RESPONSES TO PLANTING DATE AND POPULATION DENSITY BY EARLY-MATURING SORGHUM HYBRIDS IN ONTARIO

Early-maturing grain sorghum (Sorghum bicolor (L.) Moench) hybrids which have potential usefulness in southern Ontario are commercially available. One such hybrid, Pride P130, and two experimental hybrids, Pride X4043 and Pride X3160, were grown at three planting dates and three plant densities in 1975 and 1976 at Elora, Ontario. There were several hybrid × planting date and hybrid × plant density interactions for the development and yield parameters measured. However, grain yields of all hybrids decreased by about 10% per week when plantings were delayed 2 or 4 wk after mid-May in 1975. In 1976, yields declined even more after late planting. Increasing populations from 75 000 to 300 000 plants/ha in 1975 increased grain yields from 4.3 to 6.0 t/ha, averaged over hybrids and planting dates. In 1976, raising populations from 150 000 to 450 000 plants/ha increased average yields from 3.2 to 3.7 t/ha. P130 was the earliest and best-yielding hybrid in both years. In 1975, which had a warm May and near-average temperatures, the best treatment yielded 7.3 t grain/ha and hybrids at all planting dates matured. The best treatment approached grain corn (Zea mays L.) yields. In 1976 with a cool spring and late summer, the highest grain yield was 5.7 t/ha with early planting, but 11 June plantings did not mature. In south-central Ontario, sorghum grain yields appear less reliable than those of corn.