Influence of yeast on rumen fermentation, growth performance and quality of products in ruminants: A review

This review aims to give an overview of the efficacy of yeast supplementation on growth performance, rumen pH, rumen microbiota, and their relationship to meat and milk quality in ruminants. The practice of feeding high grain diets to ruminants in an effort to increase growth rate and weight gain usually results in excess deposition of saturated fatty acids in animal products and increased incidence of rumen acidosis. The supplementation of yeast at the right dose and viability level could counteract the acidotic effects of these high grain diets in the rumen and positively modify the fatty acid composition of animal products. Yeast exerts its actions by competing with lactate-producing (Streptococcus bovis and Lactobacillus) bacteria for available sugar and encouraging the growth of lactate-utilising bacteria (Megasphaera elsdenii). M. elsdenii is known to convert lactate into butyrate and propionate leading to a decrease in the accumulation of lactate thereby resulting in higher rumen pH. Interestingly, this creates a conducive environment for the proliferation of vaccenic acid-producing bacteria (Butyrivibrio fibrisolvens) and ciliate protozoa, both of which have been reported to increase the ruminal concentration of trans-11 and cis-9, trans-11-conjugated linoleic acid (CLA) at a pH range between 5.6 and 6.3. The addition of yeast into the diet of ruminants has also been reported to positively modify rumen biohydrogenation pathway to synthesise more of the beneficial biohydrogenation intermediates (trans -11 and cis -9, trans -11). This implies that more dietary sources of linoleic acid, linolenic acid, and oleic acid along with beneficial biohydrogenation intermediates (cis-9, trans-11-CLA, and trans-11) would escape complete biohydrogenation in the rumen to be absorbed into milk and meat. However, further studies are required to substantiate our claim. Therefore, techniques like transcriptomics should be employed to identify the mRNA transcript expression levels of genes like stearoyl-CoA desaturase, fatty acid synthase, and elongase of very long chain fatty acids 6 in the muscle. Different strains of yeast need to be tested at different doses and viability levels on the fatty acid profile of animal products as well as its vaccenic acid and rumenic acid composition.