RESPONSES TO A MODIFIED RECIPROCAL RECURRENT SELECTION IN 2 MAIZE SYNTHETICS

The early genotypes of maize (Zea mays L.) selected for the northern C orn Belt regions can perform poorly when grown as a delayed-sowing cro p in warmer regions. Two early synthetics, A and B, were developed and two cycles of a modified reciprocal recurrent selection were conducte d in a delayed-sowing crop by using A632 as a tester for Synthetic A a nd W117 as a tester for Synthetic B. Selection was based on a multitra it index involving grain yield, percent erect plants, and grain moistu re. This study was conducted in three environments to evaluate the eff ects of selection. The source and selected populations of the two synt hetics were tested per se and in hybrid combination to each other, to their inbred tester, and to the unrelated hybrid, A188 x W64A. The gai n in the cross between the two synthetics was 4.9% cycle(-1) (P less t han or equal to 0.01) for the performance index (calculated similarly to the selection index), and this was mainly due to the gain in grain yield (12.2% cycle(-1)). However, undesired increases were found for g rain moisture and silking date. The gains for performance index and gr ain yield were due to a balanced contribution of both changes per cycl e of populations per se and of heterosis. Synthetic B contributed more than Synthetic A to the gain in the population cross for performance index, grain yield, and to the loss of earliness. Population crosses o f both synthetics with corresponding testers and A188 x W64A showed a positive response for performance index and grain yield, and a loss of earliness for Synthetic B. The results indicated that the selection p rocedure was effective in improving the performance index of the two s ynthetics both per se and in hybrid combinations.