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Supplementing Horse Diets with Yeast Increases Fiber Digestion and Improves Feed Efficiency

A variety of livestock and companion animals, including horses have benefited from yeast in their diets for a number of years. Many research studies have been conducted to determine the efficacy and mechanism of action of yeast. A common finding in these research studies is an improvement in fiber digestion, which indicates that feeding yeast results in a change in rumen microbial activity (ruminants) or gastrointestinal tract. The first study evaluating yeast in equine diets was reported in 1983. Since then, yeast has become a common additive to horse diets.

Horse Digestive System
Horses are non-ruminant herbivores. Thus, in the wild, they consume almost all forage diets. Domesticated horses should be fed mostly forage diets with concentrated supplements fed as needed. Supplements provide additional essential amino acids, minerals, vitamins, and energy needed to support the targeted activity (performance, gestation, growth, etc.,). Supplements may also supply various feed additives targeted at improvement of overall performance or health.

Horses are different from most non-ruminant (simple-stomached) animals in that they can digest and utilize large amounts of forage. Horses have a large cecum (26-34 liters) located between the small and large intestine, and a high volume large intestine (81 liters) that provides the capacity and ability to digest fiber in forages. Similar to ruminants, horses do not produce the enzymes to digest fiber, but provide the environment and harbor the microorganisms that digest fiber. The cecum and colon of the horse has a similar microbial population to that of the rumen of cattle and sheep. The microorganisms ferment fiber and available non-structural carbohydrates to volatile fatty acids, which the horse uses for energy. These microorganisms also break down protein (that was not digested in the small intestine) to ammonia and re-assimilate the nitrogen from ammonia and urea (recycled from the blood) into microbial protein. Some of the microbial protein is digested and utilized by the horse for its own amino acid needs. This microbial population also supplies the horse with B-vitamins.

Yeast and Digestion of Feeds
Yeast appears to have an important role in the microbial digestion process. The precise mode of action has not been identified. However, the most common finding in research studies with yeast shows an increase in the fiber-digesting bacteria population and fiber digestibility.

Yeast is a potential source of vitamins and/or growth factors. For example, thiamin from yeast increased viability of a ruminal fungus in one reported study. Other researchers have isolated yeast components that stimulate growth of cellulolytic bacteria under laboratory conditions. Georgia researchers proposed that yeast was supplying certain organic acids that favor the proliferation of lactate utilizing bacteria (S. ruminantium). These scientists have shown benefits for organic acids (malate, in particular) in dairy cows, but at levels greater than that provided by the simple addition of a yeast product.

Others proposed that removal of free oxygen by yeast improves the rumen environment because oxygen is detrimental to anaerobic ruminal bacteria. The so-called "oxygen scavenging" effect could be substantially beneficial under artificial laboratory conditions, where potential harmful effects of oxygen are magnified. However, ability to prove this theory in animal tests has been difficult. The greatest problem remains explaining how a small amount of yeast produces the magnitude of responses observed in a variety of animals.

Yeast appears to improve feed palatability, which helps horses maintain a more consistent feed intake. Therefore, yeast may help overcome the negative effects of less palatable feeds.

Many yeast products are commercially available. Product characteristics range from live cell concentrates to dilute yeast cultures including the media in which the yeast is grown. Most yeast sources available use the species Saccharomyces cerevisiae. However, many different strains of S. cerevisiae have been selected for the special fermentation produced to make unique bakery or brewery products.

Dawson and Hopkins (1991) tested over 50 strains of S. cerevisiae and found only seven strains with the ability to stimulate growth of fiber-digesting bacteria. Using mixed cultures of ruminal bacteria, Newbold and Wallace (1992) demonstrated that brewers yeast strains improved a critical population of fiber-digesting bacteria more than bakers yeast strains. Thus, one critical effect in choosing a yeast strain is to demonstrate its ability to increase the fiber-digesting bacteria population or fiber digestion. ADM Alliance NutritionTM conducted numerous studies using Prosponse 
® (Table 1). Yeast products found effective in ruminants have also been effective in improving fiber digestion in horses.

Growing and Performance Horses
Several studies have shown improvements in growth of young horses fed yeast culture. Ciro (1991) reported weanling horses fed yeast culture were five kg heavier and had a 10% faster growth rate than those not fed yeast culture over a six-month period. This improvement in growth rate may be the result of improved feed digestibility.

Feeding yeast culture increased fiber digestion and nitrogen retention
(Table 2) in yearling Thoroughbred horses (Glade and Biesik, 1986). Fiber, calcium, and phosphorus digestibility, and nitrogen retention were improved in three-year-old horses fed diets containing yeast culture and 50-70% forages (Godbee, 1983). Glade and Sist (1988) found an increase in dry matter, neutral detergent fiber, acid detergent fiber, and nitrogen digestibility in yearling horses exercised and fed yeast culture. Improving fiber and nitrogen digestibility by feeding yeast culture would allow young horses to convert feed more efficiently for growth and attain faster growth rates than those not fed yeast culture.

Feeding yeast to horses during exercising and training may help condition the horse. Campbell and Glade (1989) reported lower plasma lactic acid concentrations after 35 minutes of exercise in young adult horses fed yeast culture compared to a diet without yeast culture. They also found lower heart rates during the first five and final ten minutes of a 35-minute exercise workout for horses fed a diet with yeast compared to a diet without yeast. Thus, inclusion of yeast in diets of exercising horses seemed to improve their aerobic metabolic capacity. The reason for this effect is unclear, but may be related to the improved nitrogen retention or fermentation profile of the gut when horses are fed yeast culture as shown in other studies.

Breeding and Lactating Mares
Brood mares in late gestation may have a reduced capacity to consume feed because of the large size of the fetus, additional tissue mass, and fluid volume related to the pregnancy. Some studies have shown that brood mares with ad libitum intake either lose body condition (Lawrence et al., 1992) or maintain their body weight (Kowalski et al., 1990) during the last trimester of gestation. Most of the weight gain during pregnancy occurs during the last trimester of gestation when the fetus gains approximately 60% of its total weight. The high growth rate of the fetus requires an increased protein, energy, mineral, and vitamin supply.

Pregnant mares fed yeast culture had greater digestibility of dietary dry matter, fiber (ADF and NDF), protein, calcium, and phosphorus than those not supplemented with yeast culture (Glade, 1991a). Improved digestion would help mares cope with the reduced feed capacity and increased nutrient demands during the last trimester of gestation. If mares are in adequate body condition (body condition score of 6-7 on a scale of 1-9), they can utilize stored fat as an energy source during the last trimester of gestation (Lawrence et al., 1992; Kowalski et al., 1990).

Feeding yeast in early lactation can improve early milk production of mares and increase foal growth (Glade, 1991b; Glade 1991d). Mares supplemented with yeast during the first two weeks of lactation produced 35.4 lb of milk per day compared with 31.7 lb for those not supplemented with yeast. Mares supplemented with yeast culture had foals with similar birth weights as those not supplemented with yeast culture (Glade, 1991b). However, foals at 56 days of age from mares supplemented with yeast were 57 lb heavier than foals from mares not supplemented with yeast. The foals from the yeast-supplemented mares were 3.5 inches taller at the withers than foals from mares not supplemented with yeast.

Glade and Pagan (1988) studied the effects of yeast on foals beginning at ten weeks of age and nursing their dams. The yeast culture was hand-fed daily with granulated sugar. After eight days of yeast feeding, the plasma amino acid profile was changed compared to nursing foals not fed yeast. Plasma lysine levels from day 8 to day 36 were increased in foals fed yeast compared with foals not fed yeast. The increased amino acid level in nursing foals fed yeast may be related to the improved nitrogen retention found in other studies. Foals from yeast-supplemented mares had higher concentrations of plasma glucose, triglycerides, and free amino acids at one, four, and eight weeks after birth (Glade, 1991c). These foals had greater body weight gains and wither heights than foals from mares not supplemented with yeast.

The digestive efficiency of older (20 years of age or older) horses tends to be less than young horses (Ralston et al., 1988). Pagan (1990) reported that mature horses consuming yeast culture had increased phosphorus and fiber digestion compared with horses not fed yeast culture. Based on the improved digestive efficiency with younger horses, one would expect older horses to benefit from feeding yeast.

Yeast can provide a benefit in horse diets by improving feed digestion and nitrogen retention. Increased fiber digestion and better feed efficiency are the most common benefits of yeast supplementation. Yeast products that have been efficacious at improving fiber digestion in ruminant diets have also been efficacious at improving fiber digestion in horse diets. The improved nutrient digestion and feed efficiency has a wide array of benefits for horses at different stages of life, especially growth, breeding, mid-to-late gestation, and early lactation. Performance horses during training and conditioning have also shown benefits of yeast supplementation.


Campbell, M. and Glade, M.J. 1989. Effects of dietary yeast culture supplementation during the conditioning period on heart rates and lactic acid production by horses exercised on a treadmill. In: Proc. 11th Equine Nutr. Physiol. Symp. (Okla. St. Univ., Stillwater, OK) p.72.

Ciro, T.T. 1991. Effect of Yea-Sacc1026 on growth rate and wither height of Colombian Warmblood Weanlings. In: Biotechnology in the Feed Industry. T.P. Lyons (Ed.) Alltech Technical Publications, Nicholasville, Kentucky, p.355.

Dawson, K.A. and Hopkins, D.M. 1991. Differential effects of live yeast on the cellulolytic activities of anaerobic ruminal bacteria. J. Anim. Sci. 69(Suppl. 1):531 (Abstr.).

Glade, M.J. 1991a. Dietary yeast culture supplementation of mares during late gestation and early lactation. 1. Effects on dietary nutrient digestibilities and fecal nitrogen partitioning. J. Equine Vet. Sci. 11:10.

Glade, M.J. 1991b. Dietary yeast culture supplementation of mares during late gestation and early lactation. 2. Effects on milk production, milk composition, weight gain and linear growth of nursling foals. J. Equine Vet. Sci. 11:89.

Glade, M.J. 1991c. Dietary yeast culture supplementation of mares during late gestation and early lactation. 3. Effects of mares and foals plasma metabolite, amino acid and endocrine profiles. J. Equine Vet. Sci. 11:167. Glade, M.J. 1991d. Effects of dietary yeast culture supplementation of lactating mares on the digestibility and retention of the nutrients delivered to nursing foals via milk. J. Equine Vet. Sci. 11:323.

Glade, M.J. and Bieski, L.M. 1986. Enhanced nitrogen retention in yearling horses supplemented with yeast culture. J. Anim. Sci. 62:1635.

Glade, M.J. and Pagan, J.D. 1988. Understanding protein digestibility and utilization in the young growing horse and a role for yeast culture. In: Biotechnology in the Feed Industry. T.P. Lyons (Ed.) Alltech Technical Publications, Nicholasville, Kentucky, p.113.

Glade, M.J. and Sist, M.D. 1988. Dietary yeast culture supplementation enhances urea recycling in the equine large intestine. Nutr. Rep. Intl. 37:11.

Godbee, R. 1983. Effect of yeast culture on apparent digestibility and nitrogen balance in horses. Clemson University Research Bulletin.

Kowalski, J., J. Williams and H.F. Hintz.1990. Weight gains of mares during the last trimester of gestation. J. Equine Pract. 12:6.

Lawrence, L.M., J.DiPietro, K. Ewert, D. Parrett, L. Moser and D. Powell.1992. Changes in body weight and condition of gestation mares. J. Equine Vet. Sci. 12:355.

Newbold, C.J. and R.J. Wallace. 1992. The effect of yeast and distillery by-products on the fermentation in the rumen simulation technique (Rusitec). Anim. Prod. 54:504.

Pagan, J.D. 1990. Effect of yeast culture supplementation on nutrient digestibility in mature horses. J. Anim. Sci. 69(Suppl. 1):371.

Ralston, S.L., E.L. Squires, and C.F. Nockels. 1989. Digestion in the aged horse. J. Equine Vet. Sci. 9:203.



Table 1  Effects of Yeast on Feed Digestion Under Bench Top Laboratory Conditions

24-hour Digestion


Prosponse Yeast at 2 oz

Prosponse Yeast at 4 oz

Competitive Yeast Product


NDF digestion, %



33.8 a

34.2 a



59.4 a

64.0 b

60.0 a

60.8 a


DMD, %

59.2 a

62.3 b

59.0 a

59.0 a


abMeans in the same row with different letters are different (P<.10).

NDF ? neutral detergent fiber; IVTD ? in vitro true digestion; DMD ? dry matter digestibility; SEM ? standard error of means.



Table 2  Digestibilities of Feed Fractions and Retained Nitrogen of Diets With and Without Live Yeast Culture or Urea Fed to Growing Yearling Thoroughbred Horses







Corn + Pellets

Corn + Pellets

+ Yeast

Corn + Pellets

+ Urea

Corn + Pellets

+ Yeast + Urea




Apparent Digestibility, % of Intake


Dry Matter































% of Digested Nitrogen (N)


Retained Nitrogen






aPooled standard error of a mean.

b,cMeans within a row without a common superscript differ (P<.05).

d,eMeans within a row without a common superscript differ (P<.01).

NDF  neutral detergent fiber; ADF acid detergent fiber

Glade and Biesik, 1986.



ADM Alliance Nutrition, Inc. , a wholly owned subsidiary of the Archer Daniels Midland Company