Response of Growing-Finishing Swine to Different Levels and Methods of Feeding Chlortetracycline

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Abstract
The effect on performance of different levels of chlortetracycline (CTC) in growing-finishing swine rations was observed over a 13-yr. period. In 21 dry lot comparisons, conducted in the same feeding unit, antibiotic significantly (p .01) increased daily gain only from weaning to a liveweight of 54 kg. Response in rate of gain did not significantly decrease with time and effectiveness of antibiotic supplementation decreased only when measured as the amount of feed saved per unit of gain during that portion of the feeding period from 54 kg to market weight. The daily gain of control pigs significantly (P<.01) decreased with time and while the beneficial effect of antibiotic on efficiency of feed conversion appeared to decline there was a concomitant increase (P< .01) in the efficiency of control pigs. During periods when the efficiency of feed conversion of control pigs was poorer antibiotic supplementation resulted in a greater amount of feed saved. The immediate and long term effect of intermittent short interval supplementation of chlortetracycline was also observed. Periods of supplementationrangedfrom every other weektoas infrequent as 1 out of 8 wk. The addition of 22 mg of CTC per kg of feed during alternate 1 -wk. or 2 -wk. periods significantly (P<. 05) increased rate of grain over the entire feeding periods and performance was not different from continuous supplemerilationof the same level. An effect onperformance, similarto that which resultedfromthe continuous intake of feed containing 11 mg/kg, was observed when 44 mg/ kg were included every 4th weekor88mg/kg every 8th week.

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