Saccharomyces cerevisiae in ruminants nutrition

The use of feed additives containing live microorganisms and (or) their metabolites to alter rumen fermentation and improve animal performance has increased in response to demands for using more “natural” growth-promoting substances. Microbial products used in this manner were originally called "probiotics", or products “for life”. Yeasts are unicellular eukaryotic fungal micro-organisms and their properties are completely different from those of bacteria. Among yeasts Saccharomyces cerevisiae is industrially important due to its ability to convert sugars (i.e. glucose, maltose) into ethanol and carbon dioxide (baking, brewing, distillery, liquid fuel industries). Probiotic yeast cultures based on Saccharomyces cerevisiae are currently well accepted and widely used in ruminant diets. Their nutritive value is high and rich in enzymes, fatty acids, vitamin B complex, unknown growth factors and amino acids (more than 40% of total dry matter).  The main purpose of using such additives in ruminant diets is to prevent rumen flora disorders and disturbances, especially those associated with the consumption of high energy concentrates. Inclusion of yeast in ruminant diets leads to increase of consumption of dry matter, utilization of fibre and other nutritive substances and increase of daily gains. Yeast cells also improve digestibility and absorption of minerals such as phosphorus, magnesium, calcium, copper, potassium, zinc and manganese. In recent years, due to increased consumer’s concern about safety, quality of animal products and environmental issues, the current purpose of using feed additives is not only to increase productivity but also to contribute to lower the risk of ruminant digestive carriage of human pathogens and to decrease excretion of polluting outputs like nitrogen-based compounds and methane.  S. cerevisiae is considered generally recognized as safe (GRAS) by the Food and Drug Administration (FDA) and thus is appropriate for use in animal feeds. Available products vary widely in both the strain of S. cerevisiae used, number and viability of cells present. Some products guarantee high numbers of live yeast cells and are sold as live yeast while other products are sold as yeast cultures containing both yeast cells and the media on which they are grown. Four factors can be considered to determine if a feed additive should be used anticipated response, economic return, available research, and field responses.                         

At birth the young ruminant is germ-free but with contact with his mother’s saliva, feces and that of other animals the neonate acquires a microflora rapidly. This prolonged contact between the mother and her young is more frequent in small size farming systems. In more intensive dairy systems the calf is rapidly separated from the mother and is often introduced to solid feed before the succession of all microbial populations is completed. This situation leads to an imbalanced microbial flora making the young ruminant more prone to suffer from different infections. Gastrointestinal disorders are one of the most important sources of economic loss in pre-ruminant animals. Cellulolytic rumen microorganism’s establishment is a faster in calves and lambs receiving S. cerevisiae daily. The cellulolytic population is also more stable in the supplemented animals. Protozoa appear in the rumen once the bacterial population is present as they feed on rumen bacteria. Studies have shown that protozoa appeared earlier in calves supplemented with S. cerevisiae. Also the ciliate protozoa colonised more rapidly the rumen in the presence of active dry yeast. The maturation of the microbial ecosystem is accelerated in the presence of the yeast. 

Probiotic yeasts have demonstrated their effectiveness to influence growth and activities of fibre-degrading microorganisms in the rumen. Probiotic yeasts enhance fungal colonisation of plant cell walls. Several modes of action were identified in this effect, one being the supply of thiamin, a vitamin required by rumen fungi for zoosporogenesis .The effectiveness of some yeast strains to stimulate growth or/and activities of fibrolytic bacteria has also been demonstrated. Cellulolytic bacteria become established earlier in lambs which received daily active dry yeast. Moreover most of the polysaccharidase and glycosidehydrolase activities increase in the presence of the yeast strain.  One of the main factors which could explain the beneficial effect of live yeasts on fibre degrading bacteria is the capacity of yeast cells to scavenge oxygen. Although the rumen is known to be considered as anaerobic, dissolved oxygen can be detectable and as high as 16 litres of oxygen can enter an ovine rumen daily during feed and water intake, rumination or salivation. Most of ruminal microorganisms are highly sensitive to oxygen. Yeast strains which are able to consume oxygen, stimulate bacterial activities. Redox potential of the rumen fluid is lowered in the presence of live yeasts suggesting that live yeast cells can create more favourable ecological conditions for growth and activities of the anaerobic autochtonous microflora.

In 2006, Jouany proposed a model for the first time which explains most of the positive effects that were attributed to yeasts when fed to ruminants. Briefly the model is based on the fact that yeasts are aerobic microorganisms which thrive on the oxygen trapped in the solid fraction of the ruminal content. Yeasts cells are closely associated to the solid particles around which a micro-consortium is created. A decrease in the redox potential of -20 mV was observed in the rumen of treated animals. This microenvironment simulates the growth of cellulolytic bacteria, their attachment to fibre particles and increases the initial rate of cellulose digestion. This will result in increased feed intake on supplementation of Saccharomyces cerevisiae. Selection of yeast strain as probiotic is important because there is a high variability in the oxygen scavenging property of yeasts. 

 

The stabilization of ruminal p^H provides a great advantage for lactating dairy cows.  Ruminal acidosis results from consumption of readily fermentable carbohydrates, causing lactic acid accumulation and marked post-prandial fall in ruminal p^H. As the pH falls, lactate-producing bacterial species Streptococcus bovis outnumbers the lactate utilizing species Megasphaera elsdenii and Selenomonas ruminantium.  Protozoa also disappear and bacterial diversity is largely reduced. If the pH continues to fall, lactobacilli replace S. bovis initiating a spiralling effect with excessive lactate accumulation. Yeast stimulates lactate users, increasing their numbersand serves as a competitor with lactate producers. Since lactic acid is the primary cause of acidosis in dairy cattle, reducing its concentration can have a significant effect on the p^H.           

The conversion of propionate to lactate and vice-versa is a function of the partial pressure of O₂ in the rumen. In situations where partial pressure of O₂ is low, the lactate to propionate reaction is favoured and in situations where the partial pressure of O₂ is high the propionate to lactate reaction is favoured. This explains why yeasts that decrease the partial pressure of O₂ in the rumen favour a fermentation pathway that will be less acidic, hence their effect in stabilizing rumen p^H. Rumen p^H stabilisation is also beneficial to cellulolytic microorganisms, which are acid-sensitive. The micro-consortium created around yeasts helps explain also why even if in small quantity and with a relatively short lifespan, yeasts are able to supply growth factors such as organic acids, B-vitamins and amino acids to rumen bacteria in close vicinity.

Entodiniomorphid protozoa which are known to engulf starch granules very rapidly and thus compete effectively with amylolytic bacteria for their substrate. In addition, starch is fermented by protozoa at a slower rate than by amylolytic bacteria and the main end-products of fermentation are VFA rather than lactate, which explains why these ciliates have a stabilizing effect in the rumen by delaying fermentation. Moreover, Entodiniomorphs are also able to consume lactate and thus may play an essential role in the prevention of lactate accumulation.

The concentration of somatic cells in the milk is directly related to the infection status of the udder.  If an infection occurs in the udder, it recruits leucocytes (somatic cells) to the scene in order to rid of the bacteria causing problems. The somatic cell count in milk is directly related to its quality. High somatic cell count in the milk is not desirable. Microorganisms do not adapt well to fluctuations in the rumen pH.  The bacteria dying in the rumen liberate endotoxins that cause inflammation of the hoof and udder. Since yeast stabilizes the ruminal pH, the somatic cell recruits decrease. 

In ruminants, hydrogen is produced by several hydrolytic and fermentative microbial species, and is mainly used to reduce carbon dioxide into methane by methanogenic archaea which represents the main ruminal hydrogenotrophic microbial community. The process by which hydrogen-producing and H₂-utilising microorganisms are able to interact is called “interspecies hydrogen transfer”. Hydrogen transfer towards methanogens is beneficial to the degradation of plant cell wall carbohydrates in the rumen. However, as a result of this process, methane is eructated by ruminants at 400 to 500 litres per day in adult cattle and this represents a loss of 8–12% of the energy available in the diet. Methane represents a greenhouse gas whose emissions have to be limited because it would contribute 18–20% of the global warming effect.

Various strategies have been investigated in order to mitigate ruminant methane production. It is assumed that yeast cultures reduce methane production in four ways are as follows:

Acetogenic bacteria, which produce acetate from CO₂ and H₂, appear to play an important role in the re-utilisation of fermentative hydrogen in some non-ruminant digestive ecosystems. Yeast cultures of Saccharomyces cerevisiae potentially stimulate acetogenic microbes in the rumen, consuming H₂ to form acetate and thus potentially reducing CH₄ production. Some aerobic viable yeasts or fungi (Saccharomyces or Aspergillus spp.) added in very small amounts have been reported to reduce methanogenesis in the rumen. Interesting beneficial effects of the yeast strain on growth and H₂-utilisation of acetogenic bacteria were observed in in vitro studies even in the presence of methanogens. Results of studies with different yeast strains have been somewhat conflicting, reporting either no effect in either an increase in methane production in batch cultures with mixed rumen microflora. However, the results appear to be strain dependent and variable in their impact on CH₄ production in the rumen. More research is required to screen a large number of yeast strains to isolate those that confer both a production benefit and significant CH₄ abatement potential. 

Researches have shown that in gnotobiotically-reared lambs harbouring a very simplified rumen microflora, ammonia concentration was decreased in the presence of active dry yeast. In another study with adult ruminants, the same effect on ammonia concentration was observed with daily supplementation by yeast culture. These data suggested some changes in the nitrogen metabolism of rumen microorganisms in the presence of yeasts. Recent in vitro findings suggested that one yeast strain could influence growth and activities of proteolytic rumen bacteria by limiting their action on protein and peptides.  The mechanisms of yeast action may be explained by a competition between live S. cerevisiae cells and bacteria for energy supply but also by a direct inhibitory effect of yeast’s small peptides on targeted peptidases. With an adequate balance between soluble nitrogen and carbohydrate supply Saccharomyces cerevisiae could enhance microbial growth and decrease nitrogen loss.

Fungal feed additives based on S.cerevisiae increase feed intake rather than alter feed conversion efficiency. The main effects of fungal feed additives are therefore regarded as being intake-driven. Many factors are known to influence appetite like palatability, the rate of fibre digestion, the rate of digesta flow, and protein status. The fungal products certainly have a pleasant odour and the ability of yeast to produce glutamic acid could benefit the taste of feedstuffs supplemented with yeast culture. Some studies have shown increased dry matter intake and milk production when yeast was fed during periods of heat stress, possibly reflecting the role in aiding appetite during time of stress.  

The general pattern with ruminants receiving fungal feed additives have shown to improve meat or milk production. The proper management strategies for dairy cattle are designed to prepare the cow for lactation and to minimize the incidence of metabolic diseases in the time of calving. During this time the cows go from a low-maintenance phase to a high performance period in her productive life. Proper nutrition and management during the transition period is critical to successful lactation. It is recommended to use some feed additives which are a group of feed ingredients that can cause a desired animal response in a non-nutrient role such as rumen pH shift, growth, or metabolic modifier. Dawson and Tricarico (2002) analyzed the results gained from 22 studies with Yea-Sacc®1026 (a natural feed additive containing metabolically active S. cerevisiae strain 1026) involving more than 9039 lactating dairy animals. He found an average increase in milk production of 7.3% in yeast-supplemented animals. Responses to supplementation were variable and ranged from 2 to 30% increase in milk production. Improved live weight gain has been observed in some studies.  

Sub-therapeutic use of antibiotics has demonstrated many benefits in animal production.  Antibiotics generally work by limiting detrimental microorganisms as well as growth and colonization of nonpathogenic bacteria. This in turn reduces length of intestines and overstimulation of the host immune system, all of which draw nutrients away from optimal performance. The concern associated with resistant strain development through antibiotic use has generated strong debate and objections to sub-therapeutic uses of antibiotic for promoting growth. Whether justified or not, alternatives to antibiotics must be investigated. Mannan oligosaccharide a yeast cell wall component can act as a high-affinity ligand offering competitive binding site options for gram-negative bacteria, which possess mannose-specific type-1 fimbriae. The immediate benefits are associated with pathogen removal from the digestive system without attachment and colonization. This phenomenon may elicit significant antigenic responses, thus enhancing humoral immunity against specific pathogens through presentation of the attenuated antigens to immune cells. In addition, this process may suppress the pro-inflammatory immune response, which is detrimental to production performance.  

An important part of the variability in the response to yeast supplementation is due to variation in the yeast strain used, its viability (survival during pelleting, for example), animal status and the nature of the diet.  It has been demonstrated that once the yeast product has been ingested, the same concentration of live cells can persist as long as 24–30 hours in the rumen without significant growth. If the distribution is not renewed, yeast concentration declines to undetectable levels after 4–5 days. Differences in effects on ruminal fermentations also depend on the strain of S. cerevisiae. The molecules such as organic acids and vitamins derived from yeast activity or yeast components themselves, such as peptides and amino acids, also participate in the effects observed on the rumen microflora. Another important factor to consider is the intrinsic variation between animals of a same herd. In effect, feeding behaviour factors such as rate of feed and water intake and physiological factors such as ruminal content turnover and volume of saliva production are variable from one animal to the other and may explain the discrepancies in the results reported from different trials. 

Yeast clearly affects many reactions occurring in the rumen. It stabilizes the rumen pH, stimulates certain cellulolytic bacteria, enhances fiber degradation, reduces methane production and nitrogen loss, increases milk production and thus increases gain and feed efficiency.  However, the extent and variation are the continued concerns regarding how advantageous or profitable it could be. Every animal’s response is different and can depend on their state. Indeed, field studies indicate that although positive effects on milk or meat production can be obtained, the animal response to such feed additives may be quite variable, according to various factors (nature of the diet, level of productivity, animal physiological and genetic factors, dose and strain of yeast used, etc.). A highly stressed and malnourished animal may benefit more from yeast than one which is in a well-managed production. It will be of great importance in the near future to better understand the nature of interactions between the yeast probiotic, the autochtonous flora and the dietary components in order to further predict the impact of such a probiotic in ruminant nutrition. Regardless, supplementing yeast could serve as a solution to persisting acidosis problems and provide some extra profit in cattle production.  

Bach, A., C. Iglesias, and M. Devant. 2007. Daily rumen pH pattern of loose housed dairy cattle as affected by feeding pattern and live yeast supplementation. Animal Feed Science and Technology 136(1-2):146- 153.

Chaucheyras-Durand, F.,and G. Fonty, G. 2001.Establishment of cellulolytic bacteria and development of fermentation activities in the rumen of gnotobiotically-reared lambs receiving the microbial additive Saccharomyces cerevisiae CNCM I-1077.Repord.Nutr. Dev. 41, 57- 68.

Chaucheyras-Durand, F., and G. Fonty, G. 2002. Influence of a probiotic yeast (Saccharomyces cerevisiae CNC I-1077) on microbial colonization and fermentation in the rumen of newborn lambs. Microb. Ecol. Health Dis. 14:30-36.

Chaucheyras-Durand, F., N. D. Walker, and A. Bach. 2008. Effects of active dry yeasts on the rumen microbial ecosystem: Past, present and future. Animal Feed Science and Technology 145(1-4):5-26.

Chiquette, J. 1995. Saccharomyces cerevisiae and Aspergillus oryzae, used alone or in combination, as a feed supplement for beef and dairy cattle. Can. J. Anim. Sci. 75:405-415.

Jouany, J. P. 2006. Optimizing rumen functions in the close-up transition period and early lactation to drive dry matter intake and energy balance in cows. Animal Reproduction Science 96(3-4):250-264.

Jouany, J. P., F. Mathieu J. Sénaud, J. Bohatier, G. Bertin and M. Mercier. 1999. Influence of protozoa and fungal additives on ruminal pH and redox potential. S. Afr. J. Anim. Sci. 29:65-66.

Lehloenya, K. V., D. R. Stein, D. T. Allen, G. E. Selk, D. A. Jones, M. M. Aleman, T. G. Rehberger, K. J. Mertz, and L. J. Spicer. 2007. Effect of feeding yeast and propionibacteria to dairy cows on milk yield and components, and reproduction. J. Anim. Physiol. and Anim. Nutr. 92:190-202.

Lynch, H. A. and S. A. Martin. 2002. Effects of Saccharomyces cerevisiae Culture and Saccharomyces cerevisiae Live Cells on In Vitro Mixed Ruminal Microorganism Fermentation. J. Dairy Sci. 85(10):2603-2608.

Newbold, C. J., F. M. McIntosh, and R. J. Wallace. 1996. Mode of action of the yeast, Saccharomyces cerevisiae, as a feed additive for ruminants. Brit. J. Nutr. 76: 249-261.

Nocek, J. E. and W. P. Kautz. 2006. Direct-fed microbial supplementation on ruminal digestion,      health, and performance of pre and postpartum dairy cattle. J. Dairy Sci. 89(1):260-266.

Oetzel, G. R., K. M. Emery, W. P. Kautz, and J. E. Nocek. 2007. Direct-fed microbial supplementation and health and performance of pre- and postpartum dairy cattle: A field trial. J. Dairy Sci. 90(4):2058-2068.

Robinson, P. H. and J. E. Garrett. 1999. Effect of yeast culture (Saccharomyces cerevisiae) on adaptation of cows to postpartum diets and on lactational performance. J. Anim Sci. 77(4):988-999.