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

Experimental treatments were arranged in a split-plot design with four dietary treatments evaluated across two group sizes. The dietary treatments included a negative control (NC) with no AGP, a positive control (PC) consisting of the NC diet with either chlortetracycline hydrochloride (phase 1 and 3) or tiamulin hydrogen fumarate (phase 2) added at the expense of corn, alternative diet 1 (ZA) consisting of the NC diet with zinc oxide (ZnO) plus a dietary acidifier (blend of phosphoric, fumaric, citric, and lactic acids; Kem-Gest, Kemin, Des Moines, IA) added at the expense of corn, and alternative diet 2 (DR) consisting of the NC diet with a Bacillus-based directfed microbial (DFM; BioPlus 2B, Chr. Hansen, Hoersholm, Denmark) plus resistant potato starch (MSP[RS], MSP Starch Products Inc., Carberry, Manitoba, Canada) added at the expense of corn. Combinations of AGP alternatives were used, rather than single products, to first help satisfy the objective of testing a study design, rather than focusing on evaluating specific products. The specific combinations were chosen based on results from Schweer et al. (2017a) which indicated that AGP alternatives in the categories of zinc/copper, organic acids, and probiotics were most effective. Furthermore, ZnO with an acidifier and a probiotic with a prebiotic likely have modes of action which either compliment or do not antagonize each other. Diets were fed in three phases (Tables 1 and 2) based on a feed budgeting system. When a pen consumed its entire allowance for a phase, feed for the next phase was given to that pen. In this manner, all pens were allowed to consume their entire budget for each phase before moving to the next phase. In order to associate pig weights with phase changes, weigh days were scheduled as close as possible to the first pens finishing their feed budget from the previous phase. Phase 1 was fed from days 0 to 11, phase 2 from days 12 to 24, and phase 3 from days 25 to 41.The first two phases were delivered in pelleted form, and the third phase feed was delivered as a mash. Feed was manufactured at two different commercial feed mills (phases 1 and 2 at the same mill, and phase 3 at another mill). Prior to diet manufacturing, the acidifier, ZnO, DFM, and RS products were hand-weighed on an analytical scale to the proper inclusion level, packaged in individual bags, and delivered to the commercial mill. Mix sheets used during mixing from both feed mills were validated after mixing to ensure that these bags were added to the proper batches. In all phases, the diet containing the DFM was mixed last in order to avoid contamination of the other three diets.