Experimental evaluation of the potential of tropical germplasm for temperate maize improvement

Commercial maize (Zen mays L.) in the USA has a restricted genetic base as newer hybrids are largely produced from crosses among elite inbred lines re presenting a small sample (predominantly about 6- to 8-base inbreds) of the Stiff stalk and Lancaster genetic backgrounds. Thus, expansion of genetic diversity in maize has been a continuous challenge to breeders. Tropical ge rmplasm has been viewed as a useable source of diversity, although the inte gration of tropical germplasm into existing inbred line and hybrid developm ent is laborious. The present study is an evaluation of the potential of tr opical germplasm for temperate maize improvement. All possible single-, thr ee-way-, and double-cross hybrids among three largely temperate and three t emperate-adapted, all-tropical inbred lines were evaluated in yield-trial t ests. Single-cross hybrids containing as much as 50-60% tropical germplasm produced 8.0 t ha (-1) of grain yield, equivalent to the mean yield of the commercial check hybrids. On the other hand, three-way and double-cross hyb rids with the highest mean yield contained lower amounts of tropical germpl asm, 10-19% and 34-44%, respectively. Overall, hybrids containing 10-60% tr opical germplasm yielded within the range of the commercial hybrid checks. Hybrids with more than 60% tropical germplasm had significantly lower yield s, and 100% tropical hybrids yielded the least among all hybrids evaluated. The results indicate that inbred lines containing tropical germplasm are n ot only a useful source to expand the genetic diversity of commercial maize hybrids, but they, also are competitive in crosses with temperate material s, producing high-yielding hybrids. These experimental hybrids exhibited go od standability (comparable to the commercial check hybrids) but contained 1-2% higher grain moisture, leading to delayed maturity. Recurrent selectio n procedures are being conducted on derivatives of these materials to extra ct lines with superior yield, good standability, and reduced grain moisture which can be used for commercial exploitation.