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

2000, Animal Feed Science and Technology

Four crossbred Holstein±Friesian heifers (initial live weight 306 AE 6X1 kg) ®tted with rumen and duodenal cannulae were randomly allocated to one of two dietary treatments in a double 2 Â 2 crossover design. Both diets were composed of (g/kg as fed) 250 barley straw and 750 concentrate. The concentrate 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 were given Diet C and the other two heifers were given Diet B and all four heifers were infused intraruminally, during four sequential 16-day intervals, with four levels of effective rumen degradable protein (ERDP). ERDP was given as an iso-nitrogenous mixture of urea and casein at 0, 25, 50 or 75 g/kg of concentrate intake. Animals offered Diet B ate more DM, OM and NDF than 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`0X05). Starch digestion did not differ signi®cantly between diets, but ®bre was better digested in Diet C than in Diet B, i.e. 56.5% versus 47.5%, 51.3% versus 36.4% and 50.5% versus 40.2% for arabinose, xylose and cellulose±glucose digestibilities, respectively (P`0X05). Mean rumen ammonia concentrations increased linearly from 29.1 mg/l when no ERDP was infused to 184.5 mg/l when ERDP was infused at the highest level. Ruminal pH was lower (P`0X05) in animals offered Diet B than those offered Diet C (6.29 versus 6.46) and in ERDP-supplemented rather than unsupplemented diets (6.73 versus 6.28). However, pH never fell below 5.5. There were no differences in effective rumen degradability between Diets B and C, and increasing the ERDP supply promoted an increase in straw (P`0X05) and corn (P`0X1) DM disappearance from polyester bags. The diets without ERDP infusion were apparently de®cient in degradable N because rumen microbial yield increased from 76.0 to 102.5 g N/d (P`0X05) when ERDP infusion rate was increased from 0 to 25 g/kg of concentrate, irrespective of which type of grain concentrate was used. With further increases in ERDP, microbial yield maintained constant and there was no further Animal Feed Science and Technology 88 (2000) 59±77