Effects of sodium bicarbonate on diet selection and rumen digestion by growing lambs individually fed whole barley grain and a protein supplement at their choice

A feeding and digestion study was completed to determine whether sodium bicarbonate supplementation modifies diet selection of whole barley grain and a protein supplement by lambs. The underlying hypothesis was that aversion to barley intake to avoid acidosis might explain overconsumption of protein. Sixty weaned lambs (15.1 ± 0.33 kg live weight (LW)) were allocated to one of three dietary treatments in 2 pens of 5 lambs by sex, according to a 3 treatments×2 sex factorial design. Treatments were whole barley grain and a pelleted protein supplement fed at choice either unsupplemented (U) or supplemented with sodium bicarbonate fed by free access on the side (BF) or incorporated into the protein supplement at 15 g/kg dry matter (BI). At the end of the growing period, 7 male lambs from each treatment (24.2 ± 0.13 kg LW) were moved to individual pens and slaughtered after 5 days of individual intake control to characterise rumen fermentation and determine digesta flow and microbial crude protein (CP) contribution to the abomasum, using C31 alkane and purine bases as flow and microbial markers, respectively. Intake of sodium bicarbonate was higher (P<0.005) when provided free access than when incorporated into the protein pellet (13.9 g/d vs. 5.0 g/d for BF and BI) but it did not affect dry matter intake (797 g/d) or average daily gain (328 g/d vs. 289 g/d for male and female lambs), although barley grain intake tended (P=0.052) to decrease in response to buffer addition. A lower (P<0.05) proportion of organic matter truly digested in the rumen (OMTDR; 0.53 vs. 0.61) and a trend (P=0.098) to increased efficiency of microbial CP synthesis (26.7 gN/kg vs. 21.6 gN/kg OMTDR) ocurred when the buffer was offered free access compared to incorporation into the protein supplement, respectively. Neither rumen fluid pH nor ammonia concentration differed among treatments, but the total concentration of volatile fatty acids and molar proportion of acetate tended (P<0.1) to increase with buffer supplementation (174 mM vs. 200 mM and 46.8 mol/100 mol vs. 52.7 mol/100 mol). Sodium bicarbonate can modify rumen digestion, but failed to buffer rumen fluid pH and reverse the aversion to barley grain.