Control of Silky Crazyweed (Oxytropis sericea) with Aminopyralid + 2,4-D and Picloram + 2,4-D on Native Rangeland

Techniques for preventing crazyweed toxicity in livestock have generally fallen into two categories: excluding livestock access to infested ranges during early spring and fall, and controlling crazyweed populations through herbicide application. Although picloram has been used to control crazyweed effectively in the past, aminopyralid has shown efficacy at lower application rates, exhibits less potential off-target movement, and has been classified as a reduced-risk product. Differences in the response of silky crazyweed and nontarget grasses and forbs to picloram + 2,4-D and aminopyralid + 2,4-D were investigated. Picloram + 2,4-D was applied at a rate of 0.3 kg ae ha−1 picloram + 1.1 kg ae ha−1 2,4-D, and aminopyralid + 2,4-D was applied at a rate of 0.1 kg ae ha−1 aminopyralid + 1.2 kg ae ha−1 2,4-D. Silky crazyweed canopy cover, number of flowering stalks, plant size, and biomass decreased 15 mo after herbicide treatments (MAT) with average percentage of relative reductions of 92, 95, 90, and 99%, respectively. Crazyweed density decreased by 1.5 ± 0.2 SE plants m−2 and 1.3 ± 0.2 plants m−2, a relative reduction of 95 and 80%, 15 MAT in aminopyralid + 2,4-D– and picloram + 2,4-D–treated plots, respectively. Plots treated with aminopyralid + 2,4-D had 4% lower nontarget forb canopy cover than did picloram + 2,4-D plots 15 MAT. Grass biomass remained similar within treatments over time for control, aminopyralid + 2,4-D and picloram +2,4-D plots, and was similar in all plots 15 MAT. Plots treated with herbicides had, on average, 11% greater grass cover than did control plots 15 MAT (aminopyralid + 2,4-D: 89%; picloram + 2,4-D: 85%; control: 76%). Nomenclature: 2,4-D; aminopyralid; picloram; crazyweed, Oxytropis spp. , silky crazyweed, Oxytropis sericea Nutt. Management Implications: Silky crazyweed is a widely distributed and economically damaging poisonous plant that contains the toxin swainsonine. When ingested, swainsonine causes emaciation, abortion, and birth defects in domesticated livestock as well as wildlife species. Typical toxicity prevention methods include limiting livestock access to infested ranges during early spring and fall and controlling silky crazyweed populations through herbicide application. Picloram + 2,4-D has been consistently effective in killing silky crazyweed, but a new herbicide, aminopyralid + 2,4-D, may be a better option because it is effective at lower rates, has a shorter average half-life, and has less potential for off-target movement. The effects of these two herbicides on silky crazyweed and nontarget forbs and grasses were compared at three sites in northern New Mexico. Picloram + 2,4-D and aminopyralid + 2,4-D had similar effects on silky crazyweed density, canopy cover, number of flowering stalks, plant size, and biomass. Nontarget vegetation also responded similarly to the two herbicides with grass biomass, canopy cover, and bare ground all having similar values following treatment. Forb canopy cover was lower in aminopyralid-treated plots than it was in picloram-treated plots, suggesting less selectivity with aminopyralid. This could have major implications for important wildlife forage species, but may be beneficial when multiple undesirable forb species are present. Overall, both herbicides effectively controlled crazyweed and increased grass cover.