The effects of defaunation on the metabolism of lactic acid in the rumen

Two experiments examined the effects of defaunation on the ruminal metabolism of lactate. Three rumen-cannulated sheep given a diet of molassed sugar beet pulp and barley (80:20) were used to study the effects of defaunation (with manoxol-OT) on the metabolism of lactic acid produced endogenously from rumen fermentation. Defaunation increased mean ruminal concentrations of lactate from 3.4 mmol litre−1 to 8.9 mmol litre−1 but other rumen measurements remained virtually unchanged: pH, 6.3 and 6.3; molar proportions of acetic acid, 645 and 645 mmol mol−1; propionic acid, 189 and 197 mmol mol−1 and butyric acid 142 and 115 mmol mol−1 for the faunated and defaunated states respectively. In a second experiment, two groups each of four rumen-cannulated sheep were used to study the effect of defaunation on the ruminal metabolism of added Na-D, L lactate. One group of four was defaunated using a rumen-washing technique whilst the other group of four remained faunated. The animals were given a diet of rolled barley and hay (60:40) and were given a series of intraruminal doses of Na-D, L lactate from 0 to 100 g day−1, increasing by 20 g day−1 every third day. Disappearance of added lactate was much more rapid in faunated animals: at the 100 g day−1 dose rate, L-lactate concentrations had fallen from a peak of 6 g litre−1 to >1 g litre−1 after 3 h whereas in defaunated sheep the peak of 6.5 g litre−1 was reduced to >1 g litre−1 only after 7 h. In faunated animals lactate addition caused an increase in the ruminal concentration of total volatile fatty acids (VFA) from 147 to 217 mmol litre−1 between the zero and 100 g day−1 dose rates, accompanied by an increase in the molar proportion of propionic acid from 190 to 320 mmol mol−1. However, in defaunated animals there was only a small increase in total VFA concentrations from 94 to 106 mmol ml−1 with no change in the molar proportion of propionic acid and a small increase in butryric acid from 140 to 180 mmol mol−1.