What we do know about composting is that with the exception of some herbicides, it is very effective at breaking down many different classes of contaminants. This is due in large part to the activity of bacteria and fungi. However, time, temperature, and sorbtion are also important factors. Other things to consider when evaluating the risks of anthelmitics in manure:
- Quantity and concentration: How many animals in the herd were treated, and what percentage of your compost feedstock will be made up of manure from these treated animals? As with many potential contaminants, "dilution is the solution."
- Time: How long ago were the animals treated? And similarly, how old is the manure? If the manure has been in a storage pile for several weeks or months already it is unlikely anthelmintics, or other drugs for that matter, will pose any significant risk. In fact, there will likely be a healthy earthworm population that has already moved in!
Would an active compost system that is aerated and reaches high temperatures be more effective at breaking down drugs excreted in the manure than a passive manure storage pile? Most likely it would, considering the level of microbial activity and the temperature extremes that any compound would be exposed to. However, time is also a factor and aging the manure would probably also be effective. Personally, I would have no concern about using manure in my garden from livestock treated with anthelmintics or other drugs. In other words, I would consider it a "non-issue."
There are many websites and forums that address some of the concerns about anthelmitics in manure. Sometimes it can be difficult to separate fact from fiction. Regardless, some may give you a good idea of what other people are doing to manage this risk.
Two important issues to consider when using manure are the nutrient balance of various manures (ie. horse vs. chicken) and potential to transmit human pathogens (ie. E. coli, Salmonella spp, etc). Here is a good WSU Extension Bulletin on fertilizing with animal manure.
That being said, here are some studies on the topic anthelmintics in compost or in the environment that I was able to find:
Researchers at the Cornell Waste Management Institute conducted a study using the manure collected from 60 horses over three days directly after been treated with 114 grams of ivermectin. They reported the following findings: the ivermectin did not affect the compost process; ivermectin decay occured at a rate of 1.8% per day and it had a half-life of 3.6 days; and there was no ivermectin detected in the soil below the compost pile.
A review of various studies on the toxicity of ivermectin in the environment offers the following: ivermectin is immobile in the soil with a half life of 7-14 days in soil/manure mixtures; it quickly breaks down in when exposed to sunlight (i.e. photodegrades) and does not accumulate in the environment, or non-target species; it appears to have very low toxicity to earthworms and no toxicity to plants. While I was not able to find any strong evidence that normal levels of ivermectin residue in manure will harm earthworms, there are several studies showing that in pasture systems it may reduce the activity of dung feeding insects, especially the larvae.
A study on the effects of ivermectin in goat manure found that while the rate of decomposition was significantly slower from treated than untreated goats, the number and size of worms was not effected.
A student in California came up with a good a science fair project to look at the effects of oxibendazole on red worms. She collected manure from her horse pre-worming, and 24 and 72 hours post-worming and then added 100 worms to each sample. She determined that residual oxibendazole in the manure was detrimental to the worms. You can read about her project here and see what you think!
WARNING: There are certain dog breeds that carry the MDR-1 gene defect and are highly susceptible to ivermectin toxicity if they consume more than 0.06 mg ivermectin/kg of body weight. Acute poisoning of dogs can occur when they eat manure from horses that have been recently wormed, or chew on discarded ivermectin tubes and packaging.