Determination of The Critical Period of Weed Control in Fall-grown Safflower (Carthamus tinctorius

To determine the critical period of weed control in safflower (Carthamus tinctorius), two field studies were conducted in 2002 at the Bajgah and Kooshkak Experimental Stations, College of Agriculture, Shiraz University. Each experiment consisted of two series of treatments. In the first series, weeds were kept in place until crop emergence, leaf formation, stem elongation, lateral stem emergence, head emergence, flowering and ripening, and were then removed and the crop kept weed-free for the rest of the season. In the second series, crops were kept weed-free until the above growth stages after which weeds were allowed to grow in the plots for the rest of the season. The beginning of the critical period was defined as the crop stage in which weed interference reduced crop yield by 10%. Similarly, the end of the critical period was defined as the crop stage in which the crop had to be weed free to prevent more than 10% yield loss. The Weibull and MMF models were used to determine the critical period. The critical period of weed control occurred between safflower early stem elongation and flowering stages (144-220 days after planting, DAP) at the Bajgah site and between early stem elongation andearly head emergence stages (135-184 DAP) at Kooshkak. The onset of interference seems to be less variable than its conclusion, indicating that early weed control is necessary to prevent yield loss. Also, with increase in as the weed interference period increased so yield components (number of heads per plant and number of seeds per head) decreased significantly.