I probably would blame the tarp, not the manure! It provided a rainproof cover for them to set up housekeeping. Without it, the manure pile would have just sat open to the elements like any other mulch pile. I suppose rats will be attracted to undigested oats and corn kernels passing through the horse gut, or oats left attached on bedding straw.
I wonder if using clear plastic instead of an opaque tarp would have made any difference? If it was in full sun, it might have heated up the pile to the point of discomfort? In shade it would not have made any difference.
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Dee
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