Forage Legume Establishment under Exposure to Progressive Declines in Aminocyclopyrachlor and Aminopyralid in Temperate Pastures

Abstract

Legumes such as alfalfa (Medicago sativa L.) and white clover (Trifolium repens L.) increase forage productivity and quality in northern temperate pastures, but require re-establishment following broadleaf weed control using herbicides. To quantify the residual effects of two herbicides (aminocyclopyrachlor and aminopyralid) on potential legume re-establishment we examined alfalfa and clover recruitment at two field sites over two years. Sites were over-seeded with alfalfa and clover to populate the seed bank, and then sprayed with herbicide, after which seedling densities were monitored in late summer and fall of the current growing season. Defoliation (via mowing) effects were also assessed to evaluate the role of vegetation competition on legume establishment. Herbicides were applied at recommended rates (1.0), and 0.5, 0.25, 0.125, 0.0625, and 0 times recommended field rates, emulating exponential herbicide degradation (one through four half-lives). Alfalfa and white clover seedling densities were negatively impacted by all rates of herbicide, with modestly greater negative impacts from aminopyralid than aminocyclopyrachlor, although responses to herbicides remained site and legume specific. Reductions in alfalfa and clover were particularly evident through the 0.25 (i.e., two half-life) herbicide rate, with reductions in alfalfa ranging from 78% to 95%, and in clover from 73% to 88%. Legume densities at the 0.125 (three half-life) rate were 39%–68% lower than those in nonsprayed control plots. Our results suggest that at least three half-lives of degradation must occur, and likely four or more, before these legumes can re-establish at densities acceptable for pasture production. These findings have implications for producers seeking to promptly re-establish forage legumes within pastures sprayed for broadleaf weed control in northern temperate regions.

Broadleaf weed control and white clover response to florpyrauxifen-benzyl + 2,4-D and common pasture herbicides

“…When considering fall versus spring herbicide applications, producers need to consider the weed species present to determine proper application timing, but also the specific herbicide to be used. Additionally, our findings are similar to those of other authors who have reported that commonly used pasture herbicides can result in high levels of desirable forage legume injury, and even death as was observed with aminopyralid (Beeler et al 2003;Harrington et al 2014;Mikkelson and Lym 2013;Miller et al 2020), aminopyralid þ 2,4-D (Enloe et al 2014;Payne et al 2010), 2,4-D (Payne et al 2010), 2,4-D þ dicamba (Payne et al 2010), and metsulfuron (Payne et al 2010. ) Although herbicides that contain florpyrauxifen-benzyl did significantly injure established white clover, the clover was not eliminated and recovery occurred during the trial period, indicating that this herbicide may be used in pastures containing white clover.…”

“…Another management concern when using herbicides is the elimination of desirable forage legumes such as white clover. Many common and widely available herbicides are frequently used to control broadleaf weeds in pastures and hayfields; however, the majority of these herbicides also kill desirable forage legumes (Beeler et al 2003;Miller et al 2020;Payne et al 2010). Forage legumes in pastures, including white clover, have several benefits such as increased forage quality (Posler et al 1993), which can ultimately lead to increases in livestock performance (Burns et al 1973).…”