Yeast Culture in Horse Feeds
A Research Review

A significant number of research studies have been conducted with yeast culture in evaluating the product for benefits in horses. While its nutritional benefits on growth and performance physiology are not totally conclusive, the data presents a picture of a product that is a valuable horse supplement.

Yeast culture is a unique product because it is a complex fermented feed ingredient, with a broad spectrum of fermentation factors. Live or viable bakers yeast is first allowed to ferment a liquid fermentation broth, primarily molasses, and then the entire fermented "wort" containing the live yeast is mixed with cereal grains. This wet cereal mash is allowed to ferment, similar to leavening of bread, and the entire fermented mash dried, ground and packaged. The product was officially recognized and sanctioned as an approved feed ingredient by the Association of American Feed Control Officials in 1957.

Yeast culture contains fermentative metabolites produced during the fermentation process which enhance digestion and nutrient availability in the horse. This enhancement appears to result from the product’s ability to stimulate the bacteria in the digestive tract, primarily in the hindgut. While yeast culture has been used in animal feeds for more than 50 years, quality research with the product in horses has only been completed within the last 12 years.

The earliest research with yeast culture in horses concerned its effect on apparent digestibility and nitrogen retention. Godbee (l983), while at Clemson University, used four 3-year-old Quarter Horse geldings in a 4 x 4 Latin square design to test yeast culture’s effect on the digestibility of 70% and 50% roughage diets, plus nitrogen retention. While yeast culture produced numerical improvements, no statistically significant differences were detected.

At Texas A & M University, Webb et al. (l985), used mature Quarter Horse geldings and American Miniature Horse geldings to evaluate diet digestibilities. Feeding a 50% roughage diet and using total fecal and urine collection, they were unable to detect any significant differences in apparent digestibility or nitrogen retention. They also used ileal cannulated ponies to see if yeast culture affected the site of digestion -- pre-cecal vs. post-ileal. Again, they were unable to detect significant differences.

Glade and Biesik (l986) at the University of Maryland used yearling Thoroughbreds, instead of mature horses as used in previous studies, to measure the effect of yeast culture on digestibility and nitrogen retention in growing horses. In addition to a conventional type diet, they also fed l60 grams of urea per day to provide excess "recycled urea" to the hindgut. The researchers reported numerical improvements in apparent digestibility for all fractions measured (table l), with significant differences for hemicellulose digestibility and a significant improvement in nitrogen retention when yeast culture was fed. They also found that when yeast culture was added to the diet containing urea, fecal water-soluble nitrogen was reduced and fecal cell-bound nitrogen was increased, indicating that yeast culture stimulated hindgut microbial growth with enhanced conversion of recycled urea into microbial cell mass.

Growing Horses

The first growth study with yeast culture was a Louisiana trial using wild horses from Wyoming (Mason, 1983). Growth rate, feed efficiency and gain in withers height were measured over a 120 day trial period (table 2). It was found that feeding yeast culture significantly increased both body weight gain and gain in withers height by about 60 percent. A few years later, a New York Standardbred farm completed a growth comparison trial with 38 Standardbred weanlings and found a more moderate increase in foal growth (Brown, 1985). This study found that during a 77 day trial period, the yeast culture weanlings had an overall gain in body weight of 71.5 kg compared to 57.7 kg in the control group.

Bennett-Wimbush (1991) completed a growth study with Quarter Horse yearlings as research for a Master of Science degree at the University of Missouri. While this study showed a slight numerical increase in foal growth (table 3), the only significant response to feeding yeast culture was an increase in hip height gain (11.7 cm vs. 9.5 cm). Bone density was also estimated using dorsopalmar radiography, but no significant differences were apparent.

In the area of exercise physiology, the research with yeast culture has focused mostly on blood hemoglobin and packed cell volume, and blood lactic acid levels following exercise bouts.

The first study in this area was conducted at the University of Missouri (Loch et al., 1984). Nine mature Quarter Horses were randomly split into three treatment groups and fed one of three diets: 1) basal diet, 2) basal + iron sulfate, or 3) basal + iron sulfate + yeast culture. The horses were lightly exercised daily at a fast trot on a longe line (approximately 3.6 m/sec) for 10 minutes during six consecutive weeks. Blood samples were taken weekly immediately following the last exercise bout of each week and analyzed for blood hemoglobin and packed cell volume.

The results of this trial (table 4) showed little effect due to yeast culture until the sixth week. At this time, a large but nonsignificant increase in packed cell volume was observed, as well as a significant increase in hemoglobin level when yeast culture and iron sulfate were fed. These data suggested that feeding yeast culture with supplemental iron may increase aerobic capacity of horses under forced exercise, but that the response would not be immediately realized. It appears that several weeks of yeast culture supplementation in conjunction with conditioning are required for the response to become evident.

A few years later, a conditioning study was conducted in Michigan with unconditioned horses to determine the effect of yeast culture on heart rate and blood lactic acid, indicators of aerobic capacity (Campbell and Glade, 1989; Glade and Campbell-Taylor, 1990). Following a three week diet adaptation (without exercise), the horses were started with a five minute forced exercise bout (a steady walk on an inclined treadmill). The duration of the exercise was increased one minute per day, five days per week, so that after six weeks the exercise bout was for 35 minutes at a rate of 1.53 m/sec at a 14.8 degree incline. Blood samples were taken at five minute intervals during the exercise and for the first 20 minutes of recovery following the exercise. Blood lactic acid levels in the yeast culture supplemented horses were significantly lower after six weeks of conditioning after 20 minutes into the exercise bout (figure 1). In this study, significant differences in lactic acid levels were first detected after only four weeks of conditioning.

Since the conditioning program in the Michigan study was only moderate in intensity, and blood lactic acid levels were only near the threshold of anaerobic metabolism, a more intense study was conducted at the University of Illinois (Biel et al., 1990). They used eight Quarter Horse mares in a two-period crossover design, with half of the horses receiving yeast culture in the first period and the diets switched during the second period. Each test period lasted 18 days and consisted of exercise bouts three days per week at a rate of 4.5 m/sec for 8.5 minutes on a treadmill with an 11% grade. The mares also carried 18 kg of lead weight during the exercise. Heart rate was measured and blood samples collected at 0, 3, 6 and 8.5 minutes during the exercise bout and at 5 and 15 minutes of recovery after exercise. The blood samples were measured for lactic acid, hemoglobin, packed cell volume and amino acids.

Blood lactic acid levels were much greater in this study than those in the previous Michigan study, indicating that the exercise was much more intense (figure 2). While the differences in blood lactate levels between the control and yeast culture horses during exercise were not statistically different, there was a definite numerical difference similar to that of the previous study and the differences during the recovery period tended (P=.13) to be lower during yeast culture supplementation. Hemoglobin concentrations were greater when yeast culture was fed, but the differences were not significant. No differences in packed cell volume, amino acid profile or heart rate were apparent during the study.

Yeast culture is a safe, natural nutritional supplement which has been fed to horses for decades, and recent research with the product suggests that it can improve digestion and nutrient availability when high nutritional demands are apparent. The data suggests that it can promote faster and more efficient growth in foals and possibly improve the aerobic capacity of the physically active horse. Exactly how it accomplishes this is not fully understood, but appears to be related to the product’s ability to enhance the growth and activity of digestive bacteria in the hindgut, promoting better feed digestibility and a higher plane of nutrition in the horse. As research continues on the product, a better understanding of the benefits from feeding yeast culture, and how it produces these benefits, should become more evident. In the meantime, the research reviewed in this paper suggests that yeast culture is a valuable supplement for horses, especially those with high nutritional requirements.

1. Biel, M, L. Lawrence, J. Novakofski, K. Kline, D. McLaren, L. Moserand and D. Powell. 1990. Effect of yeast culture supplementation on exercising horses. J. Anim. Sci. 68 (Suppl.1):375 (Abstr.).
2. Bennett-Wimbush, K. G. 1991. Effect of Saccharomyces cerevisiae yeast culture supplementation on weight gains, skeletal growth and bone density of the third metacarpal in yearling Quarter Horses. M.S. Thesis. Univ. of Missouri, Columbia.
3. Brown, W. S. 1985. Effect of yeast culture on growth of weanling and yearling Standardbred foals. Blue Chip Farms, Wallkill, New York (unpublished data).
4. Campbell, M. and M. H. Glade. 1989. Effects of dietary yeast culture supplementation during the conditioning period on heart rates and lactic acid production by horses exercised on a treadmill. Proc. 11th Equine Nutrition and Physiology Symposium, pp. 72-78, Stillwater, Oklahoma.
5. Glade, M. J. and L. M. Biesik. 1986. Enhanced nitrogen retention in yearling horses supplemented with yeast culture. J. Anim. Sci. 62:1635-1640.
6. Glade, M. J. and M. Campbell-Taylor. 1990. Effects of dietary yeast culture supplementation during conditioning period on equine exercise physiology. Equine Vet. Sci. 10:434-443.
7. Godbee, R. 1983. Effect of yeast culture on apparent digestibility and nitrogen balance in horses. Clemson Univ., Clemson, South Carolina (unpublished data).
8. Loch, W., L. Brockschmidt and H. Harmon. 1984. Effect of supplemental iron, live Saccharomyces cerevisiae yeast and exercise on hemoglobin and packed cell volume of the blood of horses. J. Equine Vet. Sci. 4:125-127.
9. Mason, T. R. 1983. Effect of tall oil and yeast culture on growth rate of wild horses. McNeese State Univ., Lake Charles, Louisiana (unpublished data).
10. Webb, S. P., G. D. Potter and K. J. Massey. 1985. Digestion of energy and protein by mature horses fed yeast culture. Proc. 9th Equine Nutrition and Physiology Symposium, p. 64, East Lansing, Michigan.

Conventional

Added Urea

Apparent digestibility, %

Control

Yeast Culture

Control

Yeast Culture

Dry matter

71.8

78.4

73.6

75.9

Neutral detergent fiber (NDF)

56.5

71.0

62.0

64.2

Acid detergent fiber (ADF)

58.0

68.5

58.5

54.8

Hemicellulose

51.4^a

78.5^b

53.6^a

66.8^ab

Nitrogen

52.9^c

57.6^c

76.2^d

73.5^d

Nitrogen retention, % digested

12.6^c

22.6^d

8.6^c

19.1^d

Fecal N Fraction (% of fecal N):

     Water-soluble N

30.8^c

45.2^d

59.8^e

39.6^cd

     Cell-bound N

56.4^c

42.3^d

27.9^e

46.1^d

Control

Yeast Culture

Number of foals

17

17

Days on feed

120

120

Avg. daily gain, kg

0.27^a

0.43^b

Feed efficiency, feed/gain

16.11

9.84

Initial weight, kg

203.7

203.1

Final weight, kg

236.3

254.9

Total gain, kg

32.6^a

51.8^b

Initial withers height, cm

128.5

126.2

Final withers height, cm

134.1

135.1

Gain in withers height, cm

5.6^a

8.9^b

Control

Yeast Culture

Number of yearlings

7

7

Days on feed

182

182

Initial weight, kg

228.1

228.1

Final weight, kg

359.9

364.0

Total gain, kg

131.8

135.9

Initial withers height, cm

128.4

127.9

Final withers height, cm

139.7

140.0

Gain in withers height, cm

11.3

12.1

Initial hip height, cm

134.1

133.6

Final hip height, cm

143.6

145.3

Hip height gain, cm

9.5^a

11.7^b

Packed Cell Volume, %

Weeks of Exercise

1

2

3

4

5

6

Control

43.6

39.0

42.3

45.3

43.6

40.4

Control + Iron²

40.7

38.6

42.1

45.4

44.2

43.9

Yeast Culture³+ Iron

45.1

40.8

44.3

44.7

44.6

48.5

Hemoglobin, g/dl

Weeks of Exercise

1

2

3

4

5

6

Control

12.6

12.8

13.7

14.3

13.4

12.8^a

Control + Iron

12.5

11.8

13.5

13.8

14.4

13.4^a

Yeast Culture + Iron

13.3

12.8

13.8

13.6

13.8

14.5^b