Rumen fermentation and microbial protein synthesis in cattle given intraruminal infusions of lactic acid with a grass silage based diet

SUMMARY Four Friesian bulls with ruminal and duodenal cannulae were used in a 4 × 4 Latin square experiment to study the effects of lactic acid (LA) on rumen fermentation and microbial protein synthesis. On a dry matter (DM) basis (g/kg), the basal diet comprised grass silage (700), barley (240) and rapeseed meal (60) and it was given at the rate of 7·1 kg DM/day. LA was infused continuously into the rumen at the rates of 0 (L 0 ), 40 (L 40 ), 80 (L 80 ) or 120 (L 120 ) g/kg basal diet DM. The molar proportion of propionate in the rumen volatile fatty acids (VFA) increased linearly ( P < 0.001) and that of acetate, isovalerate, caproate ( P < 0.01) and isobutyrate ( P < 005) decreased linearly with an increasing rate of LA infusion. At the same time there was a linear decrease ( P < 0.05) in the number of rumen protozoa. When the metabolic fate of infused LA was calculated on a molar basis, 0.21 of lactic acid was converted to acetate, 0·52 to propionate and 0.27 to butyrate. Infusion of LA into the rumen had no effect on the site or extent of the digestion of basal diet organic matter (OM) and neutral detergent fibre (NDF). LA diets tended to have a lower microbial N flow at the duodenum (71·4 v . 85·8 g N/day) and lower synthetic efficiency in the rumen (14·4 v . 20.4 g N/kg OM apparently fermented) when compared with the control diet. The ratio of duodenal non-ammonia N to N intake was highest with the control diet and lowest with L 40 , the effect of the LA rate being quadratic ( P < 0·05). The results suggest that propionate was the main end-product of lactic acid fermentation in the rumen with the grass silage based diet. Lactic acid had no value as an energy source for microbial protein synthesis.