Dispelling myths on probiotics for horses…

Horse catalogues and feed stores are loaded with a wide range of pre- and probiotics. But do they really help the horse?

In a presentation entitled ‘Hindgut Microbiome’, Dr Wendy Pearson, an assistant professor of equine physiology at the University of Guelph, discussed the role of microbes in digestion and the possible benefit of probiotics. Pearson’s presentation was a part of the Equine Summit hosted by Equine Guelph in Ontario, Canada.

The discussion begun outlining the anatomy and function of the horse’s digestive tract. Read more about the anatomy of the digestive tract here.

The first part, known as the foregut, consists of the stomach and duodenum. Horses are built to graze most of the time, consuming small amounts of roughage over a long period of time so the equine stomach capacity is just eight to fifteen litres. The first 8% of digestion happens in the stomach then the food moves on to the duodenum with a 6-litre capacity, the first and shortest segment of the small intestine. It carries out another 30% of the digestion process, turning food into energy without fermentation.

There is a small amount of microbial action in the foregut, but it is heavily influenced by diet, water, and the immediate environment. In this part of the gut, there is a physical and chemical breakdown of starches, and enzymes that enhance the breakdown of protein.

But in horses, most of the microbial action occurs in the hindgut: the caecum carries out 15% of digestion; the large colon 38% and the small colon 9%.

Fermentative digestion provides 35% of a horse’s daily energy needs through the production of fatty acids which are then used to produce ATP, the cell’s energy currency. The caecum is primarily responsible for fluid absorption.

Did you know? Since a horse salivates in response to chewing and horses are adapted to chew for many hours each day, an individual horse may produce up to 100 litres of saliva per day.

Fermentation begins in the large colon with the majority of cellulose fermentation occurring there.

The fibrolytic bacteria (the ones that specialise in breaking down fibre) prefer a high pH (alkaline) environment which is provided by roughage.

This is why a roughage-only diet increases the diversity of microbes in the hindgut.

In contrast, when starch is added to the diet in the form of grain or processed feed, most starch is digested in the foregut but some can travel to the hindgut. When this happens, the pH drops (becomes acidic), and fibrolytic bacteria become less efficient causing an increase in the numbers of lactic acid producing bacteria.

The build-up of lactic acid can result in a higher risk of laminitis, acidosis and colic, all of which can lead to serious health problems, or even death.

In her presentation, Dr Pearson who ran her own boarding and performance stable for many years, said that there are times when performance horses need starch (grain or processed feeds) to maintain condition. But warned that when starch is added to the horse’s natural (forage) diet, the digestive efficiency of the hindgut decreases so horses receive less nutrition from their hay and pasture. Additionally, if the amount of starch is too high, it is incompletely digested.

In draft horses and ponies, which evolved to live on low quality diets, adding starch to the diet can cause behavioural effects. Changes in diet can cause the vagus nerve to actually change taste receptors and also affect the horse’s emotional state. (The microbes can actually manipulate horse behaviour in an attempt to acquire what they need!)

Probiotics and prebiotics are advertised to increase digestive efficiency but, do they work?

Pearson said the answer is uncertain because there has not been enough research on horses. Most probiotic research has been on humans and humans have a very different digestive system.

Probiotics contain actual living microorganisms, either yeast or bacteria, which the manufacturers claim will benefit the health and digestion.

Prebiotics contain materials to enhance the bacterial performance—they are preferred feed substrates for the microorganisms. In other words, they are digested by the bacteria, not by the horse.

The first question we should ask of probiotics is whether or not they actually arrive in the hindgut alive and intact, or they are destroyed by antimicrobial enzymes.

If they survive, they could improve the nutrient absorption in low quality feedstuffs and possibly also enhance the immune system.

Anecdotal evidence would seem to imply that certain probiotics do indeed increase digestive efficiency and combat stress in animals which consume a high starch diet.

Pearson emphasises that users should follow manufacturers’ guidelines. Pearson said that a combination of probiotics and prebiotics is probably the most effective.

When asked which probiotics to use, she answered that consumers should look for companies and products with the most research behind them. She said that probiotics would be most effective for performance and possibly school horses with a high starch diet. Whereas a pleasure horse used for occasional riding needs nothing more than forage, salt and water. This is because a high forage diet enhances microbial performance, without the need for probiotics.

By understanding how digestion occurs in the hindgut, equine owners can make better decisions about feed choices. Adding starch to a horse’s diet may be appropriate to meet energy and condition needs, but it can result in a decrease in natural digestive pathways and greater risk of colic. Probiotics may, in some cases, reduce this risk.

Read more about the anatomy of the digestive tract here.