Malic acid combined with heat treatment to protect protein from soybean meal against rumen degradation

The protection of proteins from soybean meal (SBM) by treatments with solutions of malic acid (at two crescent doses) and heat was studied in a 3 x 3 Latin-square design using three diets and three wethers cannulated in the rumen and in the duodenum. Solutions 2.25 and 4.50 N of malic acid were pulverised (400 mL center dot kg(-1)) on SBM under continuous mixing. Then, the treated meals were dried in an oven at 117 degrees C for 6 hours. The diets were isoproteic and included 55% lucerne hay and 45% concentrate, with untreated (UT) and treated SBM (T2.25N and T4.50N). The diets were distributed, at 50 g center dot kg(-1) BW0.75, in six equal meals (every 4 h). The estimates of ruminal effective degradability (ED) of dry matter (DM) and crude protein (CP) of SBM samples were obtained using in situ and rumen outflow methods, whereas the intestinal digestibility (ID) was determined by the mobile nylon bag technique on residues incubated in the rumen for 8 h. The effects of the addition of malic acid in the diet on rumen fermentation patterns and fibrolitic activity (measured by the in situ degradation of the lucerne hay) were also determined. The treatments did not alter the digestive transit kinetics of SBM particles, which showed rumen outflow rates in the interval from 5.5 to 6.1% center dot h(-1). Degradation of CP showed a sigmoidal shape in all samples and was described well with a simple exponential model, based on a constant fractional degradation rate (k(d)), or using a logistic model in which the fractional degradation rate increases with time from an initial (k(o)) to a final (k(infinity)) value. According to the first model, the k(d) value corresponding to the protective treatments showed a tendency (P = 0.054) to decrease with the treatments (from 4.71 to 3.32 and 3.35% center dot h(-1) for UT, T2.25N and T4.50N, respectively). In the second model, this deceleration of CP degradation only affected k(o) (P = 0.039), whereas, no effect was observed on k(infinity). These changes reduced moderately the ED of CP of the treated samples (15-16%) in relation to the untreated SBM, although differences were not significant (46.2, 39.3 and 38.5%, respectively, as mean of both models). No effect was observed for the ID of CP with values shown between 99.0 and 99.4%. Including malic acid in the diet did not have an important effect on the degradation of DM and fibre fractions of lucerne hay or on the parameters of ruminal fermentation, except for a decrease (P = 0.050) of the acetic acid: propionic acid ratio (4.44, 3.53, and 3.78 for UT, T2.25N and T4.50N, respectively).