Abstract
A field experiment was conducted during the 1994 and 1995 wet seasons on the research farm of the Institute for Agricultural Research, Samaru (11°11'N, 0.7°E 680 m) located in the northern Guinea Savannah ecological zone of Nigeria. The experiment was laid out in split-plot design with three levels of nitrogen (0, 60 and 120 KgN/ha) in the main plot and ten periods of weed removal/infestation in the sub-plot. The productivity of both maize and rice was enhanced by increased application of nitrogen fertilizer in both years in all the plots that were kept weed free from the initial 6 weeks after sowing (WAS) up to harvest. Weed infestation for the initial 3 WAS enhanced the productivity of these crops. However, there was no significant response of nitrogen application in the plots that were kept weed-free for only the initial 3 WAS and those weeds infested throughout the crop cycle.