Development of rumen function in calves: nature of protein reaching the abomasum.

Effects of weaning age (4 and 8 wk) and ration (complete pelleted starter and unpelleted starter plus alfalfa-grass hay) on development of ruminal function were tested in a split-plot design. Maturity of ruminal function was estimated by the contribution of bacterial nitrogen to total nitrogen reaching the abomasum, essential amino acid composition of bacterial and abomasal protein, and ruminal volatile fatty acid concentrations. Sixteen Holstein bull calves were fitted with rumen and abomasal cannulas by 1 wk of age, and ingesta were sampled twice weekly from 2 to 11 wk of age. Contribution of bacterial nitrogen to total nitrogen in abomasal contents was similar to that of mature ruminants by 5 and 7 wk of age for calves weaned at 4 and 8 wk of age, respectively. Concentrations of ruminal volatile fatty acids indicative of mature ruminal function were reached by 5 wk of age. Pattern of essential amino acids in bacterial cells of the rumen was not affected by age, weaning age, or ration and was similar to that of mature ruminants. Analysis of abomasal digesta indicated no effect of starter ration and no effect of age or weaning age on the relative proportion of essential amino acids except lysine and arginine. Lysine decreased and arginine increased linearly from 2 wk until weaning.