Yeast Cultures

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 Introduction | Young growing horses | Broodmares | Performance horses | References | About the author


While yeast products have been around for many years being added quite regularly to beef, dairy and poultry feed, it is only recently that the use of yeast culture has been considered for addition to horse feeds.

What is yeast culture? Yeast culture by definition is the dried product composed of yeast, and the media it was grown on, dried in such a manner as to preserve the fermenting activity of the yeast.

The benefit of the yeast culture comes from the metabolites produced during the fermentation process. It is suggested that the metabolites stimulate the bacteria in the hind gut of the horse, increasing their activity which results in an increase in digestion of feeds by the bacteria. The increase in activity is a result of changes in the bacteria population found in the hind gut. Moore et al, (1994) found that the bacteria which digest fiber in the hind gut of the horse increased in numbers when the horses were supplemented with yeast culture. This increase in numbers can result in more nutrients from the feed being available to the horse. Research has shown that horses fed diets supplemented with yeast culture digested more of the dry matter and fiber than did unsupplemented horses. The digestion of fiber in the horse results in the production of volatile fatty acids which the horse uses as a source of energy.

In addition to increasing the digestion of dietary fiber, Glade & Biesik (1986) noted a positive effect of yeast supplementation of protein digestion in the young growing horse.

The other significant effect from yeast culture supplementation is the improvement in phosphorus digestibility. Pagan (1990) reported that the digestibility of phosphorus improved an average of 22.3% when horses were fed yeast culture.

The addition of yeast culture in feeding programs for young growing horses, broodmares and performance horses has been evaluated. While not all research has shown statistically significant improvements a number of positive results have been found when yeast culture was fed.

Young Growing Horses

A study in 1983 with weaned foals fed for 120 days found a significant increase in daily gain and wither height when foals were supplemented with yeast cultures (Table 1).

Table 1: Comparison of growth of weanlings on diets with and without yeast culture (Mason, 1983).

Form of Mineral
Daily Gain
Wither Height
Increase (inches)
With Yeast Culture
Without Yeast Culture

In an on farm demonstration, Standardbred weanlings were fed a foal ration supplement with a yeast culture or the same foal ration without yeast culture. The supplemented foals gained 0.93 kg per day vs. 0.75 kg for the unsupplemented foals. Unfortunately this study did not record changes in wither height to allow one to determine if the growth was skeletal growth or if the foals were just gaining weight.

A study with Warmblood weanlings fed a high forage diet plus 1.5 kg of concentrate with or without yeast culture found that the yeast culture foals had a 10% greater weight gain and an approximately 2% greater gain in wither height, (Ciro 1991).

A Missouri study using yearlings found no difference in daily gain, or wither height with yeast culture supplemented diets, however they noted a 10% improvement in feed efficiency, (Bennett-Wimbush, 1991).

The research suggests that modest improvements in growth are possible with the addition of yeast culture to the diet of the young growing horse. It appears that the level of improvement will be affected by the level and duality of forage in the diet, and the age of the horse when it is fed the yeast culture. If horses are fed high forage diets where the quality of the forage is questionable, a greater response may be expected.


In the mature horse, studies have shown improved fiber, protein and phosphorus digestibility due to enhanced microbial fermentation in the hind gut. Lactating broodmares fed a yeast culture supplemented diet showed a short term increase in milk production. This increase while significant persisted only for the first two weeks of lactation. However, analysis of the milk noted significant improvements in the nutrient content of the milk for the first 8 weeks of lactation. The gross energy content, protein level and other nutrient in the milk were increased as a result of the yeast culture supplementation. The increased nutrient content of the milk from supplemented mares resulted in improved growth of the foals during the first 8 weeks of life. It is important to note that while there was an improvement in foal growth rate, the trial only measured the affects of supplementation up to the 8th week post foaling.

Performance Horses

The effect of yeast culture on the performance horse is not clear. In one study where the horses were worked intensively there was a trend for a reduction in the lactic acid levels in blood during the recovery period but no significant differences due to diet during the exercise bout were noted, (Biel et al, 1990). However, in a study where the horses were lightly worked there was a reduction in blood lactate for those horses to receiving yeast culture, (Glade & Campbell, 1990).

In the performance horse it appears that the level of conditioning, the intensity of the activity and length of time the supplement is fed will affect the results obtained from feeding yeast cultures.

The use of yeast culture to supplement your horses diets has a number of benefits. The improved digestibility of the fiber portion of the diets is especially important for those horses that are fed high forage diets. The improved protein digestion is important for those horses (ie. young growing horse and the lactating mare) which have higher protein requirements and the improved phosphorus availability is useful since phosphorus can be deficient in many feeds and meeting phosphorus requirements is always a challenge.


 1. Bennett-Wimbush, K., 1991. Yeast Culture Research Report. 1994-1, Effect of Yeast Culture on Growth in Yearling Horses. Diamond V Yeast, Cedar Rapids, Iowa.
 2. Biel, M., L.M. Lawrence, J. Novakofski, K. Kline, D. McLaren, L. Moss, D. Powell, 1990.  Effect of Yeast Culture Supplementation on Exercising Horse. Journal Animal Science, 68 (supp) 375.
 3. Ciro, T.T., 1 99 1 . The Effect of Yea Sacc (1026) on Growth Rate & Wither Height of Colurnbian Warmblood Weanlings. Biotechnology Briefs in Alltech 7th Annual Symposium : Biotechnology in the Feed Industry. Lexington, Kentucky. p. 355.
 4. Glade, M.J., 1 99 1 a-c. Dietary Yeast Culture Supplementation of Mares During Late Gestation and Early Lactation.
     a) Effect of Dietary Nutrient Digestibilities and Fecal Nitrogen Partitioning. Journal of Equine Vet Science 1 1:10.
     b) Effects of Milk Production, Milk Composition, Weight Gain and Linear Growth of Nursing Foals. Journal of Equine Vet Science 1 1 :89.
     c) Effects on Mares and Foals Plasma Metabolite, Amino Acid and Endocrine Profiles. Journal of Equine Vet Science 1 1:1 67.
 5. Glade, M.J. and M. Campbell-Taylor, 1990. Effects of Dietary Yeast Culture Supplementation During the Conditioning Period on Equine Exercise Physiology. Journal Equine Vet. Science 10.434.
 6. Glade, M.J. and L.M. Biesik, 1986. Enhanced Nitrogen Retention in Yearling Horses Supplemented with Yeast Culture. Journal Animal Science 62. 1635
 7. Mason, T.R., 1 993. Effect of Tall Oil and Yeast Culture on Growth Rate of Wild Horses. Res. Bull McNeese Slate Univ., Lake Charles, L.A.
 8. Moore, B.E., K.E. Newman, P. Spring, V.E. Chandler, 1994. The Effect of Yeast Culture (Yea Sac 1026) in Microbial Populations and Digestion in the Cecum and Colon of the Equine. Abstract #969. Journal of Animal Science. Vol 72 Supp (1).
 9. Pagan, J.D., 1 990. Effect of Yeast Culture Supplementation on Nutrient Digestibility in Mature Horses. Abstract 340, Journal of Animal Science, Vol. 68, Supp. 1.

About the Author

Dr. Bob Coleman was formerly an Equine Nutritionist and Horse Specialist with Alberta Agriculture Food and Rural Development. Dr. Coleman is currently the Equine Extension Specialist with the Department of Animal Sciences at the University of Kentucky.

This information appeared in "Additives for Horse Feeds", which was presented at, and appears in the Proceedings of, the 1996 Alberta Horse Breeders and Owners Conference.

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This information published to the web on January 15, 2002.
Last Reviewed/Revised on January 22, 2015.