Residues in cattle treated with DDT for control of horn flies on pasture.

Residues were analyzed in the fat of yearlings after exposure on pasture to continuous protection from horn flies with spray treatments of DDT. Eleven applications of 2 quarts of a 0.25 per cent suspension per head produced total residues of DDT, TDE and DDE that exceeded the acceptable tolerance of 7 p.p.m. when animals had reached the prime slaughterweight on a high-energy finishing ration. More information is needed on the mobilization of fat and the DDT stored within the depot to relate an acceptable tolerance for slaughter cattle to practical methods in beef production.