Handbook of Plant Breeding Volume 3, 2009, pp 3-98


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Maize (Zea mays L.) originated from teosinte (Zea mays L. spp Mexicana) in the Western Hemisphere about 7,000 to 10,000 years ago. Maize was widely grown by Native Americans (e.g. it was the first crop in North Dakota) in the U.S. during the 1600s and 1700s. The practical value of hybrid vigor or heterosis traces back to the controlled hybridization of U.S. southern Dents and northern Flints by farmers in the 1800s. Inbreeding and hybridization studies in the public sector dramatically change maize breeding. The Long Island (led by Shull) and Connecticut (led by East) public research groups created the inbred-hybrid concept (hybrid maize) which allowed industry to exploit the practical and economical value of heterosis. The hybrid maize technology was rapidly adopted by U.S. farmers and generated genetic gains for grain yield at a rate of 1.81 kg ha−1 year−1. However, emphasis on the exploiting the inbred-hybrid concept detracted from further improvements on open-pollinated cultivars and their cultivar crosses.

Maize breeding is the art and science of compromise. Multi-trait selection, multi-stage testing, and multi-progeny evaluation are common for discarding thousands of lines and hybrids. Maize breeding has unique features that are different from any extensively cultivated self-pollinated crop. Breeding techniques from both self and cross-pollinated crops are utilized in maize. The fundamentals of maize breeding remain the same: germplasm improvement (e.g. recurrent selection), development of pure-lines by self-pollination, production of crosses between derived lines, identification of hybrids having consistent and reliable performance across an extensive number of environments, and production of the best hybrid for use by the farmer. Each successful hybrid has its own unique combination of genetic effects and allelic frequencies often limiting sample sizes for QTL experiments relative to classical quantitative genetic studies. The main limitation of traditional methods of maize breeding is to determine the genetic worth of lines in hybrid combinations. Most of the economically important traits in maize breeding are inherited quantitatively. Their importance is recognized by molecular geneticists through their emphasis in QTL experiments, molecular markers, marker-assisted selection to predict early and late generation combining abilities, and/or ultimately gene-assisted selection through specific genome selection (e.g. metaQTL analyses) and/or association mapping. Information in maize genetics has significantly expanded in the past 50 years until the unraveling of the genome sequence in 2008. However, the limiting factor for genetic improvement remains the same: good choice of germplasm. The most sophisticated breeding methods and/or technologies carrying all of the genetic information available will have limited success if poor choices of germplasm are made. Biotechnology continues to be an important addition to the breeding process for single-gene traits while conventional breeding continues to be the key for improving economically important traits of quantitative inheritance. This chapter starts with a general introduction followed by pre-breeding and the incorporation of exotic germplasm, currently led by the USDA-GEM network. The integration of recurrent selection methods with inbred line development programs follows with the classical example of B73, the public line derived from BSSS that generated billions of dollars to the hybrid industry. The chapter continues with the inheritance of quantitative traits, and methods of line development and hybrids. Finally, the concepts of heterotic groups, heterotic patterns, and inbred line recycling are detailed for exploiting heterosis and hybrid stability including multi-trait selection utilizing indices. A summary is included at the end of the chapter.