How Harmful are Vermicides in Manure?

Aged Manure

As I’ve written many times here on the blog, aged manure is hands-down one of the best materials to add to a vermicomposting system. Horse manure, in particular (especially when mixed with bedding materials), happens to be my personal favorite! Some of the most successful populations of Red Worms (highest densities, largest worms) I’ve ever encountered have been living in old outdoor heaps of horse manure, such as the one pictured above.

Manure use is not without certain “risks”, however – a prime example being the potential presence of of deworming medications (frequently given to various types of livestock). In all honesty, I’ve never really worried too much about this myself – likely due to the fact that I just haven’t I seen any evidence of worms being killed off in any of my own manure-fed systems or in the heaps where the worms “naturally” occur. Quite the opposite in fact!
Nevertheless, I’ve received a lot of emails about this issue (many of them in the past couple of months strangely enough), so it’s clear that quite a few people are concerned about this.

While there doesn’t seem to be a LOT written about this topic, I was recently pointed in the direction of two interesting info resources and thought some readers would find them valuable as well.

Firstly, I want to thank Carolyn O. for sharing the following article: Is Horse Manure Safe For Organic Gardens?

I was happy to see what the overall conclusion/recommendation was basically in-line with the advice I typically give people re: using manures – i.e. when you let manures age outdoors for a period of time before using in vermicomposting systems you are far less likely to encounter any issues (I’ll talk a bit more about some other issues with fresh manure in a minute).

Here is an interesting blurb:

Fortunately for gardeners and animal owners, most research to date indicates that Ivermectin, the vermicide most frequently given to horses, cattle and sheep, breaks down quickly once it is excreted. Several studies have shown that Ivermectin degrades rapidly when manure is hot-composted or exposed to sunlight, and somewhat less rapidly when manure is simply piled up and left to decompose.

A study of Ivermectin-treated sheep found the half-life of the chemical in sheep manure to range from seven to ten days. Because of quick decomposition, Ivermectin preparations given to animals have not been found to build up in manure-amended soil.


Thanks also to Sharon K. who recently shared this interesting document: Vermicomposting Horse Manure

It contained some reassuring words as well:

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.

It’s really important to remember that there can be other dangers associated with using fresh manure (or generally, manure that hasn’t been exposed to outdoor conditions). Likely the most significant of these is ammonia volatilization. Ammonia gas is very toxic to earthworms, and will kill your herd in no time flat even when present in relatively low concentrations (especially dangerous in enclosed systems). Another potential hazard with certain types of manure (and manure handling practices) is salt content. Earthworms also tend to be quite sensitive to relatively low salt concentrations.

In other words, don’t necessarily assume that it’s vermicides killing off your worms if you encounter problems with certain batches of manure – especially if the manure is relatively fresh!

Here are my bottom-line recommendations for using manure:

1) If you plan to use manure as a starting material (habitat) for a vermicomposting bed make sure it is VERY well aged (outdoors) – at least 2-3 months old (preferably having undergone some sort of “hot composting” stage).

2) If you are feeding manure to a well established (open) system – especially if it’s quite large – you can almost certainly get away with layering much newer manure on top since the worms can simply avoid it until it is “ready”.

3) Unless you are very experienced with using manure as a “worm food”, I don’t really recommend using it in enclosed plastic bins at all. The potential risks associated with ammonia release are just too great in my humble opinion.

Anyway – interesting topic for sure! Please share your thoughts!

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  1. I have been using horse manure for several years with no bad results. Like Bentley I find it one of the best feeds for my worms. I do make sure it has been aged for several months and that it has been outside so it gets some leaching from the rain.

    • Susan from Montana
    • February 19, 2011

    Thanks Bentley and Sharon K. for the article on horse manure. Today, I talked to someone about getting some of their manure for my worms. But since they deworm their horses, I was afraid to use it. Guess I’ll give it a try!

    • Rob Boyd
    • February 20, 2011

    Thanks Bentley for the information that you share with us all.
    I have been using horse manure for several years and the dewormers and ammonia have always been a concern.
    What I have done is taken a 5 gal bucket and drilled holes in the bottom set it inside another 5 gal bucket, fill the bucket with horse manure and then cover it with water,let it soak for a few hours,lift the bucket up and let it drain and then repeat the process.
    That seems to wash out any dewormer and ammonia that might be left and I have never had any problems loosing worms.
    The manure tea that is left over gets dumped in the garden.

    • geoff
    • February 21, 2011

    i have been succesfuly using horse and cow manure in all my beds, indoor plastic tubs, outdoor large bins etc. for over a year now. before i started raising worms i read of people leaching manure up to as many as seven times but no less than three times so i followed suite and have been leaching all of my manure. one day i decided to see if this leaching was nessesary so i set up a bin with the aged manure without leaching it. i applied about a half a pound of worms and they started squirming like they didn’t like something, but i waited. less than five minutes later the worms were all dead. i know this manure had been sitting out well better than three or four months so dont trust that all the poison is gone without testing small portions.

    • Lorne
    • February 22, 2011
  2. Great article, I am new to the blogosphere yet wanted to chime in. I want to switch the topic beyond the harmful chemicals in the manure and concentrate just on the manure as the home for your worms. Worms are amazing creatures that can withstand many harsh environments and having them in straight manure from any animals is possible, vermicides or not. However, the key is what you are trying to accomplish with your worms. For me I have worms to create the best vermicompost for my garden. Having worms only in manure will create an end result of high salt concentrated vermicompost. Worms will eat the manure decreasing the volume of the manure by 60%; however, the salt concentrations stay extremely high. I put my worms in “humus” and then feed them the most diverse diet possible to create the most diverse vermicompost. I would just say be wary of only using manure and attempt to make a “home” for your worms using manure, carbon sources, green waste, and especially foods wastes. Thanks for the great article and I will be waiting for more.

  3. Lorne- I wonder has anyone done any research on whether earthworm action increases the rate of breakdown of those broadleaf herbicides if the contaminated hay or manure is composted? I presume it doesn’t have a huge effect or well rotted horse manure would be declared ‘safe’.

    • heather rinaldi
    • February 28, 2011

    Really good info–i use aged horse stall waste in all my open systems, but not in the Rubbermaid bins. Variety of materials is also key to good vermicompost–moderation in all things is a good point for worms and us worm compsoters.

    • Lorne
    • March 1, 2011

    Catherine – as far as i know most herbicides are broken down soil microbes. apparently these pyridine carboxylic acids remain intact even after composting.

    you started me thinking and surfing – worms already use calcium carbonate in the digestive process so maybe these carboxylic acids can be nuetralised as well

    best option would be to never use them in the first place but that’s a huge multi-billion dollar discussion with some very dubious implications for the stability of global food production


  4. Currently, with alpacas, we are using the ivermectin solely for getting rid of menengeal worm from white tail deer exposure in the Northeastern US. This means we can’t use it for the other parasites they get. Ivermectin is also what is used in Heartguard for dogs. There are many more wormers than just this one being used for ruminants, including goats, sheep, llamas, alpacas, because of parasite resistance to the older drugs. Consequently, any manures I use with my worms will be composted, set up in windrows, and allowed to age and leach from the rain and the sun exposure to minimize this threat.

    • Nana
    • March 11, 2011

    I’m a horse facility manager and home worm composter, and I have been reluctant to use the plentiful manure in my worm bin for fear of killing off my little wigglers. We age our waste in large, hot manure-only piles (no urine or bedding) that are open to the elements — perhaps I should experiment a bit with months-old manure as an occasional additive in my own bin. Our horse facility is about to lose the space currently occupied by our manure aging field due to the construction of a new structure, and I’ve been debating about how to implement a space-saving compost system that can handle about 300 lbs/day of horse poop that hasn’t had the luxury of lying around in the ageing field for months. The decision to compost, rather than relocate the aging piles, is motivated by envrironmental concerns (e.g. runoff), fly control issues, and also a desire to dedicate less of our limited space to poop storage. Do you think several big (5’ x 5’), open, wood frame worm bins might work with adequate bedding/aerating materials added, or does manure vermicomposting require a less constricted system like trenches/windrows?

    • Bentley
    • March 11, 2011

    Great discussion everyone! Thanks for chiming in!

    Nana – I would DEFINITELY recommend using some of that well aged yummy goodness (sorry, that was my inner worm talking)! As for your system idea – sounds good. Just make sure they aren’t too deep – you might end up with overheating issues otherwise. If you are adding fairly fresh material just make sure to create a nice worm habitat (use some of that old stuff) beforehand, and layer the new stuff on top.

    • Rick Szekeres
    • March 30, 2011

    I have been a successful hot composter for about three years now. I hot compost the manure/bedding from 11 of my neighbors horses… I call it a backyard community horse composting operation. Using the loader on my subcompact tractor, I make windrows that are 6’x6’x24′ during the Winter months. As the pile compacts/decomposes, I add fresh manure to the top (mainly to save on space). Provided there is adequate moisture, the pile generally runs 140-170°F all Winter/Spring and eventually cools down to about 80-100°F by Memorial Day, when I harvest the whole batch and use it for various, on-site purposes: garden amendments, organic mulch, levelling the yard… and even filling in those old groundhog burrows in the pasture. I am not really satisfied with the final product and want to do additional processing… either screening through 1/4 mesh (would take a long time by hand) or better yet, vermicomposting. The question I have is when is it appropriate to add my mass of worms to the outdoor windrow? I understand that the temperature needs to be roughly 35-90°F all the time for the worms to survive, but what about adding them to a pocket that is in the optimal range, even though the rest of the pile is blistering hot? BTW, I tend not to water the pile (too far from the hose spigot) and turn it only once a month. Any advice? Also, can I use this aged manure in an indoor plasti worm bin (say 18 cf) as the exclusive food supply? I have only minimal supplies vegetable/fruit waste, altough I can get lost of coffee grounds from Starbucks for free.

    • Rich
    • September 6, 2011

    Bentley – thank you for doing so much great research, and sharing the wealth of it with us all! You and I have had this discussion before, but to go into more detail with it now – I have an idea for an outdoor system that I am potentially going to do (eventually). It will involved 3 Rabbits in cages above 3 open bin, 30gal dual systems. Essentially – the Rabbits will do direct deposit, along with a lot of other organic material that will already be in the bin.

    I know you have said that Bunny Berries are a “cold manure”, meaning that it does not need to be exposed to the sun and heat before used, but the Ammonia is the big concern I have with this idea.

    Your and anyone else’s thoughts on this would be great.


    • Rich
    • September 6, 2011

    P.S. Someone by the name of George Dickerson, (an extension horticulture specialist from New Mexico State University) has additionally made the following claim about bunny berries and their urine…

    • Adam
    • May 15, 2012

    I just picked up a ton of horse manure for free off of Craigslist today. It was still hot so I’m letting it age another few months before adding to my worm bins. I plan on testing a little at the time in a small plastic worm bin before adding to my large worm population. The lady that I picked this manure up from feeds her 6 horses “lakin lite horse pellets” ( ). I was reading the ingredients and was amazed at all of the things in the feed. Not only does it have Bermuda and Alfalfa, but it has folic acid, cane molasses, phosphoric acid (feed grade), zinc sulfate, magnese Sulfate, copper sulfate, cobalt carbonate, sodium selenite, potassium iodide, D-calcium Pantothenate, vitamin D3, vitamin E, vitamin A acetate, choline chloride, vitamin B12, Riboflavin, Thiamin Mononitrate, Niacin, Pyridoxine Hydrocloride, and D-biotin.

    Sorry for the long list of ingredients, but I’ve never ever heard of half of these supplements. My question is: does anyone know if manure from horses fed this feed will be good for the worms? I don’t want to give them manure from horses fed something to keep them healthy, but possibly harmful to the worms. The feed also has a minimum of 11% crude protein. Any help will be greatly appreciated.

    Thanks in advance,

    Adam (Phoenix, AZ)

    • Luci Blake
    • May 27, 2013

    I use bunny poo instead. My bins are inside bins (for now anyway), and fresh anything is out of the question. We bury the mostly dried bunny poo along with frozen veggies and coffee grounds. Looks like a disco in the bins with all of them undulating at once! I have access to alpaca poo, but they also receive ivermectin and other dewormers, so I stick with the bunnies.

    • Steve
    • March 28, 2015

    No manure for plastic bins unless you’re experienced?

    Maybe it’s best avoided if you’re a rank beginner, but I’ve been using horse manure in my big plastic bin ever since I started it and I’ve gotten wonderful results! I hot composted the manure, but I started using it before it had even fully cooled down (but it was broken down enough that it smelled good/earthy, not like fresh manure), and the worms were all over it! I just made sure to add small enough quantities at a time that it couldn’t heat up.

    Okay, so maybe that does require some experience to be able to judge… I wouldn’t personally discourage using horse manure though – it’s so good if done with even the slightest caution!

    • Matt
    • December 20, 2020

    Glad I found this! I have been gathering some manure for my worms, and I also wonderd about dewormer. The stuff I’ve found has been out along a dirt road for who knows how long, and its likely fine, but I’ll wait for at least an inch of rain before I put it in my bins. Hoping the salts are not too much for the worms, didn’t really see that discussed above.

    • Bentley
    • December 28, 2020

    Hey Matt – yeah, if left to sit outdoors for a while the salt levels should be just fine, especially if you are talking about horse manure. When in doubt, testing on a small scale is a good way to go (see if worms will move into the material from a safe habitat zone). As for the vermicides – this is always something I’ve felt was overblown. Definitely other manure hazards that are more likely going to cause issues.

    • Susan Haney
    • June 13, 2021

    Adam, even though that was 10 years ago, I thought for anyone else that is reading this that I would tell you the ingredients that you listed (zinc sulfate, magnese Sulfate, copper sulfate, cobalt carbonate, sodium selenite, potassium iodide, D-calcium Pantothenate, vitamin D3, vitamin E, vitamin A acetate, choline chloride, vitamin B12, Riboflavin, Thiamin Mononitrate, Niacin, Pyridoxine Hydrocloride, and D-biotin.) are vitamins and minerals, like a human would take in a multivitamin tablet. They would be typically used up by the animals, but if not, they are in very small amounts and probably actually beneficial for the plants. So I would not worry about them at all.

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