In situ evaluation of the protein value of soybean meal and processed full fat soybeans for ruminants

The rumen degradation and the intestinal digestion of dry matter (DM) and crude protein (CP) of different samples of soybean meal (SBM) and full-fat soybean (FFS) were determined using in situ techniques. Rumen effective degradability (ED) was determined in three rumen fistulated wethers on nine SBM samples, obtained by solvent extraction (SBM1 to 8) and by expelling (SBM9), and on five FFS samples, treated by extrusion (FFS1 to 4) and by toasting (FFS5). Intestinal digestibility ( ID) in the original feed and in the feed residues after rumen incubation for 8 and 24 h was determined in three duodenal fistulated wethers for three samples of SBM( 6, 8 and 9) and two samples of FFS ( 2 and 3). No differences (P > 0.05) were observed between the mean values of SBM and FFS for the degradation kinetic parameters and the ED of both DM and CP. On the contrary, differences were found within each feed category. The ED of CP of SBM9 (46.1%) was lower than that of all solvent extracted meals, which ranged from 55.8 to 67.0%. The ED of CP for FFS5 ( toasted) was equal to the minimum value of the range of the extruded samples (57.3 to 71.3%). The ED of CP was more closely correlated with the variation of the degradation rate (k(d)) (r = 0.92; P < 0.001) than with the soluble (a) or the potentially degradable (b) fractions (r = 0.67 and -0.68, respectively; P < 0.01). On the other hand, the degradation parameters of DM and CP were closely correlated (r = 0.92, 0.92, 0.81 and 0.79 for ED, k(d), b and a, respectively; P < 0.001). The effects of rumen pre-incubation time on the ID of CP, only recorded for three samples, were variable and relatively small in magnitude. The mean values of the ID of CP were similar among samples ( from 95.9 to 97.6%). The protein value of soybean products is mainly determined by the ED of CP, since the increases of by-pass digestible protein, associated with its reduction, largely exceed the reductions in the microbial protein synthesis caused by the decreased feed degradability.