Help understanding what happens to manure from wormed horses

Asked March 3, 2020, 10:14 AM EST

Hello there My garden group in central Oregon has a question about the soil built with manure from horses that have been treated with a wormer. What does it do to the soil and living organisms? How long does it stay in the soil? Thank you for your time

Deschutes County Oregon

2 Responses

Hello,Thanks for this question. I don't what brand of wormer you are inquiring about but I found a great study about this topic. I will give you the link but here is some information from it that will answer your questions.
"Composting manure and bedding from animals that have been de-wormed with ivermectin decreases the concentration of ivermectin such that the resulting compost product can be used without harm or negative effect to beneficial insects in the environment. As ivermectin has been found to persist in manure for 45 days or more, composting is a good management technique to use for manure after de-worming, or if animals are continually de-wormed. In addition, as the ivermectin concentration decreased almost entirely in the first 24 days of composting, subjecting manure and bedding from de-wormed animals to a thermophilic phase through piling would allow horse owners to field-spread the manure prior to finishing composting. "
This is directly from the findings of the study. When this study talks about composting it is referring to hot composting where the temperature reaches 131 degrees and is turned in a timely manner that all materials reach that temperature.
If the manure is only allowed to sit with no hot composting process you will get different results with the time it takes to break down but it will do so and will also have no effect on soil organisms. Here is information sited in this same study which reflects timing of cold composting or soil incorporation. " Halley, et al. (1989) studied ivermectin degradation rates in feces/soil mixtures and found that when stored indoors at room temperature, ivermectin degraded slowly with half-lives between 93 and 240 days. It was also slow when exposed to outdoor, winter weather. In contrast, when exposed to the summer weather, ivermectin in soil and feces/soil mixtures degraded rapidly, with half lives of 1 to 2 weeks."
I think this study clearly shows that either way, in time there is no adverse effect on the soil microbes.
Appreciate the question. :)

Sorry, forgot to attach the link to the study. Here it is.
http://cwmi.css.cornell.edu/ivermectin.pdf