From varieties to hybrids

​From the base varieties and races of corn distributed in the USA, and as some were mixed, by individual farmers, many small seed companies emerged to produced popular varieties for an area. By the year 1900, it was estimated that 1000 distinct varieties of corn were grown by USA farmers but most were selections from 10 basic ‘races’. From these, individual public and private corn breeders began developing inbreds for hybrid production.  Selection pressure in varieties emphasized appearance of ears in farm shows, but also for disease resistance to local diseases, pests and drought.
 
Some of the same selection pressures were applied to inbred development but ultimately it was the performance in a hybrid that became most significant. These attempts in the early 1900’s had to be disappointing to many, as the ear size reduced greatly and relationship with inbred appearance and hybrid performance was not always strong.  It presented new dynamics over variety selection but the payoff for these early efforts was great.  Realizing that making 4-way hybrids, with both parents being hybrids, made seed production cost efficient especially in comparison to the boost in grain yields over variety yields. The lack of genetic relationship between the two hybrid parents covered up the genetic deficiencies of each other, giving the double cross hybrids a boost over either parent. Hybrids were introduced in the 1920’s and gained significantly in the 30’s and by 1950, nearly all USA corn grain production was made with hybrids. Separation of male and female flowering structures encouraged genetic variability and multiples of people selecting for varieties fitting their needs, established the basis for the huge hybrid vigor revealed in corn hybrids.  
 
Corn history has intrigued many and literature is rich with details.  One comprehensive source is ‘Corn and Corn Improvement’, 3rd edition published by American Society of Agronomy in 1988. Internet search engines lead to more references including more recent literature.