Nutrition, in its broadest sense, fosters and enhances health, wellbeing, performance and behaviour; neglecting it can lead to ill health, disease, poor performance and inappropriate behaviours. Horse owners often regard veterinarians as the primary source of unbiased nutritional knowledge (Murray et al, 2015). However, many veterinarians lack confidence in their knowledge of nutrition (Roberts and Murray, 2013; Nichols et al, 2023). Even a basic understanding of equine digestive function and the facts outlined in this review can enable veterinarians to offer well-informed nutritional guidance. There are several equine diseases for which there is substantial evidence that diet plays a role in their development, including equine metabolic syndrome, laminitis, disorders linked to vitamin E and/or selenium deficiencies, exertional myopathies, nutritional secondary hyperparathyroidism, hyperkalaemic periodic paralysis and the developmental orthopaedic diseases (Secombe and Lester, 2012). Equine nutrition research has progressed significantly (Harris, 1998; Garber et al, 2020), mirroring developments in human medicine (Pérez-Cano, 2023). The Animal Welfare Act (2006) explicitly mentions nutrition as one of the five domains, directly associating it with the wellbeing of the horse and its link to health, behavioural interactions and mental state. The potential of effective nutrition as a preventative medicine is only beginning to be realised.
Levels of nutritional knowledge
Veterinarians often rely on knowledge from school, with few pursuing further nutrition education (Roberts and Murray, 2013). A survey of European veterinary schools found that graduates were dissatisfied with the nutrition education provided (Becvarova et al, 2016). Many horse owners, particularly first-time owners, rely on veterinarians for guidance, though time constraints limit the ability to educate them thoroughly. Former Pony Club members often base nutritional knowledge on horsemanship lessons (Irish Pony Club, 2024). However, evidence shows that many horse owners lack a deep understanding of equine nutrition, leading to decisions based on tradition, folklore and misinformation (Hoffman et al, 2009).
The importance of good nutrition
Nutrition is vital for growth and development, especially in young and breeding horses. A review of growing Thoroughbred horses found that horses reach approximately 96% of their mature bodyweight, 98% of their mature height and all of their peak bone mass by 24 months of age (Huntington et al, 2020; Rogers et al, 2021) (Figure 1). Most breeds attain approximately 90% of their final wither height by 12 months of age (Jackson and Pagan, 1993).
Mouncey et al (2023) followed 275 Thoroughbreds foals from birth to 18 months of age, finding that half of these animals received veterinary intervention for musculoskeletal disease at least once. Developmental orthopaedic disease was the leading diagnosis (Mouncey et al, 2023). Diet is a key factor in the occurence of developmental orthopaedic disease, as it influences the overall hormonal balance in the body and has specific effects on hormones involved in endochondral ossification (Dunnet, 2008). Although developmental orthopaedic diseases have a multifactorial aetiology (genetic predisposition, exercise, confirmation, trauma) and there is currently no clear understanding of the contribution of each factor, diet can play a major role. Overfeeding can influence the hormones involved in endochondral ossification, particularly insulin-like growth factor 1 (Torres et al, 2020), and steady, moderate growth along a typical growth curve appears to be the best method for preventing developmental orthopaedic disease (Dunnet, 2008). Dietary mineral imbalances can also have an influence on developmental orthopaedic diseases. Copper has received particular attention because of its functional role in the activity of lysyl oxidase and superoxide dismutase, which are enzymes involved in cartilage formation and turnover. Lower copper status was previously implicated in the incidence of developmental orthopaedic diseases (Harris et al, 2004). Monitoring young horses' growth rates and adjusting diets may reduce the risk of developmental orthopaedic disease (Nery et al, 2006).
The gut microbiome
The gut microbiome is a collection of microorganisms that colonise the gastrointestinal tract, containing more than 100 times the number of genes as the entire human genome (Qin et al, 2010). Understanding this hidden half of the body is key to understanding health and disease. The microbiome, often referred to as the ‘forgotten organ’, has many known functions. A balanced microbiome is essential for digestion, vitamin uptake, athletic performance, recovery, long-term health, emotional wellbeing and mental clarity (O'Brien et al, 2022). Johnson (2020) demonstrated that humans with larger social networks tended to have a more diverse microbiome, suggesting that the microbiome may also influence personality.
For equines, the microbiome may be even more important as it is responsible for up to 70% of a horse's energy supply through volatile fatty acid fermentation in the hindgut (Vermorel and Martin-Rosset, 1997). As in humans, a balanced microbiome has several functions including B-vitamin synthesis for energy metabolism and red blood cell production, fibre digestion, immunomodulation and hormone production (Kauter et al, 2019). Equine-specific research has proven that dysbiosis, or an unbalanced microbiome, can alter fermentation patterns and can manifest as colic, leaky gut syndrome, colitis, laminitis, behavioural reactivity and stereotypies (Bulmer et al, 2019; Mach et al, 2020; Boucher et al, 2024). Mach et al (2020) have shown that the faecal microbiota in healthy horses is not completely stable. Stress (such as that caused by exercise and transport) is linked strongly with dysbiosis or changes in the microbiome.
The ‘Well Foal Study’ by the ALBORADA Trust at the University of Surrey tracked 52 foals from birth to 3 years old, revealing that those with a more diverse gut microbiome at 1 month old had fewer diseases and performed better in their racing careers (Leng et al, 2024). While it is well known that healthier animals have more robust immune systems and fewer illnesses, this was the first study of its kind to highlight the emerging science of ‘sportomics’ and how early-life gut microbiota could influence future sport performance (Bongiovanni et al, 2021). However, because of the small sample size in this study, further research is warranted to confirm these preliminary findings.
How to nurture a healthy microbiome
A healthy gut microbiota is characterised by a diverse and balanced community of microorganisms that perform vital functions for the host. Microbial diversity, composition, richness, functionality and resilience are all key markers (Hills et al, 2019). Next-generation sequencing technology has started providing detailed and specific information about the taxa found in different segments of the equine gastrointestinal tract. A study examining 11 horses of varying age, breed, gender and health status reported an increase in microbial diversity at the class level from the caecum to the faeces (Costa et al, 2015). While Firmicutes was the dominant phylum across all compartments, microbial diversity increased as the sampling location moved further along the tract. The researchers concluded that faecal samples could serve as a useful model for understanding changes in the distal gastrointestinal region as a result of this rise in diversity.
Garber et al (2020) studied the foal microbiome population and its development over time. They found that the biggest change in population occurred between 30 and 60 days old and by 2 months old, the gut microbiome was relatively stable (Garber et al, 2020). By 9 months old, the microbiome populations of foals were similar to those of adult horses (Costa et al, 2016), indicating that nurturing the microbiome in early life is critical for its development.
Equine-specific research has shown that dysbiosis can manifest as colic, laminitis, obesity, stress and poor performance (Boucher et al, 2024). When horses are fed according to their natural digestive anatomy (ie consuming small, continuous amounts of slowly degraded fibrous feed), their capacity to digest starch is not overwhelmed, and the health of their gut and microbiome is maintained. Horses have existed for over 55 million years, but domestication has only taken place in the last 5000 years. In that time, humans have shifted horses from their natural fibre-rich diets to starch-heavy cereals, leading to a rise in gastrointestinal issues such as colic, equine gastric ulcer syndrome and hindgut acidosis (Durham, 2009). Kauter et al (2019) discussed these anthropogenic effects on the equine microbiome in detail, citing factors including age, housing differences, pasture access, human contact, veterinary care and medications as significant influences.
Diet and the microbiome
Several studies have demonstrated how optimising dietary fibre intake supports a diversified microbiome (Julliand and Grimm, 2017; Ramsteijn and Louis, 2024). Horses are very poor at digesting starch; they have no amylase in their saliva, and the size of their stomach and small intestine (the starch-digesting parts of the gastrointestinal tract) are relatively small compared to the rest of their digestive tract (Harris and Shepherd, 2021). High-quality forage should make up the majority of their diet, highlighting the importance of regular forage batch analyses. Forage analysis provides valuable insight into the nutritional composition of hay, allowing for precise dietary adjustments. However, this may not be practical for many horses owners as a result of limited control over forage selection and the variability between batches. If hay sources change frequently or come from mixed suppliers, achieving consistency in forage quality can be challenging, making routine analysis less feasible. Where possible, the equine industry should adopt modern practices – similar to the ruminant sectors, which routinely sample forage, soil and water to maximise yields. Additionally, any abrupt diet changes should be avoided to prevent microbiota disruption (Collinet et al, 2021).
Feeding horses cereals can upset their digestive system. Tinker et al (1997) showed that feeding horses 2.5–5 kg of cereal daily increases the risk of colic by 4.8 times, while feeding more than 5 kg raises the risk by 6.3 times. Similarly, Hudson et al (2001) found that feeding more than 2.7 kg of oats daily increases the risk of colic by 5.9 times (Table 1). Racehorses today are commonly fed higher concentrate volumes during training. Luthersson et al (2009) studied the relationship between starch volumes and the incidence of equine gastric ulcer syndrome. Their findings showed that feeding 2 g/kg of starch per meal increases the risk of squamous ulcers by 2.6 times, while feeding over 2 g/kg per meal raises the risk by 3.2 times. In light of this and other nutritional management reviews, Coenen and Vervuert (2010) recommended feeding no more than 1 g/kg of bodyweight of starch per meal and less than 2 g/kg of bodyweight of starch per day to reduce the risk of colic.
| Starch volume | Effect on colic risk | Reference |
|---|---|---|
| 2.5–5 kg of cereal per day | = 4.8×colic risk | Tinker et al (1997) |
| >5 kg of cereal per day | = 6.3×colic risk | |
| 2.7 kg of oats per day | = 5.9×colic risk | Hudson et al (2001) |
Supplements
Nutritional supplements are often advocated for on diagnosis of many equine diseases although there is generally little to no scientific evidence of efficacy for most (Geor, 2006). Oke and McIlwraith (2010) provided a useful guide for practitioners to aid them when assessing and selecting supplements for horses affected by osteoarthritis. Up to 80% of horse owners include daily supplements in their horses' diets, with many using multiple supplements (Murray et al, 2018). This has become a massive, loosely regulated industry with significant profit potential, allowing products to be sold with little to no quality testing. Supplements fall under EC 767/2009, the same regulation that applies to horse feed (UK Government, 2009). This means that any product with ‘medicinal claims’ is not a supplement and should be treated as a medicine and therefore undergo strict safety, efficacy and quality testing before being granted a license. Many horses may not need additional supplements if they are fed a complete feed and have access to ad lib forage. Adding unnecessary supplements could disrupt the balance of their diets and lead to mineral absorption issues. Veterinarians should evaluate supplements as carefully as they do medications, ensuring that claims are supported by equine-specific research and that active ingredients are present in safe and effective concentrations.
Gut supplements
Scientific consensus highlights that variations between and within individuals make it difficult to clearly define what a ‘healthy’ microbiome looks like (Boucher et al, 2024). Microbiome research in equines is in its infancy but currently, conflicting research suggests that there is insufficient understanding to confidently manipulate its composition. Despite this, the market offers a wide range of equine gut supplements with little research backing their claims, driven by consumer demand and minimal regulation.
Probiotics
Probiotics are defined as living microorganisms with the ability to confer health benefits in the host (Salminen et al, 1999). The only licensed equine probiotic is Saccharomyces cervesisae, a living fungus that acts as a prebiotic (fuel for the microbiome) in the hindgut where it has been shown to increase the concentration of lactic acid using bacteria and lactobacilli in the caecum (Grimm et al, 2020). This probiotic has been consistently shown to increase the number of cellulolytic (fibre-digesting) bacteria in horses that are fed a high concentrate diet when included at effective inclusion rates (Faubladier et al, 2013). There are a lack of scientific data supporting the use of other probiotics such as Lactobacillus, Bifidobacterium and Enterococcus. Further research is needed to confirm their ability to survive the oral route and the acidic stomach environment, ensuring they reach their site of action and successfully populate the hindgut. Concerns have also been raised about their stability as live microorganisms (Berreta and Kopper, 2022). At present, these probiotics are not permitted for use in equine feed or supplements.
Prebiotics
As opposed to probiotics, prebiotics are not live organisms. These are fibres (chains of specific types of carbohydrates) that promote and serve as fuel for a healthy microbiome. Oligosaccharides such as mannan-oligosaccharides and fructo-oligosaccharides are prebiotics, as well as other fibre forms (pectin, cellulose, lignin etc). The more diverse fibre in a horse's diet, the better the quality of its diet (Julliand and Grimm, 2017).
Postbiotics
Postbiotics are also not live organisms; they are soluble by-products with biological activities released by components of the intestinal microbiota that could confer a health benefit on their host (Hosseini et al, 2024). In simple terms, they are the by-products of a healthy microbiome. There is promising research, although funded by the manufacturer, to support the use of a fermented yeast postbiotic that has been shown to mitigate exercise-induced stress hormone levels and inflammation when fed at the recommended daily dose (Valigura et al, 2021). Another manufacturer-funded field study found that a nutritional combination containing prebiotics, digestible fibres, B-vitamins and the same fermented yeast postbiotic had a significant effect on reducing equine gut acidity in Thoroughbreds in training (O'Connor and Mulligan, 2024).
Faecal microbiota transplant
In practice, many horses with gastrointestinal issues have responded well clinically to faecal transplant treatments. It has been a proven success in colitis cases in humans (C. difficile infection; Brandt, 2012) and in dogs (parvovirus; Pereira et al, 2018). Foals practice coprophagia from their dams and the environment, which is thought to be a part of their normal development. A donor should be chosen with no history of antibiotics or anthelmintics in the past 6 months and fed a roughage-based diet. The procedure is easily done on site with softened faeces administered via nasogastric tube alongside omeprazole (to limit acid denaturing bacteria). Currently, a lack of standardisation protocols in equine practice has resulted in highly variable techniques (Long et al, 2024). Although there is a dearth of information on the indications, optimal procedure and storage for faecal microbiome transplant, many horses are reported to have responded well clinically in practice (Mullen et al, 2018).
Antibiotics and their effect on the gut
Antibiotic-induced colitis is a widely recognised phenomenon (Kabir et al, 2024). Antibiotic treatment carries significant risks, and antimicrobial resistance is one of the greatest human and animal health crises of the modern world. The World Health Organization (2023) has predicted that antimicrobial-resistant infections will cause more deaths than cancer by 2050. Judicious antibiotic use is now more critical than ever, and routine antibiotic prophylaxis is no longer acceptable. Research has demonstrated the detrimental effects of antibiotic administration on the equine microbiome population. Theelen et al (2023) showed that microbiota changes were still evident in horses that had received oral trimethoprim-sulphonamide treatment 6 months previously. Avoiding unnecessary antibiotic use is imperative to combat antimicrobial resistance and preserve the individual's all-important gut microbiota. The Well Foal Study (Leng et al, 2024) highlighted the long-term impact of antibiotic use on 52 Thoroughbred foals. Those that received fewer antibiotics in early life had a more diverse gut microbiome at 1 month old, had fewer diseases in later life and performed better in their racehorse careers.
Conclusions
Nutrition is a powerful tool in preventative medicine for equine health, directly influencing growth, performance, disease resistance and behaviour. However, veterinarians often feel unprepared to provide comprehensive nutritional guidance. This review highlights the importance of evidence-based nutritional advice and the gut microbiome's role in overall health. Advances in equine nutrition research have revealed significant links between diet and common diseases, emphasising the need for veterinarians to stay informed about nutritional science. By connecting veterinary professionals equine nutritionists registered with the Association of Nutrition, veterinarians and the horses under their care can access reliable resources and improve overall health and welfare. Ongoing education, research on microbiome health and stricter regulation of supplements are necessary to improve veterinary practices and promote long-term equine health.