Information for Horse Owners and Stables

Here are some helpful facts about aminopyralid for horse owners and those in charge of livery yards, racing stables, riding schools and other equestrian enterprises.

What is aminopyralid?

Aminopyralid is the active ingredient in Milestone®, Milestone VM, Milestone VM Plus, ForeFront® R&P, Chaparral, GrazonNext, Opensight and CleanWave herbicides. These herbicides are among the most effective for controlling and eliminating many weeds found in grassland, including thistle, ragweeds, knapweeds and star thistle. When grassland is sprayed with aminopyralid, it kills the target weeds but does not affect the grass. When this grass is eaten by horses, either out in the field or as hay or silage, a small amount of aminopyralid may pass through in the resulting manure.

Is aminopyralid harmful to my horse?

No. The herbicide passes through an animal’s digestive system unchanged and does not bioaccumulate. However, you must follow all labeling including

  • After ingestion, aminopyralid requires three days to completely pass through an animal that has grazed on grass or hay that contains aminopyralid, so manure or urine produced during this three-day period can only be used to fertilize rangeland, permanent pastures or wheat.
  • There is no aminopyralid-related restriction on the use of manure produced by animals that have grazed for at least three days on untreated forage or pasture. 

How long does it take for aminopyralid to break down?

Aminopyralid is usually decomposed over the growing season by microorganisms in soil. Residues in manure alone break down slowly, but may break down faster if incorporated into soil and rototilled or turned over regularly.

How can I tell if aminopyralid is in manure?

We recommend a bioassay testing method.

What should I do with affected manure?

Manure that may contain aminopyralid must not be given or sold to gardeners as it may affect sensitive plants. Affected manure may be spread on rangeland, permanent pastures or fields where wheat will be or is being grown.

What Can I Do?

  1. Be informed.
  2. Ask your hay or forage supplier if their product came from a farm or grassland where herbicide products, such as Milestone®, were used. Being aware allows you to be able to take the necessary precautions to prevent affected manure from reaching gardens..
  3. Test the manure.
  4. If you believe you might have manure containing aminopyralid, please contact the Dow AgroSciences Customer Information Group at 1-800-992-5994 and select Option No. 4; or e-mail