Use of NaHCO3 and MgO as additives for sheep fed only pasture for a restricted period of time per day: effects on intake, digestion and the rumen environment

Effects of NaHCO3 and MgO buffer addition on intake and digestive utilization of a pasture were studied in wethers allowed a restricted time of access to forage. Twelve wethers housed in metabolic cages and fed fresh forage (predominantly Lotus corniculatus) ad libitum for 6h/d were randomly assigned to one of the following treatments: a control forage without buffer (C) or a forage plus buffer composed of a mixture of 750g/kg NaHCO3 and 250g/kg MgO at 20g/kg dry matter intake (B). Feeding behaviour, feed and water intake and digestibility, urine output, Na urine elimination, kinetics of passage, ruminal pH and ammonia concentration, N balance and ruminal microbial N synthesis were determined in vivo, and the ruminal liquor activity was evaluated in vitro by fermentation of wheat straw. Addition of buffer increased total water intake (p=0.05), Na urinary output (p=0.01), purine derivative excretion in urine (p=0.05) and tended to decrease mean total retention time in the digestive tract (p=0.09). However, buffer addition increased ruminal pH (p<0.001) and tended to decrease the ammonia concentration (p=0.05). That use of buffer decreased ruminal activity was evidenced by a lower volume of gas produced in vitro (p=0.01) possibly due to a lower microbial concentration in rumen liquor. The higher rumen dilution rate, likely due to a higher water intake, seems to have been the key driver of the actions of buffer supplementation on the rumen environment. Moreover, addition of NaHCO3 led to an increased urinary Na excretion, which should be considered due to its likely negative environmental impacts.