Recent concerns about the use of growth-promoting antibiotics in pig diets have renewed interest in the immunologic and growth-regulating functions of the gastrointestinal (GI) tract. The numerically dense and metabolically active microbiota ofthe pig GI tract represents a key focal point for such questions. The intestinal microbiota is viewed typically as a beneficial entity for the host. Intestinal bacteria provide both nutritional and defensive functions for their host. However, the host animal invests substantially in defensive efforts to first sequester gut microbes away from the epithelial surface, and second to quickly mount immune responses against those organisms that breach epithelial defenses. The impact of host responses to gut bacteria and their metabolic activities require special consideration when viewed in the context of pig production in which efficiency of animal growth is a primary objective. Here, we summarize the working hypothesis that antibiotics improve the efficiency of animal growth via their inhibition of the normal microbiota, leading to increased nutrient utilization and a reduction in the maintenance costs ofthe GI system. In addition, novel molecular ecology techniques are described that can serve as tools to uncover the relationship between intestinal microbiology and growth efficiency.

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Six trials involving 888 pigs (Study 1) and three trials involving 360 pigs (Study 2) were conducted at various geographical locations in the U.S. and Canada to evaluate the effect of ractopamine hydrochloride on the performance and carcass characteristics of finishing swine. All trials were conducted using a randomized complete block design. Trial data were pooled within study for statistical analysis. Pigs averaged approximately 64.5 kg (Study 1) and 65.9 kg (Study 2) initially and had ad libitum access to a 16% crude protein corn-soybean meal or barley-soybean meal diet. Ractopamine was included in the diet at 0, 2.5, 5, 10, 20 or 30 ppm (Study 1), or at 0, 5, 10, 15 or 20 ppm (Study 2); diets were fed for an average of 45 d (Study 1) and 50 d (Study 2) to a final weight of about 104.3 kg (Study 1) and 106.6 kg (Study 2). Carcass dissection data were collected in three of the six trials in Study 1 (0, 5 and 20 ppm ractopamine) and in all three trials in Study 2 (0, 5, 10, 15 and 20 ppm ractopamine). All ractopamine levels improved (P less than .05) ADG and feed: gain (Studies 1 and 2) above those of control pigs. Ractopamine levels of 10 to 30 ppm (Studies 1 and 2) improved (P less than .05) dressing percentage over controls. Pigs fed ractopamine at 5 and 20 ppm (Study 1) and 10, 15 and 20 ppm (Study 2) had increased (P less than .05) dissected leanness compared with controls.(ABSTRACT TRUNCATED AT 250 WORDS)

A total of 400 barrows from Dekalb EB and 83 terminal sires mated to 43 and 45 maternal lines were used to evaluate the effects of dietary ractopamine (RAC; Paylean, Elanco Animal Health, Greenfield, IN) concentrations (0, 5, 10, or 20 ppm; as-fed basis) and feeding durations (6 to 34 d) on growth, efficiency, carcass, and meat quality characteristics of finishing pigs. Barrows were weighed and sorted into five weight blocks, each block consisting of 16 pens (five pigs per pen). Weight blocks were allocated to feeding duration treatments and assigned consecutively by weight from lightest to heaviest to represent 34, 27, 20, 13, and 6 d on test, respectively. The lightest and heaviest blocks averaged 79.8 and 103.8 kg, respectively, at the start of the test. Within a weight block, pens (four per treatment) were randomly assigned to one of four dietary concentrations of RAC in a basal diet containing 18.5% CP and 1.13% lysine. The experiment-wide target slaughter weight was 109 kg, and pigs and feeders were weighed weekly. Weight blocks (80 barrows per block) were slaughtered at a commercial packing plant after 6, 13, 20, 27, or 34 d on test. Overall, RAC supplementation improved (P < 0.05) ADG; however, ADG was not different (P > 0.08) from controls for pigs fed 5, 10, and 20 ppm RAC for 27, 34, and 6 d, respectively. During each feeding period, RAC-fed pigs had improved (P < 0.05) G:F, and, after 20, 27, and 34 d on test, pigs fed 20 ppm RAC had greater (P < 0.05) G:F compared with those fed 0 or 5 ppm RAC. Hot carcass weight was increased (P < 0.05) by RAC feeding after 13 and 27 d of feeding, and by feeding 10 and 20 ppm RAC after 20 d of feeding. After 34 d, pigs fed 20 ppm RAC had heavier (P < 0.05) hot carcass weight than pigs fed 10 ppm RAC. Fat-free lean estimates and the 10th-rib LM area were increased (P < 0.05) by feeding 10 and 20 ppm RAC after 27 d, and by feeding 20 ppm RAC after 34 d compared with controls. Japanese and American color scores, as well as L*, a*, and b* values of the LM, were not affected (P > 0.11) by 5 and 10 ppm RAC compared with controls during each feeding period. Visual marbling score for the LM was decreased (P < 0.05) when RAC was fed at 10 and 20 ppm compared with 0 ppm RAC when fed for 34 d. Dietary RAC improved growth performance at all feeding durations, whereas carcass composition was improved at longer feeding durations. In addition, 5 and 10 ppm RAC did not affect objective and subjective measures of pork quality.