EFFECTS OF COMPETITION WITH WEEDS ON THE GROWTH, DEVELOPMENT AND YIELD OF SORGHUM

In two field experiments in Suriname, competition between weeds and so rghum (Sorghum bicolor (L.) Moench) was studied in the rainy seasons o f 1982 and 1983. The crop was kept either weed-free or without weed co ntrol for each of six different periods of time from planting. Observa tions made at the end of each period allowed an analysis to be made of the growth and development of a crop with and without weed control. D ue to the crop canopy structure and fertilizer placement near the row, weed growth was concentrated in the rows. Starting at c. 20-30 days a fter planting, competition with weeds reduced ground-cover and leaf ar ea index of the crop. Competition reduced growth rates, leading to low er yields. Plant population density was not affected, but competition reduced the number of leaves present. Stem length initially increased with competition in one season, but was retarded in another. Competiti on for nutrients was strong and was found as early as 15 days after pl anting. Nitrogen was the element most competed for initially. Competit ion for water occurred, but competition for light seemed unlikely. Abo ut 20 weed-free days after planting were necessary to avoid yield loss es and around 30 weed-free days to attain negligible weed growth at ha rvest. Yield reduction was mainly due to a decrease in number of grain s per panicle. Competition during floret establishment (c. 30-40 days after planting) should be avoided.