Influence of source and micronization of soybean meal on nutrient digestibility and growth performance of weanling pigs

A total of 288 piglets weaned at 28 d and weighing 7.6 ± 0.2 kg were used in a 35-d experiment to evaluate the effect of CP content (44% vs. 49% CP) of soybean meal (SBM), micronization (fine grinding) of the 49% CP SBM (HP-SBM), and soy protein concentrate (SPC; 65% CP) on total tract apparent digestibility (TTAD) and growth performance. In phase I (d 0 to 21 of experiment), there was a positive control diet that included 6.5% of CP from a SPC with 65% CP and a negative control diet that supplied the same amount of CP as regular SBM (R-SBM) with 44% CP. The other 4 diets included the same amount of dietary CP from 2 different sources of HP-SBM that were either ground (990 μm) or micronized (60 μm). All diets were isonutritive, and the main difference was the source of SBM used. Each treatment was replicated 8 times (6 pigs per pen). In phase II (d 21 to 35), all pigs were fed a common commercial starter diet. For the entire phase I, the type of soy product did not affect growth performance of the pigs. However, from 0 to 7 d of experiment, pigs fed the micronized HP-SBM had better G:F (1.11 vs. 0.98; P0.05) than piglets fed the ground HP-SBM. Also, from 7 to 14 d of experiment, ADFI tended to be greater (P=0.08) for pigs fed the micronized HP-SBM than for piglets fed the ground HP-SBM. During phase II (all the pigs received the same diet), no differences among treatments were observed. In general, TTAD of nutrients at 7 d of experiment was greater for the SPC than the R-SBM diet, with the HP-SBM diets being intermediate. The TTAD of CP was greater (83.8% vs. 81.9%; P≤0.01) for the SPC diet than the average of the SBM diets. Also, the digestibility of OM and DM was greater (P0.01) for the HP-SBM either ground or micronized than the R-SBM diet. Micronization of the HP-SBM did not affect nutrient digestibility. It is concluded that when the R-SBM is substituted by SPC, CP digestibility is improved, but no effects are observed on growth performance. The use of the HP-SBM in substitution of the R-SBM in the diet improved nutrient digestibility but did not affect piglet performance. The inclusion of micronized HP-SBM in the diet improved G:F during the first week postweaning but did not affect TTAD of nutrients. In general, the inclusion of added-value soy products (SPC or micronized SBM) in the diet presents little advantage in terms of growth performance over the use of HP-SBM in pigs weaned at 28 d of age.