published in


August 2016

Residual Weed Control Following Aminopyralid Applications

Brent Sellers and Jay Ferrell

For questions or comments regarding this publication contact: Brent Sellers, University of Florida, IFAS

Aminopyralid is an active ingredient in Milestone, GrazonNext HL (aminopyralid + 2,4-D), and Chaparral (aminopyralid + metsulfuron). This herbicide was introduced into Florida pastures primarily for control of tropical soda apple (TSA) in 2005. While this herbicide has been used on most ranches and we have documented its activity on tropical soda apple plants and residual activity on TSA seedlings, we continue to learn about its performance. Soon after the introduction of GrazonNext, we quickly learned that tank-mix partners were needed to obtain satisfactory control of many species, including dogfennel and goatweed. What we still don’t fully understand is the effect of aminopyralid on germinating weed species other than TSA. Our observations in pastures where GrazonNext HL had been applied indicate that this herbicide has the ability to suppress weed seed germination in bare-ground areas compared to products like Pasturegard, which does not have soil activity. This has led to more recent research on the residual activity of products containing aminopyralid.

Research was conducted in 2014 in a bermudagrass pasture to compare GrazonNext HL (24 oz/A), Chaparral (2.5 oz/A), 2,4-D amine (64 oz/A), and WeedMaster (48 oz/A) by examining broadleaf weed emergence following application.  Treatments were applied in May and weed emergence was recorded 90 days after treatment. The primary weeds in untreated plots included dogfennel and spiny pigweed. Data revealed that spiny pigweed did not emerge in plots treated with GrazonNext HL or Chaparral (100% control) within 90 days after treatment, but was commonly found in plots that were treated with 2,4-D or WeedMaster (<75% control). The story with dogfennel was somewhat similar with >90% control following applications of GrazonNext HL and Chaparral, but  only 55% control following WeedMaster and 75% following 2,4-D amine. After this 90 day period, it was common to find spiny pigweed begin to emerge in all plots, but dogfennel emergence was fairly low throughout the experiment. It appears that products containing aminopyralid do in fact have residual activity on weeds other than TSA, making these herbicides an effective means to proactively manage weed species in pastures. This could be especially true early in the growing season and allow the forage grass to grow and potentially shade out any emerging weed species.

Annual applications may be necessary in some pastures where weed pressure was high prior to treatment. In cases where dogfennel density covers more than 50% of a pasture, retreatment will likely be necessary so the weed seeds in the soil are reduced to a low level. One question we often hear is “how many years do we need to apply herbicides to keep the dogfennel from reestablishing and taking over?” While many factors can contribute to dogfennel establishment, 2-3 years of applications may be necessary in some situations. An experiment was established in 2016 in a pasture treated with GrazonNext HL (24 oz/A) + Pasturegard HL (12 oz/A) in 2015. Treatments in May 2016 included no herbicide and an application of GrazonNext HL (24 oz/A) + Pasturegard HL (8 oz/A). No dogfennel were observed at the time of application in any of the plots. However, by July we had observed substantial establishment of dogfennel plants in plots that were not treated in 2016, and dogfennel density was at least 5 plants/m2 by December 2016 (Figure 1). We will continue this experiment in 2017 and 2018 so that we can continue to evaluate the effects of annual applications on dogfennel establishment; by the end of this experiment we will be able to compare dogfennel densities in plots treated 1, 2, 3, or 4 times.

Overall, we are observing fairly good residual activity on several weed species when products containing aminopyralid are applied to pastures. The length of residual activity will likely vary from location to location due to environmental conditions, the prior management level of the pasture, forage density, and the weed seed bank, among others. Regardless, the fact remains that multiple steps are often required to bring a weedy pasture back into full production. One year of herbicide usually is not sufficient to reverse several years of weedy conditions.

Figure 1. Visual comparison of dogfennel control with a single application of GrazonNext HL + Pasturegard HL in 2015 (left of pvc pipe in picture; plots were mowed prior to taking this picture, but dogfennel stems are evident) versus 2 annual applications of GrazonNext HL + Pasturegard HL in 2015 and 2016 (right of pvc pipe in picture. Photograph by J. Matson in December, 2016.

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