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Phenobarbital Metabolism in the Lactating Dairy Cow

¹ Department of Dairy Science
and² College of Veterinary Medicine, University of Illinois, Urbana 61801

ABSTRACT

Two Holstein cows were used to study the metabolism and excretion of phenobarbital in the lactating cow. Each cow received 6 g of sodium phenobarbital, intrarumenally, daily for 10 days. On the 10th day carbon 14 phenobarbital was mixed with the unlabeled material prior to dosing. Total milk, urine, and feces were collected for 6 days, and about 93% of the recovered radioactivity was in urine, 5% in milk, and 2% in feces. The biological half-lives of phenobarbital for milk, urine, and feces were 40.5, 29.5, and 46.5 h. From these values we calculated that 7 days after the last 6-g dose of phenobarbital the milk produced would contain less than .5 ppm of this compound

A second study determined how long after withdrawal from phenobarbital feeding the drug could be detected in milk. Three Holstein cows received 6 g of sodium phenobarbital, intrarumenally, daily for 10 days. Starting on the 11th day milk samples were collected at each milking and analyzed for phenobarbital by gas liquid chromatography and spec-trophotometrically. After 7 days withdrawal, phenobarbital could not be detected in milk by either method.