Nutrition of the weaned holstein calf. I. Effect of dietary protein level on rumen metabolism.

Abstract

The effect of various dietary crude protein (CP) levels on rumen metabolism was studied with four Holstein calves (avg wt, 146 kg) fitted with ruminal and abomasal cannulae and fed four corn-based diets supplemented with 0, 5, 10 and 15% soybean meal (SBM). The diets contained 10.2, 12.2, 14.1 and 16.1% CP. Apparent rumen digestion of dry matter (DM), organic matter (OM) and starch averaged 43.9, 48.0 and 68.4%, respectively; none of these was affected by diet (P greater than .05). There was a linear increase in abomasal flow of ammonia nitrogen (N) (P less than .01) and residual N (P less than .05) as SBM increased in the diet. There was no effect of diet on abomasal flow of bacgerial N. Abomasal flow of nonammonia N (g/100 g DM intake) increased in a linear manner (P less than .05) from 1.75 at 10.2% CP to 2.07 at 16.1% CP. When protein was greater than 12% of the diet, there was evidence of wastage of protein in the rumen. Bacterial synthesis in the rumen averaged 2.0 g/100 g organic matter apparently digested. There was no apparent effect of rumen ammonia concentration on rate of microbial synthesis in spite of the low levels (less than 5 mg N/100 ml) observed with the low CP diets. There was a linear increase in serum urea levels before and after feeding as dietary CP increased (P less than .001). Volatile fatty acid concentration in the rumen, turnover rate of the fluid phase in the rumen and rumen volume were all unaffected by changes in dietary protein (P greater than .05).