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Comparative Performance of Top Crosses and S₁ Progeny for Improving Populations of Corn (Zea mays L.)¹

Forty-five introduced strains, two Northern U.S.A. single crosses, and their double cross were all crossed to form a synthetic variety which was randomly mated for three generations. Randomly chosen S0 plants were selfed and top crossed to a double cross tester. The top-yielding 11% of the S₁ lines based on S₁ performance and top-yielding 11% of the $₁ lines based on top cross performance were developed into two subsynthetics in which random mating was practiced for three more generations. In the following cycle, mean yields were significantly higher in the S₁. progeny from the synthetic based on S₁ progeny performance than from the synthetic based on top cross performance. Conversely, mean yields were significantly higher in the top crosses from the synthetic based on top cross performance than from the synthetic based on St line performance.

The next generation of each sub-synthetic was progagated by intennating the top-yielding 21%, based on S₁ performance and top cross performance, respectively. Samples of seed were drawn from each generation of the subsynthetics and parent synthetic and tested at two locations. These tests indicated that a highly significant yield improvement was made with the first cycle of selection by both methods of evaluation, but there was no significant difference between the two methods. The second cycle of selection was not effective.

² Graduate Assistant (presently Research Geneticist, Department of Field Crops, University of Missouri) and Associate Professor of Agronomy, Purdue University.

Received for publication July 26, 1967.