Critical period of crop-weed competition in rainy-season sunflower (Helianthus annuus)

A field study was carried out at the experimental farm of the Indian Agricultural Research Institute, New Delhi, to determine the critical period of crop-weed competition in sunflower (Helianthus annuus L.) during the rainy (kharif) seasons of 1995 and 1996. Trianthema portulacastrum L., Digera arvensis Forsk, Acrachne racemosa Ohuy, Dactyloctenium aegyptium Beauv. and Cyperus rotundus L. were the dominant weed species associated with the crop. Trianthema portulacastrum and Digera arvensis were the predominant weeds. Seed yield loss increased with the increase in the duration of competition and maximum loss (54.6%) occurred due to full-season competition. Seed yield of sunflower increased significantly with the increase in initial duration of weed-free condition up to harvest. Critical period of weed competition was found to be 20 to 49 days after sowing.