Aminopyralid (Herbicide) Use in Pastures and Hayfields

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Aminopyralid is an effective herbicide used on pastures and some hayfields to control many of our problem summer weeds. It is found in products with the names GrazonNext HL and Milestone, as well as others, and provides excellent long-lasting control of noxious weeds such as horse nettle, pigweed (amaranth), ragweed, bitter sneezeweed, arrowleaf sida and thistles.

Aminopyralid works by mimicking the natural plant growth hormone auxin. As the herbicide is absorbed into plant tissue, rapid and uncontrollable growth occurs until the plant “grows itself to death.”  Grasses are highly tolerant to this herbicide because they rapidly metabolize the molecule and sequester it in special parts of the cell so it is no longer available for herbicide action. Many broadleaf plants cannot accomplish the metabolism/sequestration step and succumb to herbicide effects. Fortunately, aminopyralid targets a process that is unique to plants; therefore, humans and other animals are not affected by this herbicide. Livestock can actually graze treated pastures immediately after aminopyralid application because the herbicide quickly passes through the animal with no ill effects and is excreted in manure (urine and feces).

The residual effect of this herbicide continues to prevent seed germination several months after application and remains active in the manure of animals that have consumed the forage. Hay, straw, or composted manure from animals that have been fed forage (hay or fresh pasture) treated with aminopyralid or similar herbicides, such as clopyralid or picloram, should not be sold or moved from the farm from which it was sprayed and should never be used in a garden where susceptible plants- tomatoes, potatoes, lettuce, beans, and more- are to be grown. If an unwanted area has suspected aminopyralid contamination, a grass or corn crop planted, harvested and removed could help hasten the degradation of the herbicide.

Aminopyralid can be an effective tool for controlling noxious (and sometimes toxic) summer weeds in horse and livestock pastures. If buying or using composted manure from neighbors, always ask what herbicides have been used to avoid damage to susceptible plants. If the residual activity is a concern on your farm, contact the Extension office for help with selecting an appropriate alternative.