This DOES NOT MEAN that you could or should expect 30 percent of seed to germinate when planting in cool conditions. However, when comparing the cool test results for this seed lot (30 percent) to another seed lot that results in 60 percent for the cool test, THIS MEANS that when you are forced to plant in less-than-ideal conditions, you should probably plant the lot with a 60 percent cool germ, and take other precautions as well, such as planting shallower, hilldropping if crusting is historically a problem, increasing seeding rate, etc.
When planting in ideal conditions (50 or more DD-60’s expected within the first 5 days of planting into decent soil moisture, without any packing or saturating rains), the results for the cool germ test mean very little to us. In ideal conditions (daily highs exceeding 80 degrees with nighttime lows no less than 60 degrees), warm germ results are a better predictor of field performance, or at the very least, germination in the field. Emergence, as mentioned earlier, includes germination but is also dependent on other factors. It is always best to plant in ideal conditions. In reality, this is a challenge or nearly impossible in some years. When forced to plant in marginal or adequate conditions, as suggested by the Cotton Planting Conditions Calculator, that is when cool germ comes into play. Although we like to think of cool germ as a simple percentage, it SHOULD NOT BE used as a predictor of field germination in less-than-ideal conditions, but it CAN BE USED to decide what lots should be planted during those periods (high cool germ, larger seeded varieties) versus those that need to be planted only when conditions are good to excellent (decent warm germ but relatively lower cool germ). When the calculator says conditions are poor, no cotton should be planted, regardless of warm or cool germ test results.