The effects of group size and subtherapeutic antibiotic alternatives on growth performance and morbidity of nursery pigs: a model for feed additive evaluation

Olsen, Kristin M; Gabler, Nicholas K; Rademacher, Chris J; Schwartz, Kent J; Schweer, Wesley P; Gourley, Gene G; Patience, John F

The swine industry is seeking effective alternatives to AGP, and inconsistent results from AGP alternative studies has led to the need for evaluating AGP alternative testing protocols and study designs. The objective of this experiment was to evaluate the effects of AGP alternative diets and test group size on nursery pig performance. These data can then 4 Removals calculated as total number of pigs removed from study (found dead or removed for illness or injury) divided by number of pigs allotted to pen (31 or 11). 5 Means within a row without a common superscript differ significantly (P < 0.05). Interaction P value for medical treatments and removals not significant. (P > 0.10). be used to provide a better framework of standards that can be used as a model for future studies testing the efficacy of AGP alternatives that will aid in comparing and interpreting results across those studies. The majority of published studies evaluating alternatives to AGP have been conducted in academic research settings, and consequently, most studies have used relatively small groups of pigs (Schweer et al., 2017a) . The literature review conducted by Schweer et al. (2017a) showed that experiments with a positive response to an AGP alternative had, on average, more pigs per pen than studies that did not show a positive response. The observed interactions between diet and group size indicate that consideration of group size may be necessary in studies evaluating AGP alternatives. Improvements in performance due to the ZA diet were only detected when pigs were housed in large groups. Higher removal rates were observed when pigs were housed in large groups, possibly indicating a higher-stress environment. These results may suggest a greater potential for this combination of additives to be effective under higher-stress situations, which may occur in larger group sizes. Furthermore, the benefit of AGP seemed smaller when pigs were housed in small groups. Small groups fed the NC and DR diets had increased ADG compared to large groups. The PC and ZA diets seemed to somewhat compensate for slower gain in large groups as small and large groups had similar ADG when fed these diets. Improved growth performance when pigs are housed in smaller groups is in agreement with previous reports of this trend in nursery pigs (Wolter et al., 2000 (Wolter et al., , 2001 . McGlone and Newby (1994) also reported higher morbidity rates in pens of 40 pigs compared to pens of 10 or 20. These results indicate that group size may impact the outcomes of AGP alternative studies, and perhaps positive responses to specific AGP alternatives are less pronounced in studies where pigs are housed in smaller groups. The growth-promoting effects of subtherapeutic levels of antibiotics in swine diets are well documented (Cromwell, 2002) . Improvements in ADG, ADFI, and G:F observed in this study due to AGP inclusion are similar in magnitude to previous reports (Cromwell, 2002) . The current improvements are higher than the values reported by Dritz et al. (2002) which could be due to the poor performance of the NC treatment, perhaps due to health status. It should also be noted that the chlortetracycline inclusion level in the present diets is higher than some previous studies have used, but the levels of antibiotics used in this study were compliant with the 2017 VFD for this particular farm. Separately, ZnO and acidifiers have shown beneficial effects, yet results have been inconsistent; few