Studies in selective weed control I. The control of annual weeds in winter wheat

Since 1943 numerous multifactorial field experiments have been concerned with the development of methods for selective weed control in a variety of crops. Between 1943 and 1947 simultaneous comparisons have been made as to the relative effectiveness of sulphuric acid, cupric chloride, dinitro-o-cresol and the two growth-regulating substances—2-methy1-4-chloro -phenoxyacetic acid and 2:4-dichloro-phenoxyacetic acid—when applied as sprays (100 gal./acre) for the destruction of annual weeds in winter wheat. There is a highly specific relationship between the weed species and the potential toxicity of any one compound. For the eradication of Ranunculus arvẹnsis and Scandix pecten-veneris, the two growth-regulating substances, as the sodium salts, are greatly superior, but for other species, such as Galium aparine, they.are wholly ineffective or only partially effective, e.g. Matricaria chamomilla. For Papaver rhoeas and Fumaria officinalis in the young seedling stage, sodium methyl-chloro-phenoxyacetate and ammonium dinitro-o-cresylate are equally toxic, while the indications are that Matricaria chamomilla and Centaurea cyanus are best killed with sulphuric acid or ammonium dinitro-o-cresylate. All these species are resistant or partially resistant to cupric chloride. As a result of weed eradication, increases in grain yield ranging from 6 to 113% have been recorded, and over the twelve experiments the average increase for the most appropriate treatment was 23%. At the concentrations employed for sulphuric acid (max. 18.4%) ammonium dinitro-o-cresylate (max. 1.1%) and cupric chloride (max. 4.0%) direct injury to the wheat, resulting in a loss in yield, has not been found when spraying is carried out in the active tillering stage. When spraying is unduly delayed, the yield depressions may be very considerable, more particularly with cupric chloride and sulphuric acid. With the growth regulating substances, the results of two experiments indicated that the greatest selectivity is obtained when the concentration does not exceed 0.2-0.3% (2–3 lb./acre), but in the remaining trials no injury was observed up to a maximum concentration of 0.5%. In none of the trials was the formation of abnormal ears noted. When mixtures of sodium methyl-chloro-phenoxy-acetate and dinitro-o-cresol are employed,. no significant interaction on crop yield has been found.