effects of redroot pigweed (amaranthus retroflexus) on yield and yield components of soybean

in order to study the effect of different redroot pigweed (amaranthus retroflexus) density on yield, yield components of soybean cultivar and determining competitive cultivar, a field experiment was carried out at mazandaran agricultural research station 2006. experiment was set up at a randomized complete block design with five rates of pigweed (0, 4, 8, 12 and 16 plant per square meter) and three cultivars of soybean (telare, sari and sahare) in factorial arrangement with four replications. for calculating pod loss, number of seed per pod, 100 seed weight and yield; three parameter cousense equation were employed. the results showed that, there were an inverse relationship between pigweed density and number of pod, seed per pod and 100 seed weight. the number of pod and 100 seed weight of grain decreased with increasing of the weed density, but the rate of pod number and 100 seed weight decrease in low pigweed density were more, than high density of pigweed. for this reason the relation between pigweed density with pod number and 100 seed weight was not linear, because in higher densities of pigweed by initiating intra-specific competition, the rate of competition reduced between weed and soybean, but between seed per pod and pigweed density there was a decreasing linear relation. the rate of number of pod per plant and weight of 100 seed decrease reduced with increasing the weed density. the soybean yield loss in 16 pigweed plant density in sari, telare and sahare were 46.33%, 57.4% and 58.31% relative to the weed control, respectively. in conclusion the competition ability of sari was more than other cultivars, and the amount of yield loss was lesser than telar and sahar cultivar.