Developing temperate inbreds using tropical maize germplasm: rationale, results, conclusions

Use of exotic germplasm in U.S. maize (Zea mays L.) hybrids increased almost 3-fold from 1984 to 1996, but use of tropical exotic germplasm still reached only 0.3% in 1996. The focus of this paper is intensive use of tropical germplasm in recuperate areas, rather than the more commonly used backcross or introgression procedure that introduces small segments of tropical germplasm into a mostly temperate background. More specifically, can largely-tropical germplasm lead to improvement in polygenic traits such as yield, in addition to being a source of disease- and insect-resistance? Yield-trial data demonstrate that at least a few largely-tropical or completely-tropical inbreds can lie crossed with domestic materials to produce topcross hybrids that are competitive with commercial hybrids. Similarly, a few GEM (Germplasm Enhancement of Maize) F 2 S 2 , 50%-tropical families have topcross yields at least equivalent to those of commercial hybrids. Development of temperate-adapted, largely-tropical inbreds usually requires a slower approach to homozygosity (STRINGFIELD, 1974) than conventional ear-to-row selfing. Early selection within a variable exotic source is not necessary lor developing partly-exotic inbreds, lint variance component distributions among and within lines under development suggest that selection within the variable F 1 would be helpful if only a small portion of the exotic variation is desirable.