Response of Prosopis velutina Woot. to a selection of aerially applied herbicides

Prosopis velutina Woot. (mesquite) is a multi-stemmed woody weed, particularly of south-west Queensland, that has increased after substantial rain events, outstripping control efforts. Four glyphosate and four triclopyr (with and without picloram) based herbicide treatments were aerially trialled to determine their efficacy on dense infestations (~2000 plants ha [hectre]) of mesquite at three sites (dry, dry/wet and wet). Each site was infested with small, intermediate and large P. velutina. Small plants that were less than 1.5 m in height (generally less than one year old) were more susceptible to treatments. The triclopyr + picloram formulations at the higher rates with the addition of paraffinic oil and non-ionic surfactants, gave best and most uniform suppression of the weed. The triciopyr treatment with the additive alcohol alkoxylate produced similar results, but the long-term effect of this treatment, without the residual picloram component, mav be compromised. The herbicide actìvity of the triclopyr + picloram formulations was apparently enhanced in conditions where a higher water table persisted following rain immediately prior to herbicide application.