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Are weed control and wildlife mutually exclusive?

Matt Booher for Progressive Forage Published on 15 July 2020
Wedd control and wildlife

I was spot-spraying autumn olive that had encroached into the field when I noticed a bobwhite quail scurry through the grasses and into the fencerow.

Over the next weeks, my family and I watched and listened to a covey of them as they moved about our property, the first time in decades. Wildlife are abundant around my unimproved hay meadows, even though we haven’t actively managed for them. I work with lots of farmers who are in a similar situation, as well as many who are intentionally trying to improve habitat and promote wildlife.

In addition to wanting what is best for quail, pollinators, rabbits, songbirds and trout, we all seem to have one other thing in common: the threat from invasive weeds. You can probably relate.

I often see well-intentioned landowners refuse to use herbicides, under the assumption that all chemicals are equally harmful to wildlife or that their effects are simply unknown. Unfortunately, many of these people end up using a lot of herbicide to stop a small problem grown large. Some even rotate out of perennial grassland into annual crops in order to clean up out-of-control invasives.

As a former commercial applicator, and now as an extension agent, I always operated under the assumption that when used in accordance with the label, most herbicides present no risk to wildlife. I’ve come to realize that neither of these positions is good, since they are based off of assumptions rather than science. I decided to research the risks to wildlife from grassland herbicides, with a goal to identify some options that can be used with a high level of confidence in regards to their safety to wildlife.

The EPA and the U.S. Bureau of Land Management (BLM) conduct an ecological risk assessment (ERA) on any herbicide before it can be used. I turned primarily to these assessments as my source of information because of their thoroughness, and because part of the BLM’s mandate is to manage for the health and safety of wildlife.

ERAs use peer-reviewed studies to evaluate the short-term effects of various herbicides on wildlife, and they are very conservative in their approach. For example, toxicity levels are often determined in association with rates at or above the high end specified by the label. In addition to looking at acute effects on wildlife, they also attempt to evaluate longer-term impacts on growth and reproduction, the impact of chemical metabolites, as well as the possibility of chemicals to bioaccumulate in the food web. ERAs do a good job of identifying risks within the context of real-world exposure scenarios – for example, via direct spray, consumption of contaminated vegetation or insects, spray drift or runoff, or by accidental spills.

While I am not an expert on this topic, I’ve done my best to read and understand these ERAs for the most commonly used active ingredients in pasture and hay herbicides. Here is some of what I learned:

2,4-D

2,4-D is a widely used broadleaf herbicide, available in ester and amine formulations. Over the range of typical 2,4-D ester application rates, adverse effects such as reduced growth or mortality are plausible on aquatic animals (fish, insects, amphibians) in association with direct spray to water or accidental spills. (So don’t apply where spray may drift or run off into surface water.). 2,4-D amine presents less risk to aquatic life, and at typical application rates, adverse effects on wildlife are likely possible only in the event of an accidental spill.

At typical spray application rates of either formulation, non-lethal effects such as weight loss and developmental or reproductive impairment are plausible in mammals that consume contaminated vegetation or insects. Birds and terrestrial insects appear to be substantially less sensitive than mammals to 2,4-D. See Table 1 for product and formulation information.

Grassland herbicides and their active ingredients

Triclopyr

Triclopyr is used extensively for control of woody plants, and is available in ester and amine formulations. There is a potential for subclinical adverse effects (such as reduced growth and reproductive impairment) in large mammals consuming vegetation contaminated with either formulation of triclopyr. However, triclopyr is not likely to cause adverse effects in small mammals and birds. At high application rates, triclopyr ester can pose acute risks or mortality to aquatic insects, amphibians and fish; no risks are apparent for aquatic wildlife exposed to triclopyr amine across the range of labeled rates.

Dicamba

Dicamba is a widely used broadleaf herbicide. At typical rates used in grassland settings, no adverse effects are plausible for mammals, birds or terrestrial insects. While dicamba is relatively non-toxic to aquatic wildlife, very little information exists to enable a good assessment of chronic risks to them.

Aminopyralid, fluorpyrauxifen and fluroxypr

The active ingredients aminopyralid and florpyrauxifen are practically non-toxic for both terrestrial and aquatic wildlife, even under a direct spill scenario. Fluroxypr is also considered non-toxic under spray or runoff scenarios, with expected risks present to fish and aquatic invertebrates only in the case of a direct spill.

Surfactants

Fish screens and herbicides that aren't toxicSurfactants are often cited as posing greater toxicity to aquatic life than many of the pesticides with which they are used. However, the standard non-ionic surfactant used at normal rates appears fairly non-toxic to aquatic life and shouldn’t create additional risk beyond that of the herbicide it is used with. In the case of basal bark or cut stump applications, it would be wise to use a basal oil rather than diesel or fuel oil as a carrier. Basal oil is significantly more expensive, but less toxic and potentially more effective.

All of the active ingredients mentioned in this article were deemed low risk for bioaccumulation.

So are herbicides harmless tools or harmful toxins? As usual, the truth lies somewhere in the middle. We must consider scenarios where abstaining from herbicides can result in the loss of habitat for the very wildlife we are trying to protect by not spraying. Sometimes our other weed control options (e.g. burning, mowing) are less environmentally friendly and less effective than herbicides. I’m optimistic about what appears to be a growing safety of the herbicides available to us; however, we still need to be careful stewards of our land and wildlife.

Practice common sense.

1. Use typical application rates rather than maximum rates, and use individual plant treatment when possible.

2. Follow label guidelines on stream buffers.

3. Avoid conditions where spills or direct spray will contact known habitat.

4. Time spray applications to limit risk to wildlife populations, for example, outside of nesting season or during cooler weather when pollinators are less active.

I hope this has helped provide some information and confidence to help you make informed decisions on your land. end mark

PHOTO 1: Not all herbicides are threats to wildlife but knowing which ones are can help you manage weeds without harming the species you’re trying to cultivate. Photo by Lorien Koontz.

PHOTO 2: Fish screens and herbicides that aren’t toxic to aquatic life create better pastures for wildlife while also enhancing yield for cattle. Photo by Lynn Jaynes.

Matt Booher
  • Matt Booher

  • Virginia Cooperative Extension
  • Rockingham County
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