A Model for Multiple Weed Competition of Common Cocklebur and Redroot Pigweed in Soybean

The presence of weed in soybean is a serious threat to crop yield. Little research has addressed multiple weed species interference in soybean. Field experiments were conducted at the Research Farm of University of Tehran, Karaj, Iran, during the growing season of 2007 and 2008 to determine the influence of density of single and multispecies populations of common cocklebur and redroot pigweed on soybean yield and competitive abilities of weeds. Common cocklebur and redroot pigweed seeds were sown concurrently with soybean and established at selected density within 15 cm on either side of soybean row. The experimental design was factorial based on randomized complete block design with three replications. Results showed that soybean grain yield was significantly affected by weed competition. Soybean yield loss due to competition was 86% and 70% in first and second experiment, respectively. Soybean yield was affected more by common cocklebur than by redroot pigweed. Common cocklebur at a density of 2.3(in 2007) and 6.25(in 2008) plants per m-1 of row reduced soybean yield by 50%. In contrast, same reduction in soybean yield from redroot pigweed was obtained at density of 5.9 and 12.1 in 2007 and 2008, respectively. Competitive effect of common cocklebur was equivalent with 2.37 and 1.91 plant of redroot pigweed density in 2007 and 2008, respectively.