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Horse Manure Compost Q&A

Apr 14, 2008 -

Here’s a recent question from a customer that I fielded… there is some info online about this but not much. If anyone else out there has an opinion or any tips or tricks for horse manure composting… let’s hear ‘em!


“We use the manure from our horses. When we worm the horses, we stop adding their manure to the compost pile for a week because the manufacturer says the wormer is out of the horses’ systems in three days. What is your opinion/advice on the wormer/manure/compost timing issue?

Short Answer:

You should be fine not segregating your manure if you can answer yes to the following:

  • Are you willing to lose 5% of your compost pile’s red worms (worst case scenario)? 95% of the wormer never leaves the horse
  • Is your manure exposed to sunlight (from the time it hits the ground to the time it hits the garden)? Sunlight breaks down the active ingredient in the wormer.

Long Answer:

Do you know the brand of wormer you are using?

I have horses and have looked this up in the past… the page I had looked up is here.

Here is what you want to know from that page:

“Will horse dewormers kill red worms? The most common wormer used is known by the brand name Ivermectin® made by the Merial Company. Merial’s research shows that the active chemicals in Ivermectin® are deactivated when manure is exposed to sunlight. Equine studies show that 95 percent of the active chemicals in Ivermectin® are deactivated in the horse before being passed in the feces. Leading experts in vermicomposting believe that the concentration of Ivermectin® in the horse manure is not high enough to seriously injure Eisenia.”

In the past I have used wormers with the above mentioned active ingredient, I have been trying some alternative to chemical/pharmaceutical worming (tobacco and other natural alternatives). See: ‘Natural Horse Wormer - A Gentle Alternative’.

What I started to do with all my horse manure after reading that sunlight would break down the chemicals was to muck it and dump it outside my compost pile, let it sit in the sun for a week or so and then add it to my compost pile. I do this with other ingredients of my compost too. My leaves, grass trimmings, some local soil, sand and horse and chicken manure are all in separate piles near my compost enclosure. I add my food compostables directly to my bin, then add other ingredients to keep the balance.

Having said all that… I have always had high amounts of red worms in my manure piles (even when using chemical wormers) and in the past I never made a point to segregate the manure right after worming.  In fact I worked on a horse ranch for a while and we composted the manure of all 30 of the horses there and never changed routine for the wormer… always had a huge worm population (it was monitored by ranch guests who wanted to go fishing wink

Currently I segregate the manure for other reasons:

A) the balance I mentioned above

B) I like it to dry out a little prior to composting.

I’m not sure if having it dry out is good or not (now that I type that, I question it). I guess since I segregate it and it dries out in the sun I have gotten used to it being dry when I add it. I like that it’s lighter and not as smelly when it’s dry… I assume that it still has high Nitrogen content… and when it’s dry it mixes easier with other compost materials. When you add wet manure it sticks together and sometimes will not break apart or mix with other materials… when it’s dry it crumbles.

Hope that helps!

melissa Says:
Feb 20th, 2012 at 1:17 pm

if you want an organic garden you can NOT use chemical wormers/vaccinations and use manure from these animals. You can feed your horses diatomacious earth and get rid of worms and that is organinc. I went on such a learning spree when I was so sick that I thought I would die…I learned about pesticides and how we DON’T need them…i learned to do more research and become educated—not by following the herd of marketing from the drug companies—the sell drugs to make money not to heal or help…!! really. true research show that some horses are pron to worms and some are not and you can forget about what the marketing companies say because they want to sell you money. I"m talking about horses that are fed and pastured on good land and taken care of, not starvation cases on slaughter lots. I stopped worning and chemically ‘vaccinating’ my horses 8 years ago…they look AWESOME and I compete every week with them and trail ride everyday. ps…they are barefoot, too! Good luck to you, hope you will do more research and think away from chemicals and poisons…they are killers.
peace and love

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