All manures from domestic and wild animals contain the naturally occurring hormones estradiol (and its metabolites) and testosterone. Antibiotics are not routinely used for dairy cattle. If they are used for veterinary purposes, studies have shown that antibiotic residues in cow manure are in low to undetectable concentrations. Antibiotic residues in manure that is applied to pastures are degraded to non-bioactive products. Only under laboratory conditions have researchers shown that some plants (cabbage and onions) can adsorb antibiotics from soil to which unnaturally high concentrations of the antibiotics was added. Pastures to which dairy cow manure is applied will not likely be contaminated with antibiotics. Plants are not known to adsorb hormones from manure, and they will, therefore, not be a part of the pasture grass tissue. The bottom line is that applications of dairy cow manure to pastures where horses graze will not be a source of either antibiotic residues or hormones and there is no need to test hay for either of them.

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