Critical Period of Weed Competition with Sugar Beet in Qaen Region of South Khorasan

In order to investigate the critical period of weed control in sugar beet, an experiment was conducted during 2011-2012 in the experimental fields of Agricultural-Jihad training center of Qaen,. The experiment was conducted in a randomized complete blocks design with four replications using two series of different combinations of weed free and weed interference periods along with weedy and weed free checks as control. The first set of treatments were weed interference periods of 2, 4, 6, 8, 10 and 12 weeks after sugar beet planting, and the other set included weeding periods of above periods. Results showed the critical period of weed control based on 5, 10, 15, and 20% acceptable root yield reduction ranged between 10 - 106, 16 -80, 24 -78 and 31 -70 days after planting, respectively. A critical period of weed control for extractable sugar was observed to be 8 to 112 ,14 - 95, 22 - 82 and 29 - 74 days, after planting, respectively. The results showed significant difference between interference and weed free treatments on root yield extractable sugar, as well as weed dry weight. There was no significant difference for potassium content of sugar beet root among weed interference treatments, while it had significant difference among weed free treatments. In the results of this experiment the beginning of critical period control of weeds is 10 to 31 days and the end 70 to 106 days after planting.