TROPICAL CORN AS AFFECTED BY PLANTING DATE, NITROGEN, AND WATER

Tropical corn (Zea mays L.) hybrids that produce satisfactory yields o f grain (9,000, 7,600, and 5,000 kg ha-1) and silage (19, 17, 8.5 Mg h a-1) if planted 15 March, 15 May, and 1 August, respectively, under mo derate energy inputs (< 134 kg N ha-1 and no irrigation) became availa ble in the mid 70's. The objective of this study was to determine effe cts of planting date, nitrogen rate, and water regime on the productio n potential of tropical corn hybrids. Corn was no-till planted at a po pulation density of 84,968 (1988) and 44,500 (1989) plants ha-1 at Gai nesville and 44,500 plants ha-1 at Quincy (1989 and 1990), Florida. Ni trogen treatments of 268 kg N ha-1 were applied at Gainesville in 1988 ; and 0, 56, 112, 168, and 224 kg ha-1 N were applied at Gainesville ( 1989) and Quincy in (1989 and 1990). Twenty-eight kg N ha-1 was applie d as starter for all N treatment, but the 0-N treatment at planting fo r 1989 and 1990 with the remainder as a side-dress application at stag e 1.5 (0.30-m corn height). All studies were rainfed except the 1988 i rrigated study at Gainesville and half of the 1990 hybrid trial at Qui ncy which received limited irrigation prior to tasseling. Temperate co rn planted in March is very productive (13,000 kg ha-1) when fertilize d and irrigated for maximization of yield in Florida. Tropical corn ca nnot compete with it at the early planting date, but when tropical cor n is planted after 1 May, temperate corn cannot compete. Nitrogen fert ilization above 134 kg ha-1 did not significantly increase tropical co rn grain yields at Quincy in 1989, but 268 kg ha-1 in 1988 and 224 kg ha-1 in 1989 at Gainesville increased grain yields on deep sands subje ct to leaching. Because tropical corn yield potentials decrease for la ter planting dates, less N is needed. Seventeen tropical and one tempe rate corn hybrids were studied at Quincy under rainfed and limited irr igation during the ''normal'' year of 1989 and the drought of 1990. In teractions of hybrid x water regime and hybrid x planting date were si gnificant.