CORN PLANTING DATE AND PLANT-POPULATION

Predictions of yield responses to planting date and plant population i n corn (Zea mays L.) are necessary in order to match plant population to a particular planting date, and also to make replant decisions. Muc h of the published research on these factors is relatively old. This s tudy was conducted at Monmouth and DeKalb, Illinois for 4 yr (1987-199 0) in order to formulate a predicted response of two modern hybrids to planting date and plant population. The two hybrids in the study show ed no consistently different yield response to planting date, to plant population, or to the interaction of these two factors. Generally, yi elds increased from mid-April to late April planting dates, then decli ned as planting was delayed to late May. Plant population had a very l arge effect on yield, except in the very dry environments at Monmouth in 1988 and 1989. The model found to give the best prediction of respo nse 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 providi ng the highest yield was about 30 310 plants/acre, and the best planti ng 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 pl anting date and plant population responses of newer hybrids, and will be useful in making replant decisions when the initial planting is ear ly enough to allow assessment of stand damage by mid May.