A comprehensive review of the effects of yeast on the regulation of rumen fermentation, productive efficiency and health in ruminants

Due to growing demand of human communities for livestock products, livestock farms have trended towards industrialization.Therefore, with increasing livestock production, the use of highly fermentable carbohydrates has increased in livestock diets. On the other hand, the incompatibility of the gastrointestinal tract of ruminants with this feeding method causes the occurrence of various metabolic diseases and reduces the herd's economic longevity. Research has also shown that adding antibiotics to cow diet increases growth, reduces feed conversion ratio, and improves production and reproduction. But the use of antibiotics for long-term increases antibiotic resistance in livestock and consumers of livestock products. Therefore, it seems necessary to find a suitable alternative to antibiotics such as probiotics. The use of yeast in ruminant nutrition improve to establish and maintain the balance of beneficial bacterial populations, increase fiber digestion, improve ruminal function, ruminal pH and reduce the occurrence of various metabolic diseases. Yeast diets have also to improve milk production performance, persistence in dairy cattle, and milk quality. Probiotics can reduce the risk of intestinal infections and inflammation by competing for food, producing antimicrobial compounds, neutralizing toxins produced by these microorganisms. Also, the presence of effective compounds such as beta-glucans and mannas in the yeast cell wall reduces poisoning by fungal toxins, especially in ruminants, and by stimulating the immune system and the production of digestive enzymes, plays an important role in the growth performance, production, reproduction and the immune system in ruminants.