Experimental Procedures
An experiment was conducted to evaluate the effects of reducing a SBM source to smaller particle sizes and determining the low-protein adjusted digestibility of amino acids at the terminal ileum of grower-finisher pigs. Soybean meal was ground in a 60 horsepower hammer mill (model 1040; Schutte, Buffalo NY) to achieve average particle sizes of approximately 900, 600, 300, and 150-microns. With the exception of the low-protein diet, all other diets (Table 1) were formulated to 17% crude protein from SBM. The SBM diets met or exceeded the nutrient and mineral requirements for growing pigs (NRC, 1998). The low-protein 5% casein diet was adequate in its energy, vitamin, and mineral content. Chromic oxide was included in the diets at 0.5% as a dietary marker for determining amino acid digestibility. The low protein (5% casein) diet was fed to determine the endogenous secretions used to calculate low-protein adjusted digestibility of the amino acids in the SBM treatments.
Eighteen crossbred barrows that averaged 60 ± 3 days of age and a body weight of 61.7 lb were surgically fitted with a simple T-cannula at the distal ileum (Hanson, 2000). Pigs were initially allotted to one of the five treatments and then rotated to another after the collection period, such that each animal received all five diets in a different order.
Pigs were placed in individual 2 x 4 ft stainless steel metabolism crates (Rohn Agri Products, Peoria, IL) during the experiment.
The amount of feed provided during the experimental period was calculated on the basis of .090 kg.75 body weight at the beginning of the trial. Feed intake was equalized between pigs and increased by 0.33 to 0.44 lb between periods. Pigs were fed a constant amount of their treatment diet for each period of seven days. Meals were fed twice daily at 12-hour intervals (e.g. 0700 and 1900) and were offered with a 1 to 1.5 ratio of water to feed.
Digesta samples were collected from 0700 to 1900 on the 6th and 7th day to allow for a 5-day adaptation period. The ileal digesta samples for each pig was pooled for the 2-day period, mixed, freeze-dried, and analyzed for its amino acid content.
Laboratory Analysis. Particle size distribution was analyzed according to the American Society for Testing and Materials (ASTM, 1985) method using a Ro-tap style shaker (W.S. Tyler, Mentor, OH). The nine sieves used were (in microns) 2000, 1410, 1180, 991, 710, 590, 500, 350, and 105.
Soybean meal samples were analyzed for crude protein content using a Perkin-Elmer 2410 Series II nitrogen analyzer (Perkin-Elmer, Norwalk, CT).
The University of Missouri analyzed the digesta samples for dry matter, amino acids and Chromium content. Amino acids and dry matter were determined using the AOAC (1995) method. Chromium content was determined by the flame atomic absorption spectroscopy method after wet ashing.
Statistical analysis. The data was analyzed statistically using the General Linear Model procedure of SAS (SAS Inst. Inc., Cary, NC). The model included the effects of replicate, pig, period, and treatment. Orthogonal contrasts included the linear and quadratic effects of particle size. Least squared means were calculated for treatment and period.