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 mu m) or micronized (60 mu m). 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; P < 0.05) 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 (P < 0.01) 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.