摘要：Weed-crop competition is a complex field of study. The extent of competition is governed by a number of factors including crop species, crop cultivar, crop density, weed species, weed density, the relative time of emergence of the crop and weed, the duration of the weed presence, the efficiency of weed control and soil and environmental factors . The greatest single cause of economic loss is, however, a reduction in yield due to weeds competing with the crop for available light, nutrients and moisture. The effect of Phalaris minor and Avena fatua on wheat was investigated at agronomic research area, College of Agriculture, Dera Ghazi Khan during 2007-2008. The experiment was laid out according to Randomized Complete Block Design (RCBD) having four replications and a net plot size of 5x2.5m. Plant density of 2, 4, 6 and 8 m-2 for Phalaris minor and Avena fatua was maintained. The data collected were analyzed using appropriate statistical techniques. Maximum plant density of wheat (96.500) was observed in T1 (control). All other parameters like total number of tillers (403.25), number of fertile tillers (400.25) and maximum plant height (86.750) was also observed in T1(control). Growth and yield parameters like number of spike lets/spike, number of grains/spike, 1000 grain weight, grain yield and biological yield showed significant response in the presence of Phalaris minor and Avena fatua under study. Maximum grain yield (7.0500 t/ha) was obtained in T1 (control) and T2 (6.8500 t/ha) and T6 (6.8750 t/ha), respectively. Maximum biological yield (14.875 t/ha) was obtained in T1control) and T2 (15.750 t/ha) and T6 (15.975 t/ha), respectively. It can be conclude from the current study that presence of weeds ( Phalaris minor and Avena fatua ) in wheat has remarkable adverse effect on the yield and its contributing factors. As the density increases, the crop performance is gradually declined.
