Ivermectin worming medication can pass through horses and kill beneficial insects in the environment.
This was the conclusion of a very careful study
conducted by Cornell University
This study found that fresh manure from horses wormed with invermectin killed beneficial insects. Once the manure was composted to the heating stage (about 45 days) the invermectin was netralized and did no harm to beneficial insects.
Bottom line is that partially composting (45 days) or fully composting (180 days) removes any negative environmental impact of invermectin.
Much better to spread either partially or fully composted horse manure on fields than fresh horse manure to eliminate the negative impact of the invermectin worming medication.