1. Twenty-eight calves fed reconstituted buttermilk for 20 days and then concentrates, hay and water were allocated to 4 treatments consisting of the addition of chlortetracycline to both the buttermilk and the concentrate mixture, to the buttermilk alone, to the concentrates alone, or to neither.
2. Live-weight gains from 3 to 28 days and the incidence of scouring were not affected significantly by any of the treatments.
3. The addition of chlortetracycline to the concentrate mixture increased significantly: (a) rate of live-weight gain from 28 to 56 days and from 56 o t 84 days: (b) skeletal growth from 3 to 84 days: (c) the consumption of concentrates from 14 to 56 and from 14 to 84 days; and (d) the concentrate conversion rate.
4. The addition of chlortetracycline to the buttermilk had no significant effect on any of the above measurements neither did it make any significant difference to the effect of adding chlortetracycline to the concentrate mixture.
5. There was a direct relationship between quantity of concentrates consumed on the one hand, and rate of live-weight gain and efficiency of conversion of the concentrate, on the other. None of the treatment effects was significant after the data were corrected for differences in the quantities of concentrates consumed.
6. From the results of this experiment and from evidence in the literature it is postulated that one of the effects of feeding antibiotics to calves is to reduce the fermentative activity of rumen bacteria and consequently the amount of gas produced in the rumen, the net result being that the antibiotic-fed animal eats more food and hence grows more quickly.