Genetic studies in an exotic population of corn (Zea mays L.) grown under two plant densities : II. Choice of a density environment for selection

Design II progenies from an exotic population of corn (Zea mays L.) were evaluated in a randomized incomplete block design with two replications at two plant-population densities (17,222 plants/ha and 68,888 plants/ha) in 1970 and 1971, at Lincoln, Nebraska. Five traits were studied, i.e., grain weight, number of ears, days to flower, plant height, and ear height.It was suggested that a trait such as grain weight should be expressed on a unit area basis in the following situations: 1. when genetic parameters (except for correlation and the ratio between two values) obtained from experiments with different plant-population densities are to be compared, and 2. when combined analyses of variance of data from experiments involving different plant-population densities are to be performed to test the significance of genotype by density effects. The estimates of genetic parameters were used to compute the expected direct and indirect genetic changes in grain weight and other traits from selection for grain weight. Three selection schemes were compared, i.e. mass selection (M.S.), full-sib (F.S.) selection, and half-sib (H.S.) selection in which S1 seeds from the selected male parents are used for recombination. A ten year selection program was considered. F.S. selection appears to be the most promising of the three, if there are no practical limitations and if costs are disregarded. A high-density selection environment appears to be a little better than a low density one for family selection. Mass selection at low density may result in good gains when measured at higher densities.From a long term point of view, it is possible that selection at low densities could allow response to continue for a longer period of time and perhaps reach a higher selection limit than selection at higher densities. Long term selection studies are necessary to compare response to high and low density-selection.