The critical period of weed control in corn (Zea mays L.) in north of Iran condition

In order to determine the critical period of weed control in corn (Zea mays L. cv. SC 704) in north of Iran, an experiment was conducted at Sari Higher Education Complex of Agricultural Science and Natural Resources, Mazandaran University in 2006. The experiment consisted of 14 treatments which laid out in a randomized complete block design with 4 replications. The treatment included weed free and weed infested period to 4^th leaf (V4), 6^th leaf (V6), 8^th leaf (V8), 10^th leaf (V10), 14^th leaf (V14), tasseling (T) and harvest stages (Full season weed infestation and weed control were the checks). According to farmer results, weeds dry matter increased by increasing the weedy period length. Continuation of weed competition to the later stages of crop development caused significant decreasing of ear length, rows per ear and kernel per rows and grain yield so that the total yield was decreased from 9.12 ton per hectare in full season weed control to 3.5 ton per hectare in full season weed infestation treatment. Generally, the peak competition period of weeds in corn crop was between V4 to V10 stages, thus if the weeds are controlled within this period, corn yield would be protected against a significant seed yield reduction.