Inhibitory effects of isoleucine and antagonism of the other branched‐chain amino acids on fermentation parameters by mixed ruminal microbes in batch cultures and rumen simulating fermenters (Rusitec)

The inhibitory effect of isoleucine (Ile) and antagonistic effect of leucine (Leu) and valine (Val) against the inhibition by Ile were investigated using simple batch and semicontinuous cultures of mixed ruminal microbes under feeding conditions using ordinary feed (timothy hay and corn grain). In the batch culture experiment, supplementation with Ile inhibited digestion and fermentation at 8–24 h of incubation as the concentration of Ile increased up to 5 mmol/L, but these inhibitions diminished at 48 h. The inhibition caused by Ile was mitigated by the addition of Leu and Val. In a semi‐continuous culture experiment using Rusitec, digestion and fermentation of the diet was suppressed by the administration of 1 and 5 mmol/L of Ile. The daily yield of microbial‐N was also reduced by Ile, but the density of the protozoa was not. The presence of Leu and Val alleviated the inhibition by Ile. These results suggest that the dietary inclusion of Ile possibly represses the bacterial growth, feed digestion and fermentation in the rumen in ordinary feeding situations by feedback inhibition, which could be diminished in the presence of other branched‐chain amino acids. The influences of the passage and level of supplementation on inhibition by amino acids are discussed.