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
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
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
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
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
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
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.
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.
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,
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
Kowalski, J., J. Williams and H.F. Hintz.1990. Weight gains of
mares during the last trimester of gestation. J. Equine Pract.
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.
Ralston, S.L., E.L. Squires, and C.F. Nockels. 1989. Digestion in
the aged horse. J. Equine Vet. Sci. 9:203.
Yeast at 2 oz
Yeast at 4 oz
in the same row with different letters are different (P<.10).
neutral detergent fiber; IVTD ? in vitro true digestion; DMD ?
dry matter digestibility; SEM ? standard error of means.