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Herbicides in Horse Manure


February 8, 2021

Hi.  I am in zip code 87048. My question is not specific to my zip code but rather has to do with the herbicidal safety of horse manure used in compost or as a soil additive.  I do have two compost barrels that are mounted horizontally and are relatively easy to hydrate with a hose.

I have been told that using horse manure from horses not fed an organic, herbicide free type of hay may be hazardous to the plants upon which the mature compost is spread.  Is this true?  I used to have horses on my property and regularly used the manure to feed the grasses without any problem.  If herbicides are a problem, is there a way to decompose the active chemicals so that the manure can be safely used?

I would appreciate your help.


Answer by JZ: Important question. I would encourage your to read herbicide fact sheets from the National Composting Council. There is a link to that site on our Persistent Herbicides page under Composting Info in our website menu.

Persistent residual broadleaf herbicides, if there are any in the manure will usually only affect broadleaf plants, but not grasses. There are a variety of persistent herbicides in use.  The only way to know if horse fodder is contaminated with them is by going back to the farmer who grew the horse food.  And to find out which specific chemical was used.

My understanding is that it requires 2-3 years to biodegrade some of these herbicides. In our area a well managed composting setup may produce a finished compost in about 12 months, depending on what process was used. So if you are concerned that your manure is contaminated, then after it has decomposed in your composting setup, “cure” it  for 2 more years. Pile it and expose it to air and sunshine. You may use a light tarp as a drape on top of pile, uncover from time to time to expose to sun and air.   That should allow for the biodegradation of any residual persistent herbicides.  Others may have other recommendations for you.

Let us know if this has been helpful and it you have more questions.

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