The critical weed-free period in organically-grown winter wheat

Two experiments were conducted in central southern England between Septembe r 1994 and August 1996 to identify the critical weed-free period in organic ally grown winter wheat (Triticum aestivum, cv. Mercia). In competition wit h a mixed weed infestation of predominately Alopecurus myosuroides and Trip leurospermum inodorum it was found that wheat yield decreased as the durati on of the weed-infested period increased and that the crop needed to be kep t free of weeds from sowing in order to completely avoid any yield loss. Al so, weeds emerging in the wheat crop (predominately T. inodorum) during the growing season had a significant and detrimental effect on yield. The exis tence of the critical period, therefore, depends on the imposition of an ac ceptable yield loss. If a 5% yield loss gives a marginal benefit compared w ith the cost of weed control, the critical period will begin at 506 degrees C days after sowing (November) and end at 1023 degrees C days after sowing (February). This information could be used by farmers to target mechanical weeding operations to control weeds at a time that will have maximum benef it to the crop.