Using EmpiricalModels for Evaluation of Soybean Yield Loss at DifferentWeed Control Methods

In order to assess the soybean (Glycin max (L.) Merrill.) grain, oil and protein yield losscaused by interference of natural field weeds and the effects of weed management on weed dry weight loss an factorial experiment was conducted based on randomized complete blocks design with 3 replications at Research Farm of the University of Tabriz in 2008. Two factors included Johnsongrass water extract spray at four levels as No-spray, one spraying at 15 days after emergence (DAE), two sprays at 15 and 30 DAE and three sprays at 15, 30 and 40 DAE and current weed control methods in four levels as two hand-weedings, Trifluralin (pre-planting application), Trifluralin plus two hand-weedings and weedy infested. Integrated application of pre-planting herbicide and Johnsongrass water extract spray had severe effects on percentage of weed dry weight loss in comparison to application of only herbicide. Also application of water extract at three times showed considerable increase in weed dry weight loss compared to other treatments. The relative damage coefficient (q) of Kropff and Spitters model showed that weeds were more competitor than soybean (q=1.001 and 1.002). The protein yield of soybean suffered a greater reduction as weed density increased. The response of grain, oil and protein yield loss with relative weed dry weight and weed density were linear and hyperbolic, respectively. Increasing of relative weed dry weight decreased oil and grain yield and increasing of weed density decreased protein yield. Results based on model showed that weed density at 9.5 and 20 plant/m2 and relative weed dry weight at 5 and 10.2 caused grain yield loss by 5 and 10 %, respectively. This information about yield loss related to density and relative weed dry weight can be used to determine the acceptable yield loss and weed economic damage threshold.