Pigweed(Amaranthus retroflexus L.) Competition Effects on Grain and Biomass Yield of Sorghum (Sorghum bicolor L. Moench)

In order to investigate the competition effects between redroot pigweed and grain sorghum in different levels of plant density, an additive series experiment (response model) with factorial in randomized completely block design were conducted in 2005 growing season at research farm of Agriculture and Natural Resources Research Center of Khorasan Razavi (Torogh research station). Two factors were involved in the experiment, redroot pigweed density in four levels (0, 2, 8 and 16 plant m-2) and sorghum density in three levels (10, 15 and 30 plant m-2). Result showed that pigweed density had significant effects (p < 0.01) on biomass and grain yield of sorghum. By increasing pigweed density, biomass and grain yield of sorghum decreased significantly (p < 0.01), and maximum yield loss was obtain in 16 plant m-2 of weed density. Redroot pigweed growth indices and yield were significantly (p < 0.01) affected by sorghum density. By increasing density due to increased intra and inter competition, sorghum biomass yield decreased. Results of this study determined that sorghum density had significant effects on redroot pigweed and sorghum competition. By increasing sorghum density, competition ability of sorghum was increased and yield loss decreased. By increasing sorghum density, intra specific competition index of redroot pigweed decreased.