Antimicrobials As Growth Promoters

The growth-promoting effects of antibiotics and other antibacterial agents have been documented in most species of food-producing animals. Because antibiotics are used to a greater extent in the swine industry than in other animal industries, this article will focus on the benefits of antibiotic use in swine.

The efficacy of antibiotics in improving the rate and efficiency of growth in pigs is well documented in numerous research studies.[3-7] Hays[3] summarized data from 1194 experiments conducted in the United States from 1950 to 1985 and found that in studies with weanling pigs from 7- to 25-kg body weight, antibiotics improved growth rate by an average of 16.4% and reduced the amount of feed required per unit of gain by 6.9%. In studies with growing pigs from 17- to 49-kg body weight, antibiotics improved growth rate by 10.6% and feed efficiency by 4.5%.[3] In growing finishing pigs from 24 to 89 kg, growth rate was improved by 4.2% and feed efficiency by 2.2% when antibiotics were fed.[3] These results were derived mostly from experiments conducted at research stations where the environment is less stressful, pens are cleaner, and the disease load of the pigs is generally less than on typical swine farms. Thus, the responses to antibiotics under farm conditions may be twice as great as those occurring in research station environments.1-6,7-1

Even though some antibiotics have now been used for more than 50 years, they seem to be as effective as they were in the early years following their discovery. A comparison of data from the first 28 years of antibiotic usage[3'4] and during the following 8 years[5] indicated that the overall effectiveness of antibiotics did not diminish.

Antibiotic usage in swine feeds has been shown to reduce mortality and morbidity, particularly in young pigs. A summary of 67 field trials conducted over a 22-year period indicates that antibiotics reduced mortality by one-half (from 4.3 to 2.0%) in young pigs.[6'7] The reduction in mortality was even greater under high-disease conditions and environmental stress (15.6 versus 3.1%).[6'7]

The mineral element copper has antibiotic-like properties when present at high levels (100 250 ppm) in diets for swine, especially young pigs.[8] The growth responses are similar in magnitude to those resulting from the feeding of antibiotics. Interestingly, the responses to copper and antibiotics seem to be additive in young pigs; that is, copper is efficacious in both the presence and absence of antibiotics (and vice versa). Dietary zinc as zinc oxide at high levels (2000 to 3000 ppm) also has been found to stimulate growth performance in young pigs.[9]

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