Effect of age on tolerance of calves to chlorpyrifos.

Groups of five calves (1 to 2 weeks old) or older calves (4 to 6 months old) were treated topically with increasing concentrations of chlorpyrifos [O,O-diethyl-O(-3,5,6-trichloro-2-pyridinyl) phosphorothioate] to determine their susceptibility to chlorpyrifos toxicity. Newborn calves also were given oral doses of technical grade (97.4%) chlorpyrifos to estimate their susceptibility by this treatment route. Topical applications to newborn calves consisted of 0.02%, 0.06%, or 0.12% chlorpyrifos administered as a spray to the entire body surface. Oral dosages given the newborn calves consisted of 5, 10, or 25 mg of chlorpyrifos/kg of body weight. Topical applications were administered to the older calves in the same manner as were applications to the newborn calves, but at concentrations of 0.25%, 0.5%, 0.9%, or 2.3%. Minimum dosages producing signs of toxicosis in newborn calves which were orally and topically given chlorpyrifos were 25 mg/kg of body weight and 0.06%, respectively. All five newborn calves sprayed with 0.12% chlorpyrifos were severely poisoned and were treated with atropine. One of these calves died within 48 hours after exposure to chlorpyrifos despite antidotal therapy. Signs of poisoning were not observed in older calves sprayed with 0.25%, 0.5%, or 0.9% chlorpyrifos. One Brahman-cross calf sprayed with 2.3% chlorpyrifos developed signs of mild poisoning that dissipated in 72 hours without treatment. Four older non-Brahman calves sprayed with 2.3% chlorpyrifos did not show signs of poisoning. Seemingly, 4- to 6-month old calves have an increased tolerance to topically applied chlorpyrifos compared with the tolerance of newborn calves, and the magnitude of this increased tolerance is 30-fold or higher.