Abstract: On feeding ronnel, i.e., O,O-dimethyl O-(2,4,5-trichlorophenyl)phosphorothioate, to livestock and poultry selected from cloven hoof ruminant animals, swine and poultry substantially every day for at least about 3 to about 28 days depending on the animal, and more preferably for a longer period through growth and/or fattening feeding cycles or during maintenance periods, in an effective amount at a dosage below the parasiticidal level in the range of about 0.5 to about 2.5 milligrams of ronnel per kilogram (mg/kg) of body weight per day, a nutritional response is obtained exhibited as faster growth of the livestock and poultry; fiber production, such as wool in sheep, is increased; the production of milk in lactating animals is increased; and, moreover, the feed conversion efficiency of the animal is substantially improved. The ronnel is provided for oral ingestion by the animal on a voluntary basis upon incorporating sufficient ronnel into a balanced feed diet or a component thereof to provide about 0.