The production of livestock and poultry faces major challenges to meet the global demand for meat and dairy products and eggs due to a steady increase in the world’s population and the ban of antibiotics in animal production. This ban has forced animal nutritionists to seek for natural alternatives to antibiotics. In this context, the yeast Saccharomyces cerevisiae has received considerable attention in the last decade. It has been reported that feed supplementation with live yeast cells improve feed efficiency, enhance feed digestibility, increase animal performance, reduce the number of pathogenic bacteria, improve animal health and reduce the negative environmental impacts of livestock production. The current review sheds light on the effects of the use of live S. cerevisiae cells in the diets of nonruminant and pseudo‐ruminant’s animals and the mechanisms by which they exert its effects. This review work revealed that the addition of S. cerevisiae in poultry feed causes a phenomenon called competitive exclusion of pathogenic bacteria capable of causing disease adhere to the yeast surface, and so removing a large amount of harmful micro‐organisms and allowing the Animal defend more effectively, the production of antimicrobial agents, the balancing the gut microbiota and stimulation of host adaptive immune system and improving gut morphological structure, thus these benefits are reflected on the overall poultry health. In addition, in the presence of live S. cerevisiae cells, the immunity of rabbits was improved due to the high number of white blood cell. In addition, apparent digestibility of acid and neutral detergent fibre was improved in horses and rabbits. Saccharomyces cerevisiae in pig diets augment mucosal immunity by increasing IgM and IgA activity against pathogens, enhance intestinal development and function, adsorb mycotoxins, modulate gut microbiota and reduce postweaning diarrhoea.

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Summary Besides the liver, rumen is one of the most important components of metabolism in ruminants. However, the microbes residing in the rumen are influenced by several complex factors such as diet, which result in fluctuations in the rumen pH. Rumen pH affects feed intake and feed digestibility, subsequently causing microbial shift in the individual members of microbial community residing in the foregut and hindgut. This in turn causes an increase in lipopolysaccharide concentration, among other factors, in the gut fluid and animal blood. Irrespective of diet fed to animals, Firmicutes would probably be the most dominant in high grain diet while Bacteroidetes are dominant in hay diet, and both have a relative abundance of about 80% or more at times. The shift in microbial population is not limited to adult ruminants alone but also occur in calves. Firmicutes, Bacteroidetes, Actinobacteria, and Proteobacteria were the most abundant in both hay and concentrate diet of newly weaned calves. Prolonged, depressed pH, causes subacute ruminal acidosis. This leads to compromise in the integrity of both foregut and hindgut of ruminants, eventually causing structural changes in the gut physiology. Furthermore, diet containing C‐12:0 and C‐14:0, which are medium‐chain fatty acids, were toxic to rumen protozoa. Phytochemical content in some plant residues when fed to animals also causes shift in microbial population. Therefore, foregut and hindgut pH stability is important for ruminant health and for optimal productivity.

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Summary In ruminants, high fermentation capacity is necessary to develop more efficient ruminant production systems. Greater level of production depends on the ability of the microbial ecosystem to convert organic matter into precursors of milk and meat. This has led to increased interest by animal nutritionists, biochemists and microbiologists in evaluating different strategies to manipulate the rumen biota to improve animal performance, production efficiency and animal health. One of such strategies is the use of natural feed additives such as single‐celled fungi yeast. The main objectives of using yeasts as natural additives in ruminant diets include; (i) to prevent rumen microflora disorders, (ii) to improve and sustain higher production of milk and meat, (iii) to reduce rumen acidosis and bloat which adversely affect animal health and performance, (iv) to decrease the risk of ruminant‐associated human pathogens and (v) to reduce the excretion of nitrogenous‐based compounds, carbon dioxide and methane. Yeast, a natural feed additive, has the potential to enhance feed degradation by increasing the concentration of volatile fatty acids during fermentation processes. In addition, microbial growth in the rumen is enhanced in the presence of yeast leading to the delivery of a greater amount of microbial protein to the duodenum and high nitrogen retention. Single‐celled fungi yeast has demonstrated its ability to increase fibre digestibility and lower faecal output of organic matter due to improved digestion of organic matter, which subsequently improves animal productivity. Yeast also has the ability to alter the fermentation process in the rumen in a way that reduces methane formation. Furthermore, yeast inclusion in ruminant diets has been reported to decrease toxins absorption such as mycotoxins and promote epithelial cell integrity. This review article provides information on the impact of single‐celled fungi yeast as a feed supplement on ruminal microbiota and its function to improve the health and productive longevity of ruminants.