Effect of diets differing in protein concentration (low vs medium) and nitrogen source (urea vs soybean meal) on in vitro rumen fermentation and on performance of finishing Italian Simmental bulls

This research evaluated the effect of two dietary intestinal digestible protein (PDI) levels and a partial substitution of soybean meal (SBM) with urea (U) on the performance of finishing Italian Simmental bulls (Exp. 1) and on in vitro rumen fermentation characteristics (Exp. 2). In Exp. 1, eighty Italian Simmental bulls (495±58 kg of body weight (BW), 14-months old) were fed diets differing in terms of PDI concentration (85 and 72 g/kg dry matter (DM), medium (M) and low (L) diets, respectively) and nitrogen source (only SBM or SBM partly replaced by 0.5% DM of U) during the last 120 d of fattening. Animals were slaughtered at BW of 656±56 kg at 18 months of age. The average daily gain (ADG) was satisfactory (1.32 kg/d) and the medium PDI level tended (P<0.10) to increase the ADG (1.37 vs 1.28 kg/d). Apparent total tract digestibility did not differ between treatments and the feed efficiency tended to be more favourable for the M diets (0.147 vs 0.137 kg BW gain/ kg feed DM, P<0.10). Slaughter traits were unaffected by dietary treatments. In Exp. 2, continuous culture fermenters were inoculated with rumen fluid from bulls of Exp. 1 and were given the same diets. The fermentation fluid was sampled at feeding time and 1, 2 and 3 h after (T0, T1, T2 and T3, respectively) and the medium PDI level determined higher ammonia concentrations (P<0.01 at T0, T1, and T2; P<0.05 at T3). The drop of pH at T1 and T2 was less intense (P<0.05) for diets containing U, presumably due to the buffering capacity of urea. In vitro DM digestibility tended to be higher for diets containing U (P<0.10) and was unaffected by the dietary level of PDI. Differences in volatile fatty acids concentrations were limited to butyrate, which was higher for M diets (T0:13.6 vs 12.5 mol/100 mol, P<0.10; T2: 13.8 vs 12.5 mol/100 mol, P<0.01) and for diets containing U (T0: 14.3 vs 11.9 mol/100 mol; T2: 14.3 vs 12.1 mol/100 mol, P<0.01). In conclusion, L diets tended to decrease weight gain in finishing Italian Simmental bulls, but both PDI levels and SBM substitution with U had no detrimental effects on slaughter and meat quality traits. Further research efforts are required to explain the increase of butyrate in fermentation fluid of fermenters fed medium PDI diets or U diets. © 2016 Elsevier B.V.