Improvement of Corn Germplasm

Corn is the most widely and intensively grown crop species in the world and ranks third in the world, behind rice and wheat, in total production. Corn originated in southern Mexico or northern Guatemala 5,000 to 10,000 years ago. Corn is extremely variable genetically, and selection pressures were effective in developing corn strains to meet the needs of the native inhabitants. Most of the genetic variability in corn was present before the European colonists arrived in the Western Hemisphere. Native Americans had developed races that were being grown in present-day southern Canada, the United States, Mexico, Central America, islands of the Caribbean, and throughout South America by 1492. Columbus collected corn on the northern coast of Cuba on his first trip. After corn was introduced in Spain in 1493, corn became distributed throughout the world, where it could be grown and cultivated within two generations (Manglesdorf, 1974). Corn breeding includes two separate, but equally important, components: 1) germplasm improvement and 2) development of inbred lines for use in hybrids. Genetic advance depends on the systematic improvement of germplasm, and all breeding programs should include both components

Corn Breeding

The 100th anniversary of the inbred-hybrid corn concept was celebrated in 2008. It was in 1908 that G. H. Shull first presented the idea that corn could be improved by 1) selfing of plants to develop inbred lines, 2) making crosses (i.e., hybrids) among the inbred lines, 3) testing the hybrids in replicated trials to determine which hybrid has the best yield, and 4) reproducing the best hybrid and making seed available to the farmer. In the early part of the 20th century there was a rapid expansion in the interest and use of corn as a livestock feed in what we today call the “U.S. Corn Belt.” Acreages for producing corn increased, but one of the major obstacles was the relatively low yield of the open-pollinated varieties being grown at the time. From 1865 to 1935 (70 years), the average U.S. corn yields exceeded 30 bushels/acre in only four years. Hence, the question was: How can we increase corn yields? The information that G. H. Shull presented at the American Breeders Meetings in Omaha, NE in 1908, 1909, and 1910 was to have a profound affect on the type of corn grown in the 20th century. The inbred-hybrid corn concept of Shull often has been called the greatest plant breeding achievement of the 20th century

Factors Affecting Production of Corn Forage

Effects of plant densities, hybrid maturities, and harvesting dates were studied for production of forage corn (Zea mays L.) in northeastern Iowa. Biomass and grain weights increased with later harvest dates, but stover weight decreased with later harvest dates. Maximum dry weight of biomass was obtained by harvesting late-maturity hybrids at 60 days after flowering (physiolgical maturity) at the highest plant density (72.4 M plants/ha). Harvest indices decreased with higher plant densities, increased with later harvest dates, and decreased for later-maturity hybrids. The three variables studied (plant densities, hybrid maturities, and harvest dates) affected the amount of forage corn produced, but further study is needed to relate quantity of forage produced and silage quality

Recurrent selection methods to improve germplasm in maize

Recurrent selection (RS) schemes were introduced to increase the frequency of favorable alleles for quantitatively inherited traits. The main goal of RS was to genetically improve germplasm resources for breeding programs. Data were summarized for 14 intra-population and eight inter-population maize (Zea mays L) RS programs conducted in 17 genetically broad-based populations. The intra-population programs included evaluation of half-sib and full-sib families, either S1 or S2 inbred progenies, and a combination of S1 and S2 inbred progenies. The inter- population reciprocal RS programs were restricted to either half-sib or full-sib family selection. Grain yield was the primary trait considered in selection, but selection indices that include grain moisture at harvest and resistance to root and stalk lodging also were considered in making the selections that were intermated to form the next cycle population. Approximately, 10 to 20 selections were intermated for each cycle. Estimates of the genetic variation among progenies tested ( ), interactions of progenies with environments ( ), and experimental error ( ) were obtained from the combined analyses of variance for each cycle of selection and then averaged across cycles for each selection program. From the estimates of the components of variance, estimates of heritability (h2) on a progeny mean basis , the genetic coefficient of variation among progenies tested relative to their mean, (GCV), selection differentials (D), predicted genetic gains ( ), and least significant differences (LSD) were calculated and averaged across cycles of selection. The average estimates of were largest for inbred progeny selection and smallest for half-sib family selection as expected. Averaged expected genetic gain across all intra-population selection programs was 3.17 q ha-1 yr-1 vs 2.32 q ha-1 yr-1 for inter-population reciprocal recurrent selection, or 2.64 q ha-1 yr-1 across all methods. On a per cycle and per year basis the differences among types of progeny were relatively small, ranging from (3.57 q ha-1 yr-1,1.78 q ha-1 cycle-1) for half-sib family selection to (9.62 q ha-1 yr-1, 3.21 q ha-1 yr-1) for S2 inbred progeny selection. Regression analyses of the square roots of with cycles of selection suggested that genetic variation was not reduced significantly with selection. Even though RS was used to determine the primary types of genetic effects that respond to selection and contribute the expression of heterosis and could ideally support basic association and genome selection studies, the principle goal of RS is to adapt and improve genetically broad-based germplasm sources for potential use in breeding programs. Few programs have integrated RS programs with development programs to isolate unique inbred lines that have potential either as parents of hybrids or use in elite line crosses to develop recycled lines. In the past 10 years North Dakota has released 18 (out of 28) derived from RS programs, six from the NDSU EarlyGEM program (also diverse), and four from elite x elite combinations. Based on the number of progenies evaluated (25,692) in the RS programs presented and the number of inbred lines (31) that met standards for a ‘B’ designation and release to other maize breeders, the frequency of released lines was 0.12% or 1.2 lines per 1000 tested. However, the value of each line is different. B73 is a successful example of integrating recurrent and pedigree selection programs in order to develop outstanding cultivars. These significant gains can be realized with long-term RS selection programs. National support for them is encouraged in order to develop the next generation of maize products

Corn Breeding Research

The Northeast Research and Demonstration Farm is an invaluable facility for the cooperative federal-state corn breeding project at ISU. We rely on the facility as one of our main testing locations in the northern part of Iowa. One of our long-term goals is to increase our efforts in developing shorter season inbred lines, hybrids, and germplasm pools. Our research at the farm is funded primarily by the Raymond F. Baker Center for Plant Breeding