Native Varieties used for Training and as Early Checks for Pollination Purposes

The first breeding crosses for genetic and selection purposes were reported in 1901. Corn improvement was based on visual selection of old varieties for adaptation and development of new ones by crossing. The most widely adapted varieties were ‘Falconer’ and ‘Northwestern Dent’ (Olson and Walster, 1932). Other varieties were ‘Dakota White’ and ‘Gehu’ (earlier maturity), and ‘Minnesota 13’, ‘Mercer’, and ‘Rustler White’ (later maturity). Mass selection, defined at that time as ‘simple selection’ (Olson et al., 1927) was performed by selecting ears after harvest (Method I) and selecting ears from the best plants in the field (Method II). The latter tried to answer the same questions asked by Gardner (1961) when trying to reduce environmental bias selecting plants with stiff stalks and healthy leaves before looking at the ear. The ear-to-row selection method resulted in rapid improvement on non-adapted varieties (Olson et al., 1927). However, no yield improvements were obtained by continuous ear-to-row selection. Both mass selection and ear-to-row breeding became popular with primary emphasis on early maturity. The third method of improvement involved the production of self-fertilized lines that produce different strains and allowed breeders to discard undesirable recessive alleles controlling simple traits. As a consequence, improved early varieties had the yield potential similar to later maturing ones and differences in maturity among certain varieties did not lead to differences in yield. Grain yield heterosis was common among varieties, especially in the dent x flint crosses ranging from mid-parent heterosis values of 18.7% to 32.5%. Dent x flint (e.g. Rustler x Mercer) and dent x flour (e.g. Minnesota 13 x Blue Flour) crosses produced greater yield than dent x dent crosses (Hayes and Olson, 1919; Waldron, 1924). It was recognized that the mode of inheritance of the most desirable traits was complex.
 

The Faculty that Invented the Inbred-Hybrid Concept (Hybrid Corn), a Large Business still used today

The inbred-hybrid concept and its application in North Dakota

The development of the hybrid corn industry was successful because of input by both public and private sectors of corn research. The inbred-hybrid concept was defined and applied first in the public sector. The desirable consequence was hybrid corn. Hybrid corn traces back to the early 1900s. E. M. East, later replaced by H. K. Hayes and D. F. Jones, led one of the research groups that discovered and revealed the potential of hybrid corn using Leaming inbred lines. East was directly influenced by the biological principles of Darwin, Mendel, and Vilmorin in relation to plant improvement (Hayes, 1956). East related those principles to the more practical plant improvement studies to achieve his goals.  Probably the most well known public scientist in early hybrid corn research is G.H. Shull whose corn research records date back to 1904. At that time research focused on heredity as a basis for improving plants and animals and Shull studied the theory of genetics and its application to plant breeding. East at Connecticut and Shull at Cold Spring Harbor independently started studies of inbreeding and crossbreeding in 1906 (Hayes, 1963) and provided essential insight into the efforts of corn inbreeding (East, 1908; Shull, 1909). However, Shull discontinued his studies in 1916 since he concluded the inbred-hybrid concept had no practical value due to small amount of seed produced on inbred lines. Jones suggested to East a procedure that would make hybrid corn a reality for industry and farmers, using the already developed Leaming lines as females and Burr’s White inbreds as males (Jones, 1918). There were concerns that combining four inbred lines to make double-cross hybrids would be a step back to open-pollinated corn and that performance would be reduced compared to single-cross hybrids. However, producing seed on a single-cross parent provided a solution to marketing hybrids, annually to farmers. The change from open-pollinated varieties to double-cross hybrids was a significant improvement in developing maize hybrids with improved standability and grain yield performance. It had a negative impact on breeding efforts toward population improvement and genetic diversity, though (see the population-hybrid concept, publications section). After years of intensive and at times discouraging research searching for effective inbred lines, the largest corn research organization in the private sector (Funk farms) produced the first good corn double-cross hybrid (US13) using lines developed in the public sector (Crabb, 1947). The first Pioneer hybrid for sale was ‘Copper Cross’ in 1924 and included East’s Leaming lines on the female side (Hayes, 1963). These lines were developed from OP variety Leaming (Leaming, 1883; Lloyd, 1911, Carena and Hallauer, 2001).

 North Dakota

Hybrid production in ND was initiated in the 1930s. One decade later, the percentage of corn acreage planted to hybrids in North Dakota was 12% ranging from 5% in northern and western ND to 35% in southeastern ND by 1941 (Wiidakas, 1942). This percentage increased to 50% by 1948 (80% in southeastern ND). Open-pollinated varieties were rapidly replaced by hybrids in the more favorable corn growing areas of the state (SE and east central). A very positive consequence of hybrid corn was the increase of corn productivity under farm mechanization (Wiidakas, 1952). Hybrids were uniform and yielded 5% to 28% more than later and high yielding varieties such as Minnesota 13. Seed costs, state and federal subsidies, and grower migration to urban areas also increased. Early maturing hybrids were needed for the remainder of the state and popular open-pollinated varieties (e.g. Falconer) were still a safe choice for short-season areas.

H.Z. Cross (trained by M.S. Zuber of University of Missouri at Columbia, MO) followed a new approach using unique and genetically broad-based germplasm. New synthetic varieties were developed and improved in several ways. The predominant methods used were modified mass selection and modified ear-to-row selection. In the 1970s inbred lines were mainly developed from elite x elite crosses using inbreds developed in Wisconsin and ND. In the 1980s and 1990s, however, inbred lines were developed using synthetic varieties derived from Iowa, Romania, but still mostly from ND. Examples of successful lines released in the early 1980s and utilized by industry are ND246, ND252, and ND257. Cross (1975) reported that corn genotypes differed in the duration of the grain-filling period. The use of R-nj allele as a useful marker for variable expression of aleurone pigmentation was suggested as a genetic-physiologic mechanism to breed for higher grain yield without changing grain moisture at harvest (Cross, 1981). Mostafavi and Cross (1990) also suggested that R-nj color expression might be related with rate of dry matter accumulation (RDMA). In addition, Nguetta and Cross (1997) found that R-nj color expression was related to increased leaf photosynthetic area and early cob development. Moreover, selection for high-average leaf expansion rates (ALER) increased photosynthetic area without delaying maturity (Cross, 1990) confirming the hypothesis that leaf area limits grain production. General combining ability (GCA) effects between ALER and grain yield were highly positively correlated (Nevado and Cross, 1990) which was later confirmed as a method for increasing grain yield through increased ear and kernel size without increasing ear moisture at harvest and non-detectable genotype x environment interactions (Cross, 1991). A simple method of selection for low ear moisture at physiological maturity (PM) did not decrease grain yield (Cross et al., 1987). Relative rate of moisture loss (RML) or field dry-down rate, however, was recognized to be a difficult trait to select but was complementary to low ear moisture at PM (Cross and Kabir, 1989).

M.J. Carena (trained by A.R. Hallauer, Iowa State University, Ames, IA) has been the third corn breeder since 1999.