Energy

Energy is perhaps the most critical factor in a horse's diet. Energy itself is not a nutrient, but it is required for maintenance of the horse—movement, breathing, maintaining body condition, digestion, blood circulation, and many other bodily functions (3). Horses require energy to sustain basic metabolic functions, maintain body condition, and perform activities such as walking, running, and working. Energy for horses comes primarily from carbohydrates and fats, which are metabolised to fuel bodily functions (3).

Carbohydrates in a horse's diet are derived mainly from forages such as hay and pasture, as well as from grains like oats, barley, and maize. These carbohydrates can be classified into two types: structural and non-structural (3). According to Zynda et al. (2022), structural carbohydrates, found in hay and grass, consist mainly of cellulose, which is broken down by microbes in the horse's hindgut (3). Non-structural carbohydrates, like sugars and starches in grains, are digested more rapidly and provide quick energy (3). Care must be taken with grains, as excess starch intake can lead to issues such as colic and laminitis.

Fats are another important energy source for horses, particularly those with high energy demands. Fats provide more than twice (approximately 2.25 times) the energy per gram compared to carbohydrates, making them a valuable addition to diets for performance horses (3). Sources of fat can include vegetable oils or commercial fat supplements. Unlike carbohydrates, fats do not carry the risk of digestive disturbances when fed in moderation.

Protein 

Protein is essential for the growth, repair, and maintenance of tissues, including muscles, skin, hair, and hooves (4). Horses also rely on protein to produce enzymes and hormones that regulate bodily functions. Protein is composed of amino acids, some of which are essential and must be obtained through the diet, as the horse's body cannot synthesise them.

Quality protein sources include legume hays like alfalfa, soybeans, and commercially prepared feeds that contain adequate levels of amino acids such as lysine, which is often the first limiting amino acid in horse diets. Young horses, pregnant or lactating mares, and horses engaged in strenuous work have increased protein requirements. While protein is crucial, excessive protein can lead to unnecessary stress on the kidneys and increased water needs, so it is important to ensure a balanced intake.

Indicators of protein deficiency include a dull or coarse coat, weight loss, and decreased growth, milk production, and overall performance (4).

Vitamins  

Vitamins are organic compounds that play vital roles in many physiological processes. According to Kentucky Equine Research (2011), horses require both fat-soluble vitamins (A, D, E, and K) and water-soluble vitamins (B-complex and C) (5). 

Vitamin A: is important for vision, reproduction, and immune function. Horses usually obtain sufficient Vitamin A from fresh forage, but stored hay loses its vitamin content over time, so supplementation may be necessary during winter months.

Vitamin D: is essential for calcium regulation and bone health. Horses synthesise Vitamin D from sunlight, but horses kept indoors or in regions with limited sunlight may require supplementation.

Vitamin E: acts as an antioxidant, protecting cells from oxidative damage. It is also important for muscle health and immune function. Vitamin E is typically present in fresh pasture, but like Vitamin A, its levels diminish in stored feed.

Vitamin K: is required for blood clotting and is usually abundant in green forages and produced by gut microbes.

B-complex vitamins are involved in energy metabolism and red blood cell production. These are generally synthesised by bacteria in the horse's gut, but horses with poor gut health may need supplementation. Vitamin C is synthesised in the liver, and under normal conditions, horses do not require dietary Vitamin C.

Minerals

Minerals are divided into two categories: macrominerals and microminerals, both of which are essential for a horse's health (6).

Macrominerals, such as calcium, phosphorus, magnesium, sodium, potassium, chloride, and sulphur, are required in larger quantities (6). Calcium and phosphorus are particularly important for bone health, with an ideal calcium-to-phosphorus ratio of approximately 2:1. Magnesium is involved in muscle function, and sodium, potassium, and chloride maintain fluid balance and nerve function. A salt lick or electrolyte supplement can help ensure adequate intake of these minerals, especially for horses that sweat heavily.

Microminerals, including copper, zinc, selenium, and iron, are needed in trace amounts but are still critical for numerous bodily functions (6). For instance, selenium works in conjunction with Vitamin E as an antioxidant, and zinc supports skin and hoof health. Horses grazing on selenium-deficient soil may require supplementation, as insufficient selenium can lead to muscle disorders.

Water  

Water is arguably the most important nutrient for horses. An average horse requires 25 to 55 litres of water per day (or 5 litres of water per 100 kg body weight), but this can vary based on temperature, workload, and dietary factors. Horses consuming dry feed, such as hay, require more water than those on fresh pasture (7). Dehydration can lead to serious health issues such as colic and impaction, so it is essential that horses have access to clean, fresh water at all times (1).

Fibre

Fibre is a key component of the equine diet, as horses are natural grazers with digestive systems designed to process fibrous plant material (1). Forages like grass, hay, and haylage provide the bulk of a horse's fibre intake. Fibre is fermented by microbes in the horse's large intestine, producing volatile fatty acids that serve as a significant energy source.

Forage is a vital source of fibre and key nutrients, but its nutritional composition, including sugar content, can vary (1). To ensure your horse is receiving the right nutrients, you can send a forage sample to a feed company for analysis. They can provide insights on the results, which is especially important if your horse is overweight, has equine metabolic syndrome (EMS), or is prone to laminitis. In such cases, it’s typically safer to use forage with less than 10-12% sugar and starch (1).

A diet high in fibre helps to maintain gut health, prevent colic, and promote natural feeding behaviours. It is recommended that horses consume at least 1.5% to 2% of their body weight in forage daily (2). Feeding too little fibre can lead to digestive disorders, behavioural problems, and weight loss.