Rumen pH, NH3-N concentration and forage degradation kinetics of cows grazing temperate pastures and supplemented with different sources of grain

Abstract

The objective of this work was to evaluate the effect of punctual grain supplementations on rumen degradation of cows grazing temperate pastures, with emphasis on degradation of fibrous components and in relation to rumen pH and NH3-N concentrations. Two experiments were conducted with 4 cannulated cows grazing a temperate grass and legume mixture. In experiment 1, treatments were: pasture grazed as sole feed (‘pasture’) and pasture supplemented with 6 kg·day−1 of corn/wheat (50/50, w/w) (‘grain’). In experiment 2, supplementation (3.8 kg) of wheat and corn were compared. Diurnal rumen pH dynamics and nylon bag degradability of dry matter (DM) and fibre fractions of the grazed pasture were measured in both experiments. Additionally, the in vitro digestibility of a collection of feedstuffs (IVTD) was assessed using rumen contents of animals of Experiment 2. Supplementation affected the average pH (‘pasture’: 6.54 vs. ‘grain’: 6.24, P < 0.001) and the fractional degradation rate (kd) of the pasture DM, which was lower when cows were supplemented (P = 0.048). The nature of supplemented grain did not affect rumen pH, NH3-N concentration, DM or fibre degradation of the in situ incubated forage. However, IVTD was higher when cows consumed corn than wheat for most in vitro incubated substrates, with larger differences for more fibrous material.