The Effect of Different Levels and Prolonged Supplementation of Chlortetracycline upon Roughage Digestion by Sheep

Three experiments were conducted with 38 wether lambs, that weighed an average of about 65 lb. each, to determine the effect of chlortetracycline on the apparent digestibility of feed constituents of a ration which consisted primarily of low quality roughage. The rations were composed of 80% ground corncobs and 20% concentrate mixture. In Experiment 1 the basal ration was supplemented with 10 mg. of chlortetracycline and chlortetracycline as Aurofac 2A per pound of feed. In Experiment 2 the basal ration was supplemented with chlortetracycline and chlortetracycline as Aurofac 2A, each at the 1 and the 2-mg. levels per pound of feed. Experiment 3 was a repetition of Experiment 2, except the levels of antibiotic fed were increased to 5 and 10 mg. per pound of feed, and the collection period consisted of three consecutive 10-day collection periods. In all experiments the initial supplementation of 5 and 10 mg. of chlortetracycline or Aurofac 2A caused a loss of appetite within 40 to 72 hours. All levels of chlortetracycline supplementation significantly decreased the apparent crude fiber digestibility except the 1 mg.level of chlortetracycline as Aurofac 2A in Experiment 2. This same trend was present for dry matter, organic matter, and nitrogen-free extract. As the level of chlortetracycline was increased from one to two and from 5 to 10 mg. the decrease in apparent digestibility of ration constituents became greater. In Experiment 3 there was a definite period effect in the chlortetracycline supplemented groups with a recovery of apparent digestibility of ration constituents in Period 3. The average apparent digestibility of crude fiber for both sources of chlortetracycline at the 10-mg. level for Periods 1, 2 and 3 in Experiment 3 were reduced by 18.4, 11.5 and 5.4%, respectively. This same trend of recovery was present for dry matter, organic matter and nitrogen-free extract. The implications of these findings are discussed.