I'm not really the brightest person, and I have not really thought about the manure of other animals except for chicken. I
So, I have been familiar with the idea that manure is primarily that of an hen. I have seen cow's dung, but never that of an horse.
I'll say manure is manure, it's not necessary important to require a certain animal compulsorily. I know they have almost the same function, if not the same. Primarily it helps plants grow. I saw this online about horse manure, "Horse manure can give new plants a jump start while providing essential nutrients for continual growth. It contains adequate amounts of organic matter and can be applied in various ways. It's also slightly higher in nutritional value than cow or steer manure."
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