Feeding the microbiome
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As herbivores and hindgut fermenters, horses are particularly reliant on the microbes that live in the large intestine. Plant fibre (known as structural carbohydrates, the most abundant being cellulose) contains large amounts of energy-providing glucose, however the way these molecules are structured means that digestive enzymes are not able to break it down. Gut microbes however, do have the necessary enzymes to break fibre such as cellulose down into glucose. The bacteria use the glucose for their own energy needs, and in turn produce volatile fatty acids (VFAs), which the horse can absorb and use for energy through a number of mechanisms. Without this symbiotic relationship, the horse simply would not be able to survive on grasses and shrubs alone.
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With domestication and the advent of agriculture, we have found other ways to sustain horses – preserving grass in the form of hay and, crucially, concentrated forms of energy such as grains. Grains contain high amounts of non-structural carbohydrates – starch predominantly. Again, these are long chains of glucose molecules, but they can be broken down by mammalian digestive enzymes – in the horse this occurs in the small intestine. If large amounts of starch are fed, it can overwhelm the ability of the small intestine and instead enter the hindgut, where microbes can ferment it. The type of bacteria that prefer utilising starch, compared to cellulose or other fibres, are different, and those that use starch (amylolytic bacteria) produce different substances – rather than beneficial VFAs, they can produce lactic acid. This can make the hindgut more acidic, which can lead to digestive upset, laminitis and, very importantly, alters the microbial population
Whilst this idea has long been understood, it is only in recent years that scientists have begun to uncover a more detailed picture of the microbiome – how sensitive the bacteria are to change, and what sort of dietary changes and components affect the diversity and structure of the overall population.
Researchers now understand that forage-only diets (pasture or hay) result in the most diverse and stable microbial populations. Diversity in bacteria means that the horse will be able to adapt to acute changes in diet more easily – an important ability when sugar and starch levels in grasses can fluctuate not only seasonally, but day to day and hour to hour.
Researchers now understand that forage-only diets (pasture or hay) result in the most diverse and stable microbial populations. Diversity in bacteria means that the horse will be able to adapt to acute changes in diet more easily – an important ability when sugar and starch levels in grasses can fluctuate not only seasonally, but day to day and hour to hour.
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Despite this, the equine gastric microbiome is acutely sensitive to changes in diet – an abrupt change from a fibre-based pelleted feed directly to pasture can result in increases in the relative abundance of the lactobacillus and streptococcus bacteria that are linked with colic and laminitis, however within days on pasture there is a rapid increase diversity. The bigger dietary issue is with concentrate feeds, particularly grains. Horses fed only forage have very low numbers of lactobacillus and streptococcus bacteria, however when fed 50% of the diet as (grain-based) concentrate, this can quickly change, even when the overall starch levels in the diet are considered “safe” (2g starch/kg bodyweight per day).
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Bacterial populations also appear to fluctuate more when horses are fed concentrate, which leaves them less able to cope with stressors and potentially trigger gastric disorder or laminitic events. This fluctuation can be reduced if concentrate is fed “little and often”, as has so long been advised.
Recent work has looked at the source of starch – corn, oats or wheat middlings, as the starch in these different grains have varying properties and characteristics. They found that corn and wheat middlings had a much greater effect on potentially negative amylolytic bacteria than did oats – where feeding oats resulted in a 10 times increase, there was a 100,000 times increase when corn (fed to provide the same amount of starch) was fed! Fibre-consuming bacteria (cellulolytic) were also decreased when corn, wheat middlings or high amounts of oats were fed, however there was no effect over a lower amount of oats, compared to forage only. There are two possible explanations for these results – firstly, the amount of starch reaching the hindgut is known to vary between grains – corn is known to be particularly resistant to digestion in the small intestine, whilst oats are well digested by enzymes in the small intestine. This will mean that less oat starch gets to the hindgut to cause problems. Alternatively, there may be differences in the grain components that affect the bacteria themselves.
It’s clear that minimising and carefully choosing dietary starch is beneficial to for the equine microbiome, but what about newer trends in feeding, such as “super fibres” like soybean hulls and beet pulp, or adding fat sources?
Research is ongoing – stay tuned, and if you'd like to get involved personally, please visit equiGI or their Facebook page. This small but dedicated research group is based in Sydney and looking for horses owners to help their research into the equine microbiome.
References
Dougal, Kirsty, et al. "Characterisation of the faecal bacterial community in adult and elderly horses fed a high fibre, high oil or high starch diet using 454 pyrosequencing." PloS one 9.2 (2014): e87424.
Fernandes, Karlette A., et al. "Faecal microbiota of forage-fed horses in New Zealand and the population dynamics of microbial communities following dietary change." PloS one9.11 (2014): e112846.
Harlow, Brittany E., et al. "Effect of dietary starch source and concentration on equine fecal microbiota." PloS one 11.4 (2016): e0154037.
Venable, Erin B., et al. "Effects of feeding management on the equine cecal microbiota." Journal of Equine Veterinary Science 49 (2017): 113-121.
Willing, B., et al. "Changes in faecal bacteria associated with concentrate and forage‐only diets fed to horses in training." Equine veterinary journal 41.9 (2009): 908-914.
Recent work has looked at the source of starch – corn, oats or wheat middlings, as the starch in these different grains have varying properties and characteristics. They found that corn and wheat middlings had a much greater effect on potentially negative amylolytic bacteria than did oats – where feeding oats resulted in a 10 times increase, there was a 100,000 times increase when corn (fed to provide the same amount of starch) was fed! Fibre-consuming bacteria (cellulolytic) were also decreased when corn, wheat middlings or high amounts of oats were fed, however there was no effect over a lower amount of oats, compared to forage only. There are two possible explanations for these results – firstly, the amount of starch reaching the hindgut is known to vary between grains – corn is known to be particularly resistant to digestion in the small intestine, whilst oats are well digested by enzymes in the small intestine. This will mean that less oat starch gets to the hindgut to cause problems. Alternatively, there may be differences in the grain components that affect the bacteria themselves.
It’s clear that minimising and carefully choosing dietary starch is beneficial to for the equine microbiome, but what about newer trends in feeding, such as “super fibres” like soybean hulls and beet pulp, or adding fat sources?
Research is ongoing – stay tuned, and if you'd like to get involved personally, please visit equiGI or their Facebook page. This small but dedicated research group is based in Sydney and looking for horses owners to help their research into the equine microbiome.
References
Dougal, Kirsty, et al. "Characterisation of the faecal bacterial community in adult and elderly horses fed a high fibre, high oil or high starch diet using 454 pyrosequencing." PloS one 9.2 (2014): e87424.
Fernandes, Karlette A., et al. "Faecal microbiota of forage-fed horses in New Zealand and the population dynamics of microbial communities following dietary change." PloS one9.11 (2014): e112846.
Harlow, Brittany E., et al. "Effect of dietary starch source and concentration on equine fecal microbiota." PloS one 11.4 (2016): e0154037.
Venable, Erin B., et al. "Effects of feeding management on the equine cecal microbiota." Journal of Equine Veterinary Science 49 (2017): 113-121.
Willing, B., et al. "Changes in faecal bacteria associated with concentrate and forage‐only diets fed to horses in training." Equine veterinary journal 41.9 (2009): 908-914.