Increasing lease and purchase costs for irrigated pasture are encouraging producers to use herbicides to control broadleaf weeds and maximise forage production on their current land base, as opposed to seeking out additional pastureland. There are two challenges to using herbicides to control broadleaf weeds in irrigated pasture. One challenge is that producers often encounter weed species for which herbicide information is limited or non-existent. Another challenge is that broadleaf herbicides tend to damage desirable legumes. We evaluated the efficacy of several broadleaf herbicide options for Symphyotrichum divaricatum, a species not previously the subject of research in irrigated pasture. Additionally, we monitored herbicide impacts on desirable legumes, principally Trifolium repens. We also evaluated control of Convolvulus arvensis and Plantago lanceolata, subjects of considerable herbicide research. Our goal was to identify herbicide options that controlled all three weeds while causing the least damage to legumes. Several rates and combinations of dicamba, diflufenzopyr, triclopyr, MCPA, and 2,4-D provided statistically indistinguishable control of C. arvensis and P. lanceolata, with point estimates suggesting 66–88% control in 2011 and 43–86% control in 2012. 2,4-D triclopyr appeared to provide the best control of P. lanceolata and C. arvensis, with point estimates for high rates indicating nearly 100% control in both years. However, this combination was particularly damaging to legumes. Based on these findings, we recommend 2,4-D triclopyr where desirable legumes are absent. Where desirable legumes are present, we recommend 2,4-D alone. Whereas 2,4-D alone appeared not to perform as well as 2,4-D triclopyr at controlling S. divaricatum in 2012, it was less damaging to desirable legumes.
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Vol. 66 • No. 9