Genetic uniformity in maize

​When varieties, not hybrids, were common on the USA corn belt, average yields were frequently less than 50 bushel/acre.  Part of the reason for the low yields was the lack of uniformity of pollination timing and plant height within the fields, mostly due to genetic variability among plants within the variety.  Significance of obtaining genetic uniformity within a field became apparent in the early 1900’s as inbreds were combined to make specific hybrids, although the seed production quantity on the inbreds forced the use of double crosses.  As inbred seed yields became more commercially acceptable in the 1960’s, the advantage of single crosses became obvious at least partly because of genetic uniformity among plants in the field. 
Variability in plant height, especially when a shorter plant is adjacent to a taller one, regardless if it is due to genetic variability, or uneven emergence, penalizes the performance of the smaller plant.  If a corn plant is slower to push out silks than surrounding plants, it will not match the yield of the majority.
 
Producing genetically pure single crosses has several challenges for seed companies.  Haploid-di-haploid breeding schemes produce genetically pure, homozygous individual plants. However, the small quantity of seed needs to be increased, initially to make test hybrids and later, if successful to produce commercial hybrids.  Each increase carries some risk of contamination that can be mostly controlled by techniques but even with a low mutation rate the eventual inbred line will have the essence of the parent seed but also include some plants slightly different from the original.  Seed producers make considerable effort to maintain genetic uniformity in parents and single cross hybrids but evaluation is difficult.  DNA analysis methods may identify presence of DNA differences but may have difficulty in realizing the significance.  With 32000 genes in corn, the few changes may not matter in important aspects of performance.
 
If a seed production field is contaminated with hybrid, resulting ‘outcrosses’ are distinct from the correct hybrid plants for many phenotypic characters expressed as seedlings as well as mature plants. Pollen from hybrid plants have more than 1024 different genetics from the two hybrid parents.  Consequently, outcross plants within a single cross seed production will differ from each other and from the intended single cross for many characters.  Several years ago, we intentionally made outcrosses by crossing hybrid pollen with two popular female corn parents. We planted the resulting seed for our standard seedling growout test and later transplanted to grow the plants to maturity.  Seedlings expressed the differences as predicted.  Mature outcross plant showed considerable variation in plant height, flowering time, silk color and tassel shape as predicted.  If the outside pollen was placed on a female in which the normal male was short, more of the outcrosses were taller than the expected hybrid.  If placed on a female in which the correct male was tall, very few of the outcrosses were taller than the correct hybrid.  In other words, a higher percentage of outcrosses were taller than the canopy if the correct male was short. In both cases, a few plants were very short and inferior.
 
Ultimate field performance of a hybrid is affected by any factors affecting uniformity.  Genetics is one, but germination quality uniformity and multiple micro-environmental factors also are large.  Seed producers attempt to limit negative effects of the first two.