THE INFLUENCE OF JOHNSONGRASS INFESTATION AND NITROGEN RATES ON GRAIN-SORGHUM

Grain sorghum [Sorghum bicolor (L.) Moench] is grown on marginal land and johnsongrass [Sorghum halepense (L.) Pers] is the most common and hard to control weed in this crop. The agronomic performance of grain sorghum at different nitrogen (N) and johnsongrass infestation levels in the field has not been adequately investigated. Therefore, research was conducted on a Decatur silty clay loam soil (Rhodic Paleudult) to determine the influence of N rates (0, 40, 80 and 120 kg ha-1) and jo hnsongrass infestation levels (0, 2, 4, 6 and 8 plants per 5 m crop ro w) on the growth and yield of grain sorghum (cv. GK522G) and johnsongr ass. Increase in N rates increased seed weight, seed yield and percent protein in grain sorghum as well as dry matter and percent protein of johnsongrass. The increase in johnsongrass infestation (from 0 to 8 p lants per 5 m crop row) significantly decreased the performance of gra in sorghum and johnsongrass at all the N rates. The results suggest th at 40 kg N ha-1 with up to 2 johnsongrass plants per 5 m crop row or 8 0 kg N ha-1 with up to 4 johnsongrass plants per 5 m crop row produced grain sorghum yields equivalent to absence of johnsongrass in the cro p.