Why feed a horse?

By Anna Welch BVSc, BSc, MRCVS. Veterinary Nutrition Director, TopSpec.

Feeding a horse is often seen as a routine management task, yet it is an essential factor influencing equine health, welfare and performance. Water, forage, and the nutrients supplied through appropriate hard feeds, are essential not only for survival, but for supporting healthy physiological functions through metabolism. 

As equine retailers are frequently the first point of contact for owners seeking advice, it is important to understand why horses require feed and water, and how the nutrients supplied underpin healthy physiological activity and metabolic processes. 

The equine digestive system

Horses are grazing animals adapted to consuming small amounts of fibrous forage for approximately 17 hours per day. This feeding pattern has shaped both the anatomy and physiology of their digestive tract.

Horses rely on microbial fermentation in the hindgut to extract energy from fibre. This process requires a continuous supply of fibre (from forage, such as grass or hay) to maintain a stable microbial population (largely bacteria). Healthy fermentation produces volatile fatty acids (VFAs), which are a horse’s primary source of ‘Non-Heating’ energy. 

Prolonged periods (more than 4 to 6 hours) without forage affect the delicate microbial balance of the hindgut, reduce gut fill and compromise motility, as well as increase the risk of gastric ulcers. These effects increase physiological stress and the likelihood of problems such as acidosis, colic and loose droppings.

Water 

Water is the essential base for all physiological functions. An adult 500kg horse, would typically consume between 25 and 50 litres per day, depending on their diet, workload and environmental conditions.

Fresh grass contains a significant proportion of water, whereas conserved forages increase reliance on voluntary drinking because of their much higher dry matter. Feeds with a high water-content, such as mashes, can also contribute to overall water intake. 

Water plays a critical role in: - 

• Hydration
• Digestion and nutrient absorption
• Thermoregulation
• Blood circulation and oxygen transport
• Waste elimination via urine and faeces
• Cellular metabolism and biochemical reactions

Even mild dehydration can alter physiological function, contributing to decreased gut motility, drier intestinal contents and an increased risk of impaction colic. Reduced hydration also affects blood volume, influencing cardiovascular function and heat dissipation. As little as 4% dehydration can reduce performance.  

Vitamins and minerals

Horses require a balanced supply of vitamins, minerals and trace elements to maintain health. They do not provide energy themselves but are essential for the metabolic processes that allow nutrients absorbed from digestion to be used effectively by the body. These micronutrients contribute to all aspects of equine metabolism, including for example: -

• Tissue growth, maintenance and repair 
• Muscle and nerve function
• Circulation and oxygen transport
• Immune function
• Hormone synthesis and regulation
• Enzyme activity
• Electrolyte balance and hydration
• Reproductive function

Even when a horse appears healthy, insufficient intake of vitamins or minerals can compromise physiological function. 

Protein

Protein is the second major constituent of all tissues in a horse’s body, second only to water, and makes up approximately 20% of both muscle and bone, and 15% of total body mass. Protein provides amino acids required for: - 

• Muscle maintenance and repair
• Enzyme and hormone synthesis
• Immune function
• Growth and development
• Nutrient transport

While mature horses in light work may obtain enough protein from forage alone, performance horses, breeding animals, and those recovering from illness have increased demands. Inadequate protein intake can impair cellular processes resulting in a loss of muscle mass, poor tissue repair, and compromised immune responses. 

Energy

All horses require energy to fuel basic physiological functions such as breathing, circulation, thermoregulation, and tissue repair. Additional energy is required for increased condition, growth, pregnancy, lactation, and exercise. Energy is supplied through: -

• Structural carbohydrates (fibre)
• Non-structural carbohydrates (NSCs - sugars and starch)
• Fats

Energy intake directly affects body condition and overall health. Excess energy intake relative to expenditure leads to an increase in condition, while insufficient intake results in mobilisation of body reserves and weight loss.

Any change in energy intake alters metabolic pathways. Therefore, feeding decisions must always consider the horse’s workload, breed/ type, age, health status, and metabolic sensitivity.

Nutrient gaps from a forage-only diet

Forage supports normal gastrointestinal function and can supply much of a horse’s energy, but alone it cannot meet all nutrient requirements, particularly for horses with higher physiological demands (e.g., growing foals, lactating mares, and performance horses).

Grass

The nutrient content of pasture is highly variable. Spring grass is usually high in energy, protein, water-soluble carbohydrates (WSCs) and vitamins (particularly A and E) while summer and autumn grazing can be lower and winter grazing is often sparse and less nutrient-dense. Trace elements (such as selenium, zinc, copper, and iodine) tend to be low in most pasture year-round, depending on soil composition. 

Hay and stabled horses

Conserved forages, such as hay or haylage, provide fibre and some protein but they are usually deficient or depleted in vitamins (e.g. vitamin A) and trace elements, as well as essential amino acids. 

By understanding these potential gaps, owners can appreciate that feeding is not optional, whether the horse is at grass or stabled. For example, supplementing with zinc and copper contribute to coat and hoof quality, selenium supports antioxidant defences in combination with vitamin E, iodine is necessary for thyroid hormone synthesis, vitamin A contributes to vision and epithelial (e.g. skin) health, and vitamin E is important for muscle and immune function. 

Physiological consequences of inadequate feeding

While overfeeding and obesity are recognised welfare concerns, underfeeding or prolonged restriction can have serious consequences, including: -

• Weight loss
• Increased risk of gastric ulcers
• Disrupted hindgut microbial populations
• Reduced gut motility
• Impaired immune function and slow tissue repair
• Reduced bone strength, poor hoof and coat quality

In severe cases, excessive fat mobilisation through underfeeding (starvation) can result in hyperlipaemia, a potentially fatal metabolic condition. Other chronic health issues can also stem from micronutrient deficits. These outcomes demonstrate again that feeding is not optional, it is a continuous requirement for maintaining healthy physiology and efficient metabolic function resulting in long-term health.  

Rescue horses

Malnourished rescue horses provide a striking example of how feeding directly affects wellbeing. Those rescued from neglect often present as emaciated, with poor coat condition, poor muscle development and topline, behavioural signs of stress, and metabolic systems that are severely compromised.  

Carefully reintroducing a balanced diet, providing consistent forage, fresh water, and essential micro- and macronutrients, allows gradual restoration of physiological function. Improved hydration supports circulation and digestion, while nutrients provide the building blocks for tissue repair, immune function, and energy metabolism. Over months, appropriate feeding helps restore body condition, muscle tone, and overall health because it is supplying the nutrients required for normal physiology to operate effectively.

Conclusion

Feeding a horse is central to supporting the physiological systems that allow him to function, adapt, and thrive. Water, forage and an appropriately balanced hard feed provide the nutrients needed for digestion, cellular activity and energy use. Horses at grass, or stabled on hay or haylage, usually require supplementation to meet micronutrient needs. Underweight horses, and those with higher requirements, benefit from additional provision of protein and calories. 

Equine retailers play a key role in reinforcing how feed supports normal physiology and how inappropriate feeding can exacerbate health problems. Encouraging owners to seek guidance from experienced equine nutritionists and veterinary professionals is also important, particularly when managing horses with health or metabolic challenges.