Probiotics are live organisms (either yeast or bacteria) thought to have a beneficial effect on gut bacteria when consumed. The use of equine probiotic supplements is common practice but research is limited and has produced varied results. However some studies have suggested that lactobacilli bacteria and some other probiotics may have beneficial effects in horses and other species. A recent study conducted in collaboration with the WALTHAM Equine Studies Group aimed to determine whether lactobacilli bacteria could reduce hindgut imbalances as result of starch overload and produced some very interesting results…

The consequences of starch overload

Horses evolved to consume a high fibre, low starch diet. However in order to meet their energy requirements, some horses are fed large amounts of cereal grains which are high in starch. When the amount of starch fed exceeds the small intestine’s capacity to digest it, undigested starch overflows into the large intestine or ‘hindgut’. Microbes in the hindgut include cellulolytic or ‘fibre digesting’ bacteria, amylolytic or ‘starch and sugar digesting’ bacteria, lactate producing bacteria and lactate utilising bacteria.


When undigested starch reaches the hindgut it is rapidly fermented by amylolytic bacteria resulting in an increase in lactic acid. Lactic acid is fermented by the lactate utilising bacteria to produce short chain fatty acids which can be absorbed and utilised as an energy source. However if lactic acid production exceeds the capacity of lacatate utilising bacteria, lactate levels remain high causing a drop in pH (referred to as hindgut acidosis) and subsequent upset to the microbial population of the gut.

New Research

This study evaluated the effect of 3 different species of lactobacilli bacteria (probiotics) on pH, amylolytic (starch and sugar digesting) bacteria and lactate utilising bacteria. Faecal bacteria were incubated in test tubes with ground corn, wheat and oats to mimic the hindgut environment following a starch overload. Tubes were then subjected to one of the following ‘treatments’

  • No bacteria (controls)
  • acidophilus (LA)
  • buchneri (LB)
  • reuteri (LR)
  • An equal mix of LA, LB & LR bacteria
  • Dead L. reuteri


Key Results

  • All 3 cereal grains/ starch sources caused a drop a pH with the largest reduction seen in corn and wheat.
  • In some cases, the addition of lactobacilli bacteria at lower concentrations caused a further drop in pH which was not surprising given that lactobacilli are acid producing bacteria. However at higher concentrations, the addition of lactobacilli reduced the drop in pH for all grains regardless of the species
  • Overall LR bacteria was most consistently effective species at reducing microbial and pH changes
  • Corn was fermented most rapidly followed by wheat and then oats. The addition of LR bacteria reduced the speed of starch fermentation for wheat (but not corn or oats)
  • Mixing the bacteria species had no additional affect
  • Dead and live LR bacteria had the same effect



What does this mean?

  • Some probiotic supplements contain a blend of bacteria species which are claimed to work in synergy thus offering greater benefit to the horse. However the results of this study suggest that targeted probiotics could provide a better treatment/ prevention strategy for hindgut acidosis.
  • It is currently accepted that in order to be effective, ‘probiotic’ bacteria must reach the hindgut alive. However a very unexpected but fascinating finding in this study was that ‘dead’ bacteria had the same effect. Although the reason for this is unclear, it raises interesting questions for future studies to explore.
  • Reducing the rate of starch fermentation could improve the stability of hindgut microbes by allowing them more time to adapt although this may be dependent on the type of cereal fed
  • Whilst the results of this study are exciting, it was completed ‘in vitro’ (in a test tube) and further research ‘in the horse’ is required.
  • It is also important to note that live bacteria are not currently licenced for use in feeds and supplements in the EU.


For more advice on feeding to support your horse’s digestive health contact the SPILLERS Care-Line.