Postweaning age effects on rumen fermentation end-products and digesta kinetics in calves weaned at 5 weeks of age.

Four rumen-cannulated calves were used in a block design to study changes in rumen fermentation end-products and rumen digesta kinetics after weaning. Calves were weaned at 5 wk of age and had access to dry feed from d 3 of age. Dietary DM consisted of 85% concentrate and 15% forage. The experiment was divided into three periods: 2, 4, and 8 wk after weaning. Rumen fluid samples collected each period were analyzed for pH, VFA, NH3 N, bacterial DM concentrations, and protozoa numbers. Calves were dosed ruminally with Co-EDTA and Yb-mordanted grain and then sampled for estimation of rumen fluid and particle turnover rates during each period. Average daily gain, DMI, and BW gain increased over the three periods by 106, 81, and 42%, respectively. Rumen pH increased .32 units, and rumen volume increased twofold, during the three periods. Rumen NH3 N concentration and diurnal NH3 N variation changed significantly with age. Bacterial DM concentrations, VFA, and digesta kinetics in the rumen did not change with age. We conclude that the metabolic and physical changes that take place in the rumen of young calves after weaning are evidence of gradual development and yield important information for further study on degradation and metabolism of dietary components in the rumen.