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Effects of Long-Time Feeding of Chlortetracycline to Dairy Cattle^{1, 2,}

Department of Animal Husbandry, Iowa State University, Ames

ABSTRACT

Forty dairy heifers paired by age and breed were divided into two groups, with one of each pair receiving 240 mg. chlortetracycline for a minimum of 15 mo. before parturition and subsequently except when dry. The mean weight for the antibiotic-fed group exceeded that of the controls at 12 mo. of age by 38 lb., at 18 mo. of age by 39 lb., and at ten days after calving by 44 lb. Age at first breeding was less for the chlortetracycline group, but since this group required more services per conception the ages at first calving for the two groups were similar. Calves from antibiotic-fed heifers were slightly larger at birth, but response of the calves to chlortetracycline feeding appeared to be independent of treatment of the dam. A total of 30 and 32 records were made by treated and control animals, respectively, in three lactations. Mean total pounds milk and fat during the first lactation (adjusted to 2x, 305-day, M.E.) were 11,088 and 435 for treated cows and 10,615 and 399 for controls, respectively. Only the difference in fat yields was significant statistically (P < .05). Average adjusted milk and fat productions, in pounds, for all lactations were 10,531 and 409 for treated animals and 10,869 and 407 for controls. These differences were not significant (at P = .05). Although culling procedures limited comparisons, there apparently was little difference between groups in reasons for leaving the herd.

¹ Journal Paper No. J-3931 of the Iowa Agricultural and Home Economics Experiment Station, Ames, Iowa. Project No. 1124.

² This study was supported in part through funds provided by the American Cyanamid Company, New York, N. Y.