CORN PLANTING DATE AND PLANT-POPULATION

Predictions of yield responses to planting date and plant population in corn (Zea mays L.) are necessary in order to match plant population to a particular planting date, and also to make replant decisions. Much of the published research on these factors is relatively old. This study was conducted at Monmouth and DeKalb, Illinois for 4 yr (1987-1990) in order to formulate a predicted response of two modern hybrids to planting date and plant population. The two hybrids in the study showed no consistently different yield response to planting date, to plant population, or to the interaction of these two factors. Generally, yields increased from mid-April to late April planting dates, then declined as planting was delayed to late May. Plant population had a very large effect on yield, except in the very dry environments at Monmouth in 1988 and 1989. The model found to give the best prediction of response to planting date and plant population was Yield (bu/acre) = 23.96 + 1.659D - 0.0303D2 + 7.673P - 0.1266P2, where D = days after 1 April and P = plant population in thousands per acre. The population providing the highest yield was about 30 310 plants/acre, and the best planting time was found to be about 27 April. The table of predicted values from this study can be used to provide more accurate information on planting date and plant population responses of newer hybrids, and will be useful in making replant decisions when the initial planting is early enough to allow assessment of stand damage by mid May.