herbicides in compost

Asked June 3, 2020, 10:07 PM EDT

Can using horse manure (aged 2-3 years) in garden compost injure plants if herbicides had been used where the horse's feed was grown?

Umatilla County Oregon

1 Response

I'll be candid with you: I'm unsure.

I did find this publication from our friends at North Carolina State - the information would apply here as well:
Herbicide Carryover in Hay, Manure, Compost and Grass Clippings. https://lee.ces.ncsu.edu/2016/03/herbicide-carryover-in-hay-manure-compost-and-grass-clippings/

Very interesting and includes plenty of other information cited at the end.

For herbicides, you could run a small scale test by planting a few seeds of subject vegetables, like tomato, bean or marigold. WSU has developed some testing protocols, find them here: https://lee.ces.ncsu.edu/2016/03/herbicide-carryover-in-hay-manure-compost-and-grass-clippings/

I have several other concerns about horse manure:
  • Horses do not have a compound stomach, as cows do, and weed seed seems to survive their digestive process more than from animals with compound stomachs.
  • In the last 20-30 years or so, many horses are fed a daily worming dose, rather than a quarterly dose. It's possible that some vermifuge residues will pass the gut and exit with the manure. If present in sufficient concentrations, these could harm the worm population in your soil, if present.
  • ANY manure should be evaluated for storage conditions before being applied to gardens. Again, weed seed is the concern - if not covered, weed seed can blow onto the manure pile.
As with so many things in life, caveat emptor!