The critical period of weed competition and its application in organic winter wheat

Two experiments were conducted in central southern England between September 1994 and August 1996 to identify the critical weed free period in organically grown winter wheat. In competition with a mixed weed infestation of predominately Alopecurus myosuroides and Tripleurospermum inodorum it was found that wheat yield decreased as the duration of the weed infested period increased and that the crop needed to be kept weed free from sowing in order to completely avoid yield loss. Also, weeds emerging in the wheat crop (predominately T. inodorum) during the growing season had a significant and detrimental effect on yield and there was no point when weeding could be relaxed to avoid a yield penalty. Therefore. the existence of the critical period depends on the imposition of an acceptable yield loss. If a 10% yield loss gives a marginal benefit compared with the cost of weed control, the critical period will begin at 645 degrees Cdays after sowing (November) and end at 1223 degrees Cdays after sowing(March). This information could be used by farmers to target mechanical weed control operations to control weeds at a time which will have maximum benefit to the crop.