Prediction of Redroot pigweed (Amaranthus retroflexus) competitive effects on yield of grain sorghum (Sorghum bicolor L.Moench) by relative leaf area

In order to prediction competitive effects of redroot pigweed on yield and yield components of grain sorghum by observation of relative leaf area an additive series experiment (response model) with factorial in randomized completely block design (RCBD) were conducted in 2005 growing season at farm research of khorasan Agriculture and Natural Resources Research Center (Torogh Research Station). Two factors were involved in the experiment, redroot pigweed density in four levels (0, 2, 8 and 16 plant/m2) and sorghum density in three levels (10, 15 and 30 plant/m2). kropff three parameter model was used for data analysis and predicting of competition and relative yield loss coefficient (q). Result of this study indicated that sorghum is sensitive in competition and has sever damage. The economical yield reducing at 10, 15 and 30 plant/m2 density due to red root pigweed competition was 0.11, 0.38 and 0.22 kg/m2 respectively, where as the biological yield loss was 0.6, 1.27 and 1 kg/m2. Prediction of model from relative yield loss coefficient (q) indicated high competitiveness of pigweed and also faster leaf area spreading. Result showed that this model is very suitable to describe competition effects and to determine relative yield loss coefficient (q) by observation of relative leaf area.