Corn populations

​Separation of male and female flowers in corn allows for spreading of pollen among corn plants and, consequently, some mixing genes. However, much considerable amount of pollen falls on the silk of the same pollen producing plant, resulting in increasing homozygosity among the genes on seed saved from from each generation. Although those saving seed from desirable plants, the next season would have some of the desirable characters recognized by the seed savers but would also show the affect of homozygosity of some genes that would detract from maximum performance. Thus, this practice of annually saving seed in the various areas tended to select for characteristics favored but also resulted in eventual selection of less less obvious genetics inhibiting maximum expression of the capacity of grain yield in this species.

Some of the seed savers witnessed the extra vigor when some of these isolated populations were mixed, showing the advantages of intentionally crossing between them. James L. Reid’s father, Robert Reid used a seed variety called Gordon Hopkins adapted to their Ohio area in the mid 1800s. He and his son James L. Reid migrated to Illinois in the late 1800s and brought along some of the corn seed. Robert Reid started intentionally crossing and selecting seed of desirable plants in Ohio, and later with his son in Illinois, developing a popular variety known as Reid yellow dent corn. It included some New England Flint and Southern Dent corn genetics as well. This variety was sold and distributed to many areas of the USA but was especially adapted to the environments of the Eastern half of the USA.

The detrimental affect of inbreeding and the advantage of crossing between between varieties became clear to students of corn breeding in the early 1900’s. Inbreeding allowed the reliable reproduction of some traits but with the depression of others.

In 1935, George Sprague began intercrossing 16 inbreds and 4 inbred parents that were mostly from these many sub-varieties of Reid dent corn. This became known as Iowa Stiff Stalk Synthetic population (SS). New inbreds from this populations were test crossed with Lancaster inbreds, the successful SS inbreds were intercrossed, new selfs made, crossed again to Lancaster inbreds. These cycles were continually resulting in more populations at Iowa State University. Other public and private corn breeding efforts built on Stiff Stalk Synthetic, selecting from portions of it as well as constructing their own genetic populations. Inbreds from these efforts continuously improved hybrid performance as national corn yields increased. A third heterotic group, Iodent, was developed by mostly private breeders in the USA. Internationally, similar processes of identifying inbreds that combined with unrelated inbreds to give hybrid vigor.

The advantage of crossing of inbreds derived from different populations was becoming understood but would take some time and effort to economically produce hybrid seed.