With more and more awareness of how important gut health of the horse is – especially at this time of the year – I am asked a lot about gut supplements.

There are many out there and a lot are combinations of different additives. One such additive that is available on its own or as part of a combination product is yeast.

Horses have benefited from yeast in their diets for a number of years, many commercially available horse feeds (especially balancers) contain yeast of some description.

Much research has been conducted to determine the efficacy and mechanism of action of certain strains of yeast and therefore it should be given consideration above those additives that do not have research available to back up any claims.

A common finding in these studies is an improvement in fibre digestion, which indicates that feeding yeast results in a change in microbial activity in the horse’s hindgut. The first study evaluating yeast in equine diets was reported in 1983. Since then, yeast has become a common additive to horse diets.

Fibre digestion

The horse’s hindgut is where fibre digestion takes place.

Since its domestication, with the way it is fed and managed sometimes being at odds with the way it was designed, the horse has had various 'issues' with gut health.

Horses are different from most non-ruminant (single-stomached) animals in that they can digest and utilise large amounts of forage. Horses have a large caecum (26-34 litres) located between the small and large intestine, and a high volume large intestine (81 litres) that provides the capacity and ability to digest fibre in forages.

Similar to ruminants, horses do not produce the enzymes to digest fibre, but provide the environment and harbour the micro-organisms that digest fibre. These ferment fibre to volatile fatty acids, which the horse uses for energy and 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 utilised by the horse for its own protein needs. This microbial population also supplies the horse with B-vitamins.

Good or bad bugs?

There are various things that will have a detrimental effect on the good bugs in the horse’s digestive system some of which are:

* Stress

* Rapid feed changes (including sudden access to spring grass or hidden changes in grass from day to day)

* Wormers

* Antibiotics

* High use of starch

* Hormone imbalance

The impact of these can be minimised via management changes, but sometimes the good bugs need a little help.

As with most types of supplements for horses, there is an array of digestive enhancing additives to choose from. The jury is currently out as to how effective pre-biotic and pro-biotic supplements are in horses.

Research that has actually been carried out in horses (as opposed to humans) is scant, although anecdotal evidence is abundant. However, many of the yeast products, eg Yea-sacc do have EU registration for use in horses which means that a myriad of research into its efficacy and safety has been carried out.

This is a live yeast which promotes the good bugs, increases digestion of fibre and reduces the effects of lactic acid on both the gut lining and the good bugs.

The stabilising effect of Yea-sacc on hind-gut fermentation means that larger amounts of cereal can be fed to provide energy with a lower risk of colitis, acidosis, enterotoxaemia or laminitis. Most importantly, it also helps stabilise the hindgut during grazing when the carbohydrate profile of the grass can be very different from one day to the next.

Performance horses

Feeding yeast to horses during exercising and training may help condition the horse.

Lower plasma lactic acid concentrations after 35 minutes of exercise were observed 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 10 minutes of a 35-minute exercise workout for horses fed a diet with yeast, compared to a diet without it.

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.

Brood mares

Pregnant mares fed yeast culture had greater digestibility of dietary dry matter, fibre, protein, calcium, and phosphorus than those not supplemented with yeast culture.

Improved digestion helped mares cope with the reduced feed capacity and increased nutrient demands during the last trimester of gestation. Feeding yeast in early lactation can improve early milk production of mares and increase foal growth.

Mares supplemented with yeast during the first two weeks of lactation produced more milk than non-yeast fed counterparts. Those supplemented with yeast had foals with similar birth weights as those not supplemented with yeast culture, but foals at 56 days of age from yeast-fed mares were heavier than other foals.

Foals from the yeast-supplemented mares were also taller at the withers than foals from mares not supplemented.


There are many things we can do to keep our horse’s gut healthy – the most effective is to feed as much fibre as possible in the form of hay/haylage and chops and, also to ensure that concentrates are fed little and often to avoid overloading the gut with starch.

Yeast can provide a benefit in horse diets by improving feed digestion, nitrogen retention and reducing the risk of dysfunction due to acidosis.

Increased fibre digestion and better feed efficiency are the most common benefits of yeast supplementation along with a general improvement in the gut environment (less loose droppings etc).

Always remember; happy bugs, happy horse!