Interactions between reduced rate of imazethapyr and multiple weed species-soyabean interference in a semi-arid environment

Field experiments were conducted to evaluate Xanthium strumarium and Amaranthus retroflexus competitive ability with soyabean under unsprayed and sprayed (half the recommended rate of imazethapyr) conditions. Weed density combinations of 0, 4, 8 or 12plantsm -1 of row for A. retroflexus and 0, 2, 4 or 8plantsm -1 of row for X. strumarium were established alone, and in all possible combinations. When X. strumarium and A. retroflexus were grown at the same density, X. strumarium contributed 60-69% of the total weed biomass depending on weed density. The half rate of imazethapyr reduced the biomass of both weed species by 90% as compared with untreated plots. Untreated A. retroflexus and X. strumarium reduced soyabean yields to 1911kg and 1656kgha -1, respectively, while with the reduced rate of herbicide, the yield only decreased to 3612kg and 3453kg ha -1 respectively. The total weed density required to cause a 5% soyabean yield loss was 0.16 and 0.32plantsm -1 of row with no-herbicide treatment, while with the half rate of imazethapyr, it increased up to 2.78 and 4.32 plantsm -1 in 2007 and 2008 respectively. Owing to the higher competitive ability of X. strumarium, it should be given a greater priority for control than A. retroflexus in soyabean grown in a semi-arid environment. The half rate of herbicide significantly decreased the competitive ability of X. strumarium and A. retroflexus indicating that, in soyabean, application of this rate might be more cost-effective and more environment-friendly than the recommended ones. © 2012 The Authors. Weed Research © 2012 European Weed Research Society.Peer Reviewe