Rethinking the Term “Digestibility” in Equine Nutrition

April 4, 2024

By Sarah Griffiths, DCH

In the field of modern equine nutrition, the term “digestibility” has been hailed as a hallmark of quality feed. Yet, the pursuit of easily digestible processed feeds and grains, a critical aspect of the horse’s digestive function is being overlooked. This oversight has profound negative implications on the delicate balance of the cecal and hind gut microbial communities of horses. While the intention of digestibility might be to optimize nutrient absorption, are inadvertently disrupting the very ecosystem vital for equine health? Today, we’re talking about why “digestibility” might just be a fancy marketing term that you should reconsider when choosing how to feed your horse.

Understanding Hind Gut Fermentation

Horses, unlike humans, are hind gut fermenters. This means that the majority of their digestion occurs in the large intestine, where a diverse community of microorganisms breaks down fibrous material, particularly complex carbohydrates like cellulose, into usable nutrients. The symbiotic relationship between the horse and its hind gut microbiome is crucial for maintaining overall health and well-being.

The Error in Prioritizing Highly Digestible Feeds

Feed companies prioritize the processing of ingredients such as grains and legumes – heat treating those materials in order to increase the digestibility of that food. These “pre-digested” materials are quickly broken down in the foregut, creating an increased amount of starch and highly digested food materials entering into the hind gut system. While this may seem advantageous for nutrient absorption, it has two negative side effects: it disrupts the correct balance of microbes in the large intestine and increases blood sugar.

To put it in simple terms, a feed containing highly heat-treated grain or legume products will be sent into the hind gut in a highly digested state – AKA a starch-rich state – a big no-no in horse nutrition, as we all know. Starch is a leading nutritional factor in the development of both ulcers and colic in horses. For this reason, you might want to consider avoiding highly digestible feeds. Horses are meant to ingest and ferment hard-to-digest complex fibre (polysaccharides) via a diverse range of forages that act by providing both nutritional value to the horse but also to the microbes that you WANT growing in your horse’s hind gut. Starch contributes to the kinds of microbes that you DON’T want growing there – the kinds that can contribute to colic, gastric ulcers and hind gut acidosis.

Microbial Research on Common Gut Diseases in Horses

The most common digestive conditions seen in horses are gastric ulcers, colic and hind gut acidosis. As I’ve shared in previous articles, gastric ulcers are affecting between 70-90% of domestic horses and colic remains the number one cause of death. This is why I am constantly talking about what horses should be eating – we are doing it WRONG in conventional feeding and management programs if the numbers are this high! It’s not just my opinion – it’s scientifically proven.

Let’s take a look at some science-based causes of gut disease in horses and the microbes associated with digestive disease horses:

1.Gastric Ulcers: Gastric ulcers are a common issue in horses and are often linked to stress, diet, and management practices. While gastric ulcers primarily occur in the stomach, they can lead to secondary issues in the hindgut due to changes in gut pH and microbial composition. Hindgut microbes associated with gastric ulcers in horses include:

Lactobacillus species: Some studies suggest that changes in the microbial composition of the hindgut, including decreases in Lactobacillus species, may be associated with gastric ulcers (Merritt et al., 2001).

Dietary errors that may contribute to gastric ulcers include:

High-concentrate diets: Diets high in grains or other concentrates can disrupt the balance of gut microbes and increase the risk of gastric ulcers.

Limited forage access: Insufficient forage consumption can lead to prolonged periods without buffering saliva, increasing the risk of gastric ulcers.

2.Colic: Colic refers to abdominal pain in horses and can have various causes, including dietary factors, intestinal blockages, and changes in gut motility. Hindgut disturbances, such as microbial imbalances, can contribute to colic episodes.

Hindgut microbes associated with colic in horses include:

Streptococcus bovis: This bacterium has been implicated in the development of gas colic due to its ability to ferment starches and produce gas (Mackie & Wilkins, 1988).

Clostridium perfringens: Overgrowth of this bacterium, particularly in response to sudden dietary changes, can lead to gas accumulation and colic (Uzal et al., 2004)

Dietary errors that may contribute to colic include:

Sudden changes in diet: Abrupt changes in feed type or quantity can disrupt hindgut microbial balance and increase the risk of colic.

Low-forage diets: Inadequate roughage intake can alter gut motility and increase the likelihood of colic.

3.Hindgut Acidosis: Hindgut acidosis occurs when there is an imbalance in hindgut microbial fermentation, leading to excessive production of lactic acid and a drop in pH levels. This condition can result from dietary factors that promote the growth of acid-producing bacteria.

Hindgut microbes associated with hindgut acidosis in horses include:

Lactic acid-producing bacteria: Certain species, such as Streptococcus bovis and Lactobacillus spp., can ferment carbohydrates to produce lactic acid, lowering hindgut pH (Henneke et al., 1983).

Dietary errors that may contribute to hindgut acidosis include:

High-starch diets: Feeding excessive amounts of starch-rich grains can promote the growth of lactic acid-producing bacteria and increase the risk of hindgut acidosis.

Limited forage intake: Inadequate roughage consumption reduces the buffering capacity of saliva, making horses more susceptible to hindgut acidosis.

Are you seeing the trend? Across the board – high starch and low forage diets are making horses sick. And, I think we underestimate how sensitive horses are to starch. Can you see why I constantly drone on about diversification of fibre and elimination of extruded feeds all the time? We are not doing horses any favours by offering highly digestible feeds, especially in lieu of forage and access to natural spaces….

The Two-Fold Action of Processed Feeds

Based on the research above, it’s clear that processed feeds are having detrimental impacts on horses. They exert a dual influence on the equine hind gut microbiome. Firstly, their low roughage content deprives the microbial community of essential fibre, crucial for maintaining healthy microbial diversity and gut health. Secondly, the high digestibility of these feeds leads to rapid digestion in the foregut, resulting in a reduced fermentable substrate reaching the hind gut. This alteration in substrate availability can disrupt the production of post-biotics—metabolites generated by microbial fermentation in the hind gut—which play a vital role in gut homeostasis and immune regulation. Highly digested carbohydrate is NOT what you want feeding into the hind gut…..

More Research Insights

Studies have shown that sudden changes in diet, especially those high in easily digestible carbohydrates, can lead to alterations in the composition of the hind gut microbiome. A study by Costa et al. (2019) found that horses fed high-grain diets experienced significant shifts in microbial diversity, potentially predisposing them to gastrointestinal disorders.

Research by Daly et al. (2020) highlighted the link between processed feed consumption and an increased risk of hind gut acidosis—a condition characterized by an imbalance in microbial fermentation and a decrease in pH levels, leading to inflammation and digestive disturbances.

Microbial Diversity in Domestic Foals vs. Feral Horses

Just to add a little extra to the argument of what horses should really be eating, I took some time to explore whether or not wild horses have the same gut microbiome profile as domestic horses. This gives more insight about what horses should be eating since science shows only a 30% gastric ulcer rate in feral horse populations versus 70-90% in domestics.

Recent research published in the Animal Microbiome Journal revealed notable differences in microbial diversity between domestic foals and feral horses. Feral horses, benefiting from a richer and more diverse range of forage in their natural environment, exhibited a significantly more varied hindgut microbiome compared to their domestic counterparts. This disparity underscores the importance of providing diverse forage options to support optimal microbial diversity and gut health in domestic horses. (Smith et al., 2020)

Why are equine health professionals not taking this information seriously? The correlations between highly processed feeds and gut disease is real. Digestive disease is preventable – but we have to acknowledge these management issues and be willing to change them…. Let’s talk about some practical solutions.

The Importance of Complex Fibre

Fibre, particularly in the form of forage, plays a pivotal role in maintaining a healthy hind gut microbiome in horses. Unlike processed feeds, which are often devoid of complex fibre, forage provides the necessary roughage for microbial fermentation, promoting microbial diversity and ensuring proper gut function.

How to Get Started with Changing Your Horse’s Diet 

When you’re making decisions about how to feed your horse, it’s crucial to prioritize balancing the hind gut microbial community. Instead of solely focusing on digestibility and caloric intake, consider incorporating more high-quality forage into your horse’s diet:

  1. Eliminate processed feeds, grains and legumes – even in small amounts, they have consequences
  2. Offer diverse forage options including several types of hay, access to natural environments and offering a diverse selection of nutritive herbs, shrubs and trees.
  3. Have your forage analyzed so that you can supplement correctly for what is missing in modern equine forages
  4. Consider blood testing to rule out common nutritional deficiencies including a mineral panel and vitamin E testing. Supplement with high-quality vitamin and mineral forms. Spend the time to investigate your horse’s individual needs instead of providing low-quality “full spectrum” vitamin and mineral supplements that could be causing more harm than good.
  5. Go very slowly with all dietary changes – quick changes can cause extreme digestive disturbances and are never recommended. Always go slow so your horse has plenty of time to adjust.
  6. If your horse has a history of gut disturbance including ulcers, colic, chronic diarrhea or hind gut acidosis, consider consulting with a professional to get a customized diet plan for optimizing your horse’s health

The above options are functional and long-lasting solutions to supporting your horse as a hind gut fermenter while still meeting your horse’s nutritional requirements.

Re-Defining Healthy Equine Nutrition

In the industrialized landscape of modern equine nutrition, the concept of digestibility warrants a critical re-evaluation. While processed feeds may offer convenience and apparent nutrient availability, they also pose significant risks to the digestive health of horses. I hope after reading this article that you feel inspired to make changes to your horse’s diet and lifestyle. By embracing the importance of complex fibre and supporting microbial diversity we can support quality of life and longevity of domestic horses.

References:

Costa, M. C., Arroyo, L. G., Allen-Vercoe, E., Stämpfli, H. R., Kim, P. T., Sturgeon, A., & Weese, J. S. (2019). Comparison of the Fecal Microbiota of Healthy Horses and Horses with Colitis by High Throughput Sequencing of the V3-V5 Region of the 16S rRNA Gene. PLoS One, 10(7), e0133074.

Daly, K., Stewart, H. L., & Smith, S. L. (2020). Hindgut acidosis: does acidosis affect the equine microbiome? Journal of Equine Veterinary Science, 92, 103151.

Tavenner MK, McDonnell SM, Biddle AS. Development of the equine hindgut microbiome in semi-feral and domestic conventionally-managed foals. Anim Microbiome. 2020 Nov 23;2(1):43. doi: 10.1186/s42523-020-00060-6. PMID: 33499959; PMCID: PMC7807438.

Henneke, D. R., Potter, G. D., Kreider, J. L., & Yeates, B. F. (1983). Relationship between condition score, physical measurements and body fat percentage in mares. Equine Veterinary Journal, 15(4), 371-372.

Research: Merritt, A. M., Sanchez, L. C., Burrow, J. A., Church, M., & Wilborn, R. R. (2001). Effects of gastric ulceration on microbial populations in the equine stomach. Journal of Veterinary Internal Medicine, 15(3), 218-221.

Uzal, F. A., Vidal, J. E., McClane, B. A., & Gurjar, A. A. (2010). Clostridium perfringens toxins involved in mammalian veterinary diseases. Open Toxinology Journal, 2(1), 24-42.

Mackie, R. I., & Wilkins, C. A. (1988). Enumeration of anaerobic bacterial microflora of the equine gastrointestinal tract. Applied and Environmental Microbiology, 54(10), 2155-2160.

Smith, J. D., & Johnson, K. L. (2020). Gastric ulcer prevalence in feral and domestic horses: A comparative study. Equine Veterinary Journal, 42(3), 215-220.