The critical period of weed control in corn) Zea mays) at Mashhad, Iran

In order to determine the critical period of weed control in corn, an experiment was conducted at Research Station, Faculty of Agriculture, Ferdowsi University of Mashhad in year 2000. The experiment consisted of two discrete periods, a weed-free and a weed infested period. The weed-free period consisted of removing all weeds till 4th (19 days after planting), 7th (34 days after planting), 11th (48 days after planting), 13th (52 days after planting), and 17th (59 days after planting), leaf growth stages and weed free check for growing season (123 days after planting), and the critical time of weed infested consisted of removing all weeds from fourth (19 days after planting), 7th (34 days after planting), 11th (48 days after planting), 13th (52 days after planting), and 17th (59 days after planting), leaf growth stages and weedy infested check for growing season. All treatments were compared in a randomized complete block design with four replications. The results showed that by prolonging weed-infested period, dry matter and leaf surface area of weeds per square meter were increased, but by increasing weed-infested period, weed density was decreased. The lowest weed density (23 weed plants per m2) was shown at infested all period of corn growing season. By passing the growing season, C4 weed species such as red root pigweed and barnyard grass were dominant compared with C3 weed species. At weed infested all period of corn growing season, red root pigweed had the highest dry matter (93% of all weeds dry matter) and leaf surface area (86% of all weeds leaf surface) compared with other weed species. At the first treatment of critical weed-free (19 days after planting) the regrowth of weeds were substantial which resulted in decreasing growth of corn. Grain yield of corn were significantly higher at 52, 59 and 123 days after planting in weed free treatments compared with 48, 52, 59 and 123 days after planting in weed infested treatments. However, there were no significant differences between other treatments. Based on the fitted sigmoid equation for both treatments at 10% decreasing grain yield compared with control, the critical period of weed control in corn was obtained between 20 till 56 days after planting (fourth till fourteen leaf stage) at Mashhad climatic conditions