Identifying Resources In Ex-PVP Maize Germplasm for Improving Nitrogen Use Efficiency.

High yielding maize hybrids are created from elite inbred germplasm; however, academic scientists have limited elite germplasm resources for use in cultivar improvement and gene discovery research. Currently, maize breeding in the United States is concentrated in the private sector, and the resulting maize inbreds are protected by the Plant Variety Protection Act (PVP) which provides legal protection for elite germplasm for a period of 20-years. However, once the 20-year period expires, PVP-protected germplasm becomes public domain and seeds can be freely acquired by any public or private breeder.

Improving nitrogen (N) use has important implications for improving the sustainability, profitability, and yield potential of maize. Previous reports have suggested that maize heterotic groups respond differently to nitrogen inputs, and ex-PVP germplasm provides an excellent resource to study N use traits in commercially representative maize hybrids. The goal of this study was to demonstrate how different heterotic groups used by breeders in the United States respond to N, and to establish the contributions of general combining ability (GCA) and specific combining ability (SCA) to low N tolerance and responsiveness to N fertilizer. Twelve inbred lines (10 ex-PVP, B73, and Mo17) were evaluated in a partial diallel mating design at Champaign, IL in 2011. Single-cross hybrids were created by combining inbreds in the usual stiff-stalk by non stiff-stalk heterotic pattern. Hybrid performance was evaluated under 0, 67, and 252 kg N ha^-1 to characterize low N tolerance, and initial and maximum responses to N. General and specific combining abilities were calculated for grain yield, kernel number, and individual kernel weight at each level of N fertilizer, as well as initial N response at 67 kg N ha^-1, and maximum N response at 252 kg N ha^-1. Grain yield was significantly correlated with kernel number (0.79 to 0.91), while kernel number had a significant negative correlation with individual kernel weight (-0.56 to -0.63). Specific combining ability had greater importance at low N (0 and 67 kg N ha^-1), while GCA tended to contribute more at 252 kg N ha^-1. In general, yield rankings of hybrids were established at the 0 kg N ha^-1 level, and were maintained with increasing fertilizer application, suggesting that the highest yielding hybrid under optimum conditions is often the best hybrid under resource-limited conditions. LH1 and PHG39 had the highest initial and maximum responses (2.28 kg ha^-1 and 3.66 Mg ha^-1, respectively) of the stiff-stalk group of inbred parents. In contrast, the non stiff-stalk group of inbred parents resulted in no significant differences for initial N response in their hybrid progeny, but PHG84 contributed the highest average maximum N response (4.42 Mg ha^-1).  Collectively, the differences in N use traits between heterotic groups indicate that it is possible to create more N use efficient hybrids by combining inbreds from complementary heterotic groups.