Each seed in a single cross hybrid corn bag is slightly different in its biological and even genetic history. Rarely are the two inbred parents in a seed field 100% homozygous plus some of those plants may have their own mutations. Fortunately, drastic mutants are usually caught by the seed producer before pollination. An individual female plant in the seed field may produce 200-300 seed. And one 80000 kernel bag of seed corn must represent seed from 300-400 individual plants.
We want to think of all single cross seed in a bag are the same, but they are not identical genetically or in germination quality. Even with multiple generations of selfing in development of the parent seed, some mutations occur with each generation of seed increase prior to planting in the hybrid seed field. Most often these mutations are non-consequential to hybrid performance and especially not visible in the field where many small environmental effects are affecting appearance of the plants. The closer we get to discerning DNA differences the more difficult it becomes to distinguish inconsequential differences from the drastic ones.
Seed quality differences among the seed in that one bag of hybrid seed also shows differences. A warm test may show 95% germination but even beyond the non-germinating ones, there will be some that are slower to germinate than others even when all environment is uniform. As the percent germinated gets lower, more late ones become evident. A cold test, especially like ours at Professional Seed Research, Inc. in which we cover the seed with 3/4 inch of artificial soil, nearly always show lower percent germination and more late emerging plants than the warm test. Why are all the seed not with the same quality when produced in same field?
Unfortunately, not all the seed on a single seed field ear have the same environment. First silk, coming from the ovules near the base of the ear, emerge 3-6 days before the final silk. Successful pollination by the correct male parent is dependent of many variables, including factors associated with maturity for the male and female in the seed field. In general, pollen timing is affected more by accumulation of heat units whereas silking is favored by water. Cool wet pre-flowering weather can lead to silks being exposed before pollen. This not only makes the seed more vulnerable to contamination by outside pollen from field corn, resulting in outcrosses, but also to infection from fungi such as Fusarium or Diplodia species traveling down the silk channel before it closes after pollination. The opposite can happen with hot dry weather, in which the silk emergence is delayed, causing the pollen to be spent before all the silk emerges. Consequently, there often are differences in germination (and outcrosses) among seed positions on the ear.
About Corn Journal
The purpose of this blog is to share perspectives of the biology of corn, its seed and diseases in a mix of technical and not so technical terms with all who are interested in this major crop. With more technical references to any of the topics easily available on the web with a search of key words, the blog will rarely cite references but will attempt to be accurate. Comments are welcome but will be screened before publishing. Comments and questions directed to the author by emails are encouraged.