Modern Maize Breeding

Maize breeders during the hybrid era (1939 to present) have been extremely successful in making continuous genetic improvement in commercial grain yield (Fig. 1). Commercial grain yield in the US increased from about 1,300 kg ha⁻¹ in 1939 to 7,800 kg ha-1 in 2005, about 99 kg ha⁻¹ year⁻¹, with similar gains observed in Canada during the hybrid era (80 kg ha⁻¹ year⁻¹) (Lee and Tollenaar 2007). This 6-fold increase in grain yields over a 60 year period is unprecedented among cereals or oil seeds.

The USDA began collecting data on yield in 1862 and from that time maize yields did not change until the 1930s when they began an upward trend which has showed no signs of abating (Tracy et al. 2004). Prior to 1909, nearly all maize breeding was done by farmers or farmer/seedsmen, who used mass selection as their main breeding method (Hallauer et al. 1988). Mass selection is a method in which the best ears from the best plants would be selected from a population of maize plants. While this technique has been used since the domestication of plants, it is not very effective for traits with low heritability such as yield. Some farmer/ breeders experimented with ear-to-row selection which entails growing and evaluating families derived from individual open-pollinated ears. While this method can be successful in improving traits of low heritability, the lack of knowledge of statistics and experimental design limited the success of this method and yields continued unchanged. Also contributing to static yields were the maize shows that were prevalent during the late 19^th and early 20^th centuries. In these shows, 10 ears were shown by each exhibitor and those groups that were most uniform and conformed to some ideal type in the mind of the judge were chosen as the winners, with no attention paid to yield or other economic traits. It is likely that the emphasis on uniformity in these competitions contributed to inbreeding and further suppressed any yield increases (Hallauer et al. 1988). A third breeding approach attempted during the 19th century was varietal hybridization in which two open-pollinated cultivars would be hybridized and the resulting hybrid would be grown for seed (Beal 1877). Some crosses did reveal reasonable amounts of heterosis (10–15%). But this yield increase was not enough to justify the extra energy and expense of producing the hybrid seed.