Effects of Feeding Low-Level Chlortetracycline to a Dairy Herd Continuously for One Year1

The effects of feeding chlortetracycline at the low level (0.1 mg. per pound of body weight, or 0.2 p.p.m.) were studied for a continuous period of 12 mo. on 34 pairs of Guernsey cows in one herd. Under the conditions of this experiment, the incidence of diseases such as foot rot, bloat, metritis, and milk fever was low in both treated and untreated groups. There was a moderate number of mastitis cases in both groups, with no significant difference between them. Differences found in the production of milk, fat, 4% FCM, 4% FCM-ME, fat percentage, persistency of lactation level, daily peak production, or average yield per month were statistically not significant. A comparison of the initial 15 cow pairs showed tendencies in milk, fat percentage, and 4% FCM production in favor of the chlortetracycline treatment, but differences were statistically significant only for fat and 4% FCM production. This experiment with lactating cows and a low-level oral dosage of chlortetracycline did not seem to add new support to a concept of direct metabolic action by chlortetracycline, on the basis of the criteria used for evaluation, even though there was a tendency for the chlortetracycline-fed cows to have an increased fat content in the milk.