Live yeast for horses
Dr Gérard Bertin, Dr Elinor McCartney


A specific strain of Saccharomyces cerevisiae already was widely used in animal feeds in many countries and had been awarded provisional European Union approval for food-producing ruminants, including calves, cattle for fattening, and dairy cows. However, a separate, dossier was required for provisional EU approval for use in horses (see: Live yeast in EU: Specific strain, specific approval... at the end of this article). Further efficacy studies are now running under EU conditions in order to achieve permanent EU authorisation for this live yeast strain in horses. These studies are an opportunity to build on previous research in this area, as well as in obligatory requirement for final EU approval.

The basic EU efficacy claim for this yeast strain as a live microbial product is increased fibre digestion – cellulose and hemicellulose – in the caecum of the horse. The supporting data indicate an increase in acetate (C2) concentrations and an increased ratio of acetate + butyrate (C2+C4) to propionate (C3).

In addition, there is an increase in fibrolytic activity, which seems unrelated to the enzyme activities identified. The product reduces the risk of dysfunction due to acidosis in the hindgut, and allows flexibility to increase energy incorporation in the diet.


Live yeast – modes of action in ruminants

Research of live yeasts in ruminants is much further advanced than in equines, which are non-ruminant herbivores. Several milestones appear in the published literature:

-Yeast cultures must be metabolically active to be effective (Dawson et al., 1990; El Hassan et al., 1993);

-Live yeast strains vary in efficacy, as in efficiency, effectiveness, and breadht of action (Dawson and Hopkins, 1991; Newbold and Wallace, 1992; Newbold et al., 1995);

-Live yeast optimises rumen pH by stimulating lactic acid utilising bacteria (Williams et al., 1991; Girard et al., 1993; Chaucheyras et al., 1995);

-Live yeast scavenges oxygen in the rumen, reducing redox potential (Jouany et al., 1994; Newbold et al., 1996);

            -Live yeast provides peptides and amino acids and increases the numbers of cellulolytic bacteria (Dawson and Hopkins, 1991; Chaucheyras et al., 1993; Girard and Dawson, 1994; Girard, 1996);

-Live yeast promotes the growth of beneficial rumen bacteria, including cellulolytic and lactic acid utilising bacteria (Matthieu et al., 1996; Newbold et al., 1996; Wallace and Newbold, 1992);

-Live yeast improves dry matter (DM) intake by accelerating fibre digestion in the rumen (Dawson and Hopkins, 1991; Smith et al., 1993; Kumar et al., 1997); and

-Live yeast supports protein synthesis by boosting microbial protein turnover in the rumen and flow into the small intestine (Erasmus et al., 1992).


But, exactly how does a specific live yeast strain work effectively in ruminants which are not natural hosts? To produce yeast biomass, optimal conditions for growth are pH 4.5, at 32ºC, in a medium rich in sugars, vitamins, trace elements, and with high oxygen concentration. The biomass production is largely exothermic due to production of ATP (38), and the biggest challenge is maintaining the temperature.

By contrast, when a live yeast is administered to ruminants, very different, non-optimal conditions prevail – pH 6-6.6, 39ºC, and low redox potency (-250-300 mV). How does the yeast survive and provide benefit to the animals?

Current research-based hypotheses suggest that the yeast survives through uptake of glucose and oxygen, and that certain live yeast strains can stabilise rumen function by these means (Table 1).


Table 1.

How do live yeasts act in ruminants?


Research-based hypothesis


Uptake of glucose

Live yeasts uptake glucose from rumen fluid thus inhibiting lactic-acid-producing bacteria (such as Streptococcus bovis).

Less lactic acid is produced.

Scavenge oxygen in the rumen

(reduction of redox potential)

Oxygen is toxic to many beneficial rumen cellulolytic bacteria, which are strict anaerobes.

Creation of a stable microclimate

and substrate

for growth of desirable microbes

Live yeast cells are huge compared with many rumen micro-organisms, so could provide a useful ”physical platform” for microbial growth.

Provision of nutrients

(e.g. peptides, vitamins, amino acids)

Yeast cell wall breakup may partly explain positive effects, but not completely, because dead yeasts also provide nutrients but are not as effective as live yeasts, which boost the activity of lactic-acid-consuming bacteria, such as Megasphaera elsdenii.

Produce specific peptides

and nutritional cofactors

Research suggests a dynamic equilibrium where yeast peptides boost logarithmic growth of micro-organisms.


Under these conditions, however, there is a secondary effect that also helps to stabilise the rumen. The double-walled yeast cell breaks down, releasing peptides and vitamins into the rumen medium. These nutrients or nutritional co-factors promote growth of bacteria, particularly increasing the numbers of cellulolytic bacteria.


Modes of action in horses

How does a specific live yeast strain work effectively in horses, which, like cattle, are not natural hosts of yeast?

Several papers published in the late 1980s and early 1990s in breeding mares and yearlings in the USA showed that live yeast improves feed digestibil­ity (Glade and Biesik, 1986; Glade and Sist, 1988; Glade 1991a; Glade 1991b; Glade 1991c). Researchers speculated that improvements in digestibility are linked to the capacity of yeasts to im­prove microbial fermentation of fibre, and in horses such processes take place in the hindgut.

More than a decade later, French scientists provided convincing evidence to confirm this hypothesis, demonstrating numerous similari­ties between the modes of action of live yeasts in horses and ruminants (Medina et al., 2000, 2001, 2002). The Saccharomyces cerevisiae with provisional approval in the EU increased the concentration of total anaerobic bacteria and lactate users in the hindgut. This was concordant with an increase in lactate utilisa­tion. Enzymatic activities, such as xylanase, CMCase, and xylosidase were significantly increased in the caeco-colonic system, especially in the caecum, when the live yeast was added to the feed. Essentially, live yeasts act primarily on caeco-colonic microflora and digestion in equines, but on ruminal microbes and digestion in the rumen of ruminants.

Specifically, the French studies contributed useful insights into the problem of high-starch diets in horses. Race horses and other equine athletes require high-energy diets to meet the demands of training and perform­ance. However, cereal-rich diets are frequently implicated in colic or laminitis, which at worst mean death or euthanasia for the affected animal, and often result in long periods off work, thus ruining a promising equine career.

The main findings of the French research point to how live yeast may help equine nutritionists, owners and trainers to meet the equine athlete's need for energy, without provoking disastrous incidents of colic of lam­initis (Table 2).


Table 2.

Summary of French research in horses


Effects of high starch diets

in equine hindgut


Increases caecal anaerobes

and lactic acid-using bacteria

When excess starch is fed, residual starch in the hindgut provokes a dysfunction in the caecum and colon, resulting in lactic acid accumulation, low (acidic) pH, a decrease in cellulolytic bacteria, and digestive disorders such as colic and enterotoxaemia.

Decreases cellulolytic bacteria in caecum

Increases lactobacilli and streptococci

Increases lactic acid, decrease pH

Decreases ratio of



Effects of live yeast supplement

in equine hindgut, primarily caecum


Stimulates total anaerobes

and increases % of cellulolytic bacteria

Main effects occur in high fibre diets, thus improving fibre digestion.

Modifies ratio of

acetate-butyrate/propionate in colon

Stimulates lacting acid-using bacteria

Reduces lactic acid, increases pH

Main effects occur in high starch diets, thus reducing the risk of digestive or related disorders.

Reduces caecal ammonia


Increases caecal cellulolytic activity

Effects occur in both high fibre and high starch diets

Reduces variation in microbial numbers

(lower SD*)

Results in a more stable hindgut bacterial balance and fermentation.

Notes: * SD = standard deviation


These findings also illustrate the parallel with ruminants. Whereas live yeasts fed to ruminants help to reduce the incidence of acidosis and bloat, in horses there seems to be a preventive effect in relation to colic and other problems which also result from excess dietary starch. In horses, a diet with high starch and/or rapid fermentable sugars (RFS) leads to sub-clinical acidosis in the caeco-colonic section of the gut. Growth of certain microbial populations - including same pathogenic populations - accelerates to the detriment of other populations, resulting in a reduction of fibrolytic activity. Live yeast supplementation influences microbial populations in the caecum, increasing cellulolytic activity and reducing negative effects of sub-clinical acidosis and the incidence of colitis.

The stabilising effect of live yeast on hindgut fermentation in the equine allows trainers to feed a larger amount of starchy cereals to provide that energy, with a lower risk of colitis, enterotoxaemia, or laminitis. Thus equine athletes have a better chance to stay in training, keeping fit and hea1thy, so that they can fulfill their genetic promise.


Mares with nursing foals

The most consistent effect of live yeast supplementation in horses is improved feed digestibility, particu­larly in mares and yearlings. American research has evaluated the benefits of such improve­ment in mares and foals (Glade 1991a; Glade 1991b; Glade 1991c). Live yeast supplementation gave a 12% boost to milk production in mares, and the milk produced was more nutritious for the foal, since it contained more energy and protein, resulting in improved nutrient intake and better growth.

Thus supplemental live yeasts help nursing foals start life with optimum growth. This is particularly important for thoroughbred foals, which are officially classed as yearlings on 1st January, and which will be racing as three-year-olds, long before they reach physical maturity.

In comparison with the vast number of research papers published in rela­tion to live yeast supplementation of ruminant feeds, there is relatively little published data on horses. These initial results are encouraging and scientists are now targeting research at defining more clearly the mechanisms by which live yeasts work in the equine hind­gut. This is the subject of the second phase of work with the first strain of S. cerevisiae to achieve provisional EU approval for use in horses, since for definitive EU authorisation additional significant and convincing data will be required.


Dr. Gérard Bertin is a specialist in live yeasts as feed additives, with 37 publications in peer-reviewed scientific journals. He worked previously for Lallemand Animal Nutrition and Health and currently directs European Union regulatory affairs for Alltech.

Dr. Elinor McCartney manages Pen & Tec Consulting, helping companies wishing to register feed additives in the EU.

Complete references for this article are available from Dr. Gérard Bertin,


Feed International, august 2005, p 12-17, Watt Publishing Co., Mt. Morris, IL, USA


Live yeast in EU: Specific strain, specific approval

The European Union's 2000 White Paper on Food Safety led to sweep­ing EU legislation affecting the food chain, with a strong focus on health, welfare and safety of all components of that chain, including animals, workers, consumers, and the environ­ment. The EU aspires to the highest standards of food safety, globally, and EU feed additive registration is now the toughest in the world. By way of contrast, in the USA, live cultures such as bakers’ or brewers’ yeast, for example, may be fed to all animals as ”generally recognised as safe” or GRAS feed ingredients, requiring no special approval process. However, in the EU, an exacting, strain-specific registration process is required, cov­ering safety, identity, and efficacy for each target animal category.

Therefore, even though a specific strain of Saccharomyces cerevisiae already had been awarded provisional EU approval for food-producing ru­minants, including calves, cattle for fattening, and dairy cows, a separate, equine dossier was required for provi­sional approval for use in horses. This equine live yeast dossier included both safety and efficacy sections, with an abridged identity section covering stability in feeds and premixes.

Thus, the product S. cerevisiae NCYC1026-CBS 493.94-EC n° 5 (Yea-Sacc® 1026 from Alltech) was the first live yeast culture to achieve provisional EU approval in horses (OJ Regulation n° 490/2004). To date, this yeast strain has been pro­visionally approved in the EU for all classes of horses from 2 months post-weaning onwards, including pregnant mares.

Permanent EU authorisation of this product requires further significant ef­ficacy studies under different EU hus­bandry conditions. Fortunately, this work is another opportunity to build on current knowledge concerning the use of live yeasts in horses.