Using tropical maize germplasm for temperate breeding

Despite many efforts, little truly exotic germplasm is represented in U.S. maize hybrids. A wide array of tropical materials is available and use of elite tropical materials that have been through the inbreeding process is the quickest way to use tropical maize germplasm for temperate breeding programs, but how does one choose which material to use? Because of reaction to photoperiod, evaluating tropical germplasm sources is virtually impossible in the U.S. Corn Belt. N. C. State is involved in the development of inbred lines with high percentages of exotic germplasm via a process called incorporation by Simmonds, rather than the backcross introgression procedures adopted by most breeders. The advanced, largely-tropical lines at N.C. State all trace back to intercrosses among elite tropical hybrids. Results of second generation lines derived by phenotypic selection while selfing are reported here, including the results of testing under temperate and tropical conditions using both a temperate and a tropical tester. The results suggest that selection in either a very different location or with a very different tester can result in definite improvement in breeding value. Rarely would more extreme comparisons be made, yet all the rank correlations between testers and between environments were consistently positive and significant. Six lines have been released from these studies: NC298 and NC300 in 1991, NC304 in 1994, NC340 in 1997, and NC348 and NC350 in 1998. A joint public-private program called GEM (Germplasm Enhancement of Maize) also has been developed to move tropical germplasm more rapidly into breeding programs. The first widespread testing of family topcrosses from 50%-tropical materials has now been completed, and those results are summarized here.