Two experiments were conducted to determine the standardized ileal digestibility (SID) of crude protein and amino acid (AA) and concentrations of digestible (DE) and metabolizable (ME) in soy protein concentrate (SPC) ground to three particle sizes and in soybean meal and fish meal when fed to weanling pigs. In the first experiment a N-free diet and diets containing soybean meal, fish meal, or SPC ground to a mean particle size of 70, 180, or 700 μm as the only source of AA were fed to six barrows weighing about 13kg with T-cannula installed in the distal ileum.

In the second, 36 barrows weighing about 14kg were allotted to a randomized complete block design with 6 diets and 6 replicate pigs per diet. A corn-based diet and diets containing corn and soybean meal, fish meal, or SPC ground to the three particle sizes were used. In the third experiment, 160 pigs weighing about 7kg were allotted to four dietary treatments with eight pens per treatment and five pigs per pen. The pigs were fed one of four diets during phase 1 (0 to 14 days post-weaning), and a common diet in phase 2 (days 14 to 28 post-weaning). The four diets used in phase 1 included a control diet containing fish meal and spray-dried plasma protein, and diets in which fish meal, spray-dried plasma protein, or both fish meal and spray-dried plasma protein were replaced by SPC ground to 180 μm.

Results of the first experiment indicated that the SID of His, Lys, and Thr tended (P ‹ 0.10) to be greater in SPC ground to 180 μm than in soybean meal, and the SID of Arg, Ile, Phe, and Trp was greater (P ‹ 0.05) in SPC ground to 70 or 180 μm than in soybean meal. In the second study, there were no differences in the digestible energy and metabolizable energy (DM-basis) among corn, soybean meal, fish meal, and SPC ground to 70, 180, or 700 μm. Results of the third experiment indicated that growth performance of pigs fed the four experimental diets in phase 1 were not different in phase 1, phase 2, or in the entire experiment. The conclusion of these three experiments was that grinding SPC to approximately 180 μm maximizes SID of indispensable amino acids without impacting metabolizable and diets based on soybean meal and SPC as the main protein sources may be fed to pigs during the initial two weeks post-weaning without affecting growth performance.