ZEA MAYS دوره بحراني كنترل علف هاي هرز دو رقم ديررسي و متوسط رس ذرت دانه اي

The critical period of weed control in two late and medium maturity grain maize(Zea mays L.) cultivars

To determine the critical period of weed competition and adequate time for weed control an experiment was conducted with two factors including maize varieties (SC-704 and SC-604) and weeding times (12 times of hand weeding) in Kermanshah province in I999 cropping season. The weeding times were divided into two sets; including weed interference util 0, 2, 5, 10 leaf stages, tasling and silking stages and weed removal at these stages. These two sets of data were analysed and Logistic and Gompertz equations were fitted, respectively. With a 5% yield reduction tolerance, SC-704 had a critical period of 5-8 leaves stages (26-37 days after emergence) for grain yield and a period of 4-12 leaves stages (23-47 DAE) for dry mass. In SC-604 the critical periods for grain yield and dry mass were 3-9.5 leaves stages (20-41 DAE) and 2-13 leaves stages (17-51 DAE), respectively. Based on accumulated degree days (GDD), the critical period of weed competition for grain yield and dry mass in SC-704 and SC-604 were 318-471 and 286-644 and 256-556 and 206-676 GDD, respectively. Wild Licorice (Glycyrrhiza lepidota), field Bindweed (Convulvulus arvensis) and Common purslane (portulaca oleracea) were the dominant weeds in the early growing season, while Johnsongrass (Sorghum halepense) and Camelhorn (Alhagi camelorum) were dominant at the end of the growing season. The number of weeds and its dry weight in weed free treatment decreased by 75 and 81% in comparison with control for 35 days after crop emergence. The whole season weed interference with maize reduced the yield and dry mass by 80 and 86% of weed free control.