Effect of intraruminal acid or buffer infusion on the rate of neutral detergent fibre digestion in highly digestible pastures in dairy cows

Abstract

Control, acid or buffer solution was infused intraruminally into cows eating highly digestible grassbased pasture and the rates of neutral detergent fibre (NDF) degradation of 4 highly digestible ryegrasses were determined using the nylon bag technique. The control solution was water, the acid solution was a mixture of acetic and hydrochloric acids and the buffer solution contained sodium hydrogen carbonate and disodium hydrogen orthophosphate. The average daily ruminal fluid pH of cows receiving acid infusion was lower, and the diurnal variation in ruminal fluid pH was greater, than that of cows receiving the control or buffer infusion. Cows receiving acid or buffer infusion had a slower rate of NDF degradation than cows receiving the control infusion. The rate of NDF degradation did not decrease as the proportion of NDF in the grass increased. The composition and structural arrangement of NDF was likely to have differed between pasture sources. The cellulolytic pH threshold for highly digestible pasture in grazing cows remains poorly defined because variations in pH are likely to be as, or more, important than a mean value.