Influence of dietary rumen-degradable protein supply on rumen characteristics and carbohydrate fermentation in beef cattle offered high-grain diets

Four crossbred Holstein-Friesian heifers (initial live weight 306 +/- 6.1 k g) fitted with rumen and duodenal cannulae were randomly allocated to one o f two dietary treatments in a double 2 x 2 crossover design. Both diets wer e composed of (g/kg as fed) 250 barley straw and 750 concentrate. The conce ntrate consisted of (g/kg as fed) 655 corn and 225 barley (Diet C) and 225 corn and 655 barley (Diet B), respectively. During Period 1, two heifers we re given Diet C and the other two heifers were given Diet B and all four he ifers were infused intraruminally, during four sequential 16-day intervals, with four levels of effective rumen degradable protein (ERDP). ERDP was gi ven as an iso-nitrogenous mixture of urea and casein at 0, 25, 50 or 75 g/k g of concentrate intake. Animals offered Diet B ate more DM, OM and NDF tha n those offered Diet C (97.6, 89.9 and 37.6 g/kg versus 94.4, 87.3 and 31.9 g/kg metabolic live weight W-0.75), respectively (P < 0.05). Starch digest ion did not differ significantly between diets, but fibre was better digest ed in Diet C than in Diet B, i.e. 56.5% versus 47.5%, 51.3% versus 36.4% an d 50.5% versus 40.2% for arabinose, xylose and cellulose-glucose digestibil ities, respectively (P < 0.05). Mean rumen ammonia concentrations increased linearly from 29.1 mg/l when no ERDP was infused to 184.5 mg/l when ERDP w as infused at the highest level. Ruminal pH was lower (P < 0.05) in animals offered Diet B than those offered Diet C (6.29 versus 6.46) and in ERDP-su pplemented rather than unsupplemented diets (6.73 versus 6.28). However, pH never fell below 5.5. Then were no differences in effective rumen degradab ility between Diets B and C, and increasing the ERDP supply promoted an inc rease in straw (P < 0.05) and corn (P < 0.1) DM disappearance from polyeste r bags. The diets without ERDP infusion were apparently deficient in degrad able N because rumen microbial yield increased from 76.0 to 102.5 g N/d (P < 0.05) when ERDP infusion rate was increased from 0 to 25 g/kg of concentr ate, irrespective of which type of grain concentrate was used. With further increases in ERDP, microbial yield maintained constant and there was no fu rther effect on intake and digestion of DM, OM and fibre components. The ru men ammonia concentration promoting maximum microbial yield under the condi tions of this experiment was approximately 81 mg/l. (C) 2000 Elsevier Scien ce B.V. All rights reserved.