During the 1988 wet season (May‐September), an experiment was conducted at the experimental farm of Abubakar Tafawa Balewa University, Bauchi, to investigate the effects of nitrogen and different weed competition durations with an improved sorghum(Sorghum bicolor(L.) Moench), variety BES, on the crop growth and yield, and determine the critical period of weed competition. The results showed that fertilized sorghum tolerated 2 weeks of early weed competition with minimal yield reduction. Weedy sorghum yield decreased as weed competition period lengthened from 2 to 7 weeks, primarily because sorghum plants under competitive stress had less panicle weight, fewer spikelets per panicle, lower 1000 grain weight and conseqently greatly reduced grain yield. Crop height was not significantly increased by nitrogen nutrition. However, sorghum plants under competitive stress were significantly taller than the weed‐free plants, probably as a result of severe competition with weeds for sunlight. Panicle weight, number of spikelets per panicle, weight of 1000 grains and grain yield were significantly increased by nitrogen nutrition. The mean reduction in grain yield due to weed competition decreased from 51% at zero nitrogen to 37.8% and 32.2% at 60 and 120 kg N/ha respectively, indicating that yield reduction decreased at medium and high fertility. Unweeded sorghum suffered a yield loss of 65% from severe competition with weeds that were predominantly made up ofRottboellia cochinchinensis. Setaria viridis, Cynodon dactylon, Striga hermonthica, Ipomoea cairica, Dichromena ciliata, Tridax procumbensandCyperus rotundus.