The Effects of Group Size and Sub-Therapeutic Antibiotic Alternatives on the Performance of Nursery Piglets: A Model for Feed Additive Evaluation

The objectives were to develop guidelines for studies evaluating alternatives to sub-therapeutic antibiotic growth promoters (AGP) and to evaluate the effects of AGP alternatives in 2 group sizes on nursery pig performance. A 41-d experiment was conducted in a commercial wean-to-finish barn; 1,300 piglets weaned at 21-d of age (weaned 2 or 4 days prior to experiment; 6.14 ± 0.18 kg BW, PIC 1050 sows and multiple sire lines) were blocked by sire, sex, and wean date, and assigned to 8 treatments: 4 dietary treatments each evaluated across 2 group sizes. The four diet treatments were: negative control (NC), positive control (PC; NC + in-feed antibiotics (chlortetracycline-HCl in phases 1 and 3 and tiamulin hydrogen fumarate in phase 2), zinc oxide with a dietary acid blend (ZA; NC + ZnO + acid), and a bacillus-based direct-fed-microbial combined with resistant potato starch (DR; NC+DFM+RS). The two group sizes were 31 (large pens) or 11(small pens) pigs per pen; floor was modified so area/pig was not different (0.42m²/pig). There were 7 pens/diet with 11 pigs/pen and 8 pens/diet with 31 pigs/pen. Data were analyzed as a randomized complete block design using SAS PROC MIXED with pen as the experimental unit. Pigs naturally experienced acute diarrhea and septicemia in week 1 and PRRSV in weeks 4-6. There was a significant interaction between diet and group size for ADG (P = 0.012); PC compared to NC increased ADG in large and small pens (0.33 vs. 0.26 and 0.33 vs. 0.29 kg/d, respectively; P < 0.05) and ZA increased ADG only in large pens (0.28 vs. 0.26 kg/d; P < 0.05). Small pens had increased ADG compared to large pens when fed NC or DR diets (0. 29 vs 0.26 and 0.28 vs 0.25 kg/d; P < 0.05). Similarly, PC increased ADFI (0.47 vs. 0.40 and 0.47 vs. 0.43 kg/d for large pens and small pens fed PC vs. NC, respectively; P < 0.05). Compared to NC, ZA increased ADFI (0.43 vs. 0.40 kg/d) in large pens only (P < 0.05; diet*group size P = 0.015). Pigs fed PC had greater G:F than NC (0.69 vs. 0.66; P < 0.05), and small pens had greater G:F than large pens (0.67 vs. 0.65; P < 0.05). In conclusion, compared to NC, ZA increased ADG and ADFI in large pens and PC improved performance in both pen sizes. Small pens had increased G:F compared to large pens.