Influence of Type of Antibiotic and Length of Antibiotic Feeding Period on Performance and Persistence of Antibiotic Resistant Enteric Bacteria in Growing-Finishing Swine

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Abstract
Five trials involving 300 growing-finishing swine were conducted to determine the effects that the type of antibiotic fed and the length of the feeding period had on rate and efficiency of gain, number of fecal coliform and lactobacilli, and antibiotic resistance patterns of lactose-positive fecal isolates. Pigs which had no previous exposure to antibiotics were fed a control diet (without antibiotics) or diets supplemented with 44 mg of chlortetracycline (CTC), bacitracin, tylosin or virginiamycin per kilogram. Antibiotic supplemented diets were fed for approximately 6 or 11 weeks followed by the control diet or the antibiotic supplemented diets were fed for the entire 16 week test period (14 to 98 kg body weight). During the initial 6-week period, pig gains were improved by feeding diets supplemented with virginiamycin (P<.01) or tylosin (P<.05) and feed required per unit of gain was improved by virginiamycin (P<.05) addition, compared with values for pigs fed the control diet. Feeding of CTC or bacitracin resulted in slight but nonsignificant improvements in growth rate. Pigs fed diets supplemented with tylosin or virginiamycin for the entire experiment (16 weeks) gained faster and more efficiently (P<.05) than pigs fed the control diet. For the entire growing-finishing period, gain .and feed efficiency were improved by 5.6% (P<.01) and 3.3% (P<.05), respectively, when antibiotics were fed continuously compared with values of control pigs. The early advantages in performance from antibiotic feeding were lost when the antibiotics were withdrawn from the diet at 6 or 11 weeks, as reflected in final gains and feed/gain response that were not different (P<.05) from those of pigs fed the control diet throughout the test. Marked differences in performance as a result of antibiotic supplement would not be expected as extreme care was taken to avoid exposure of pigs to other pigs, to sanitize pens between trials and to avoid stresses of crowding. Coliform counts from pigs fed the control diet decreased (P<.01) with the length of time that the pigs were on test. Except for tylosin fed pigs, counts were higher for pigs fed diets supplemented with antibiotics for 6 weeks than for pigs fed antibiotic diets for 11 or 16 weeks. Counts were higher from the pigs fed virginiamycin for 6 (P<.01) or 11 (P<.05) weeks than from the pigs fed the control diet. The percentage of the coliform population which was resistant to 25 μg CTC/ml was less in samples from pigs fed the control diet than from pigs fed antibiotics. Pigs fed antibiotics tended to have a higher percentage of CTC-resistant coliforms following antibiotic withdrawal. Average percentages of the coliform population resistant to CTC were 39, 90, 60, 50 and 73%, respectively, for pigs in control, CTC, bacitracin, tylosin and virginiamycin treatments. Resistance to tetracycline was similar for isolates from the control, CTC and virginiamycin fed groups and lowest for the tylosin group. A greater percentage of isolates from the virginiamycin and tylosin fed groups was resistant to ampicillin. No definite trends in percentage of isolates resistant to a given antimicrobial fed or the duration of feeding. Though isolates from the control group contained the fewest number of agents in their resistance patterns, no definite trend in the number of agents in the resistance patterns could be associated with the antibiotic fed or with the duration of feeding. There was no apparent relationship among either the coliforms counts or the resistance patterns of coliforms and the growth performance of the pigs. Copyright © 1978. American Society of Animal Science . Copyright 1978 by American Society of Animal Science.

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