Equine nutrition
DENVER, CO − The feeding of horses is an intricate science that requires feeding the precise amount and balance of nutrients, explained Kara M. Burns, MS, MEd, LVT, speaking at the American College of Veterinary Internal Medicine Forum. Companion horses today consume a variety of feeds ranging from forage with a high content of moisture to cereals with a high amount of starch, and from hay in the form of physically long fibrous stems to salt licks and water.
Horses are non-ruminant herbivores that naturally spend 60 to 75% of their day grazing. With domestication came alterations in feed, feeding times, and feeding methods – more in line with domesticated dogs and cats. Unfamiliar materials such as starchy cereals, protein concentrates, and dried forages have been introduced. Today we see horses spending more of the time in stalls or smaller pastures, and fed 1-2 times per day. As a result they spend only about 40% of the day eating.
Diets formulated for horses contain on average 5% fat and 7-12% protein, with carbohydrate being the major source of energy. Grass and hays serve a strong foundation although protein is required mainly in the building and replacement of tissues.
Key nutritional factors for horses
Nutritionists are mainly concerned that horses get enough water, energy, protein, minerals, and vitamins. Ms. Burns stressed that there should always be fresh, abundant water available since horses, on average, drink 25 litres per day. She added that technicians need to remind owners that the more grain their horse eats the more water intake the horse will need.
Energy is measured in terms of digestible energy (DE) and fed in kilocalories. The amount of DE horses need is dependent upon physiologic state, activity level, environment, and the size of the individual horse. Most of the energy utilized by the horse is from carbohydrates that are ingested through the horses’ natural feed.
Fats provide the horse with high-density energy and should not exceed 20% of the total diet or 30% of concentrate. Exceeding these percentages will likely result in decreased palatability and loose stools.
Protein amounts are typically expressed as ‘crude protein’ and are expressed as % dry matter. Again the amount of protein needed by an individual horse is dependent upon its physiologic state, type of diet, age, and quality of diet. The closer the proportions of each of the various indispensable amino acids in the dietary protein conform to the proportions in the mixture required by the tissues, the higher the quality of the protein.
Calcium and phosphorus are considered together because of their interdependent role as the main elements of the crystal apatite, which provides the building blocks for the skeletal system. The requirement of calcium and phosphorus is dependent upon the physiologic state of the horse. The average adult horse weighing approximately 500 kg will need approximately 20g of calcium and 14 g of phosphorus per day. Ms. Burns said that it is important to balance the ratio of calcium to phosphorous with a mature horse needing a ratio of 1.1:1 to 6:1. The ratio for a growing horse is recommended to be 1.1:1 to 3:1.
Sodium is the principal determinant of the osmolarity of extracellular fluid and as a result, the volume of that fluid. Chloride concentration in the extracellular fluid is directly related to that of sodium. If the requirements for sodium are met, seldom will a deficiency of chloride occur. Good sources of sodium and chloride can be found in grains with premixture and salt blocks.
Potassium deficiencies or excesses in equines are rare; however, excess potassium can lead to hyperkalaemic periodic paralysis, a syndrome of episodic weakness in horses accompanied by elevated serum potassium concentration.
Selenium is a trace element needed to aid in antioxidant defense. Selenium deficiencies produce pale, weak muscles in foals and a yellowing of the depot fat, know as ‘White Muscle Disease’. It is imperative that pregnant mares receive adequate amounts of selenium in their diet. Selenium is highly toxic to animals; the minimum toxic dose through continuous intake is 2 to 5 mg/kg feed. Skin, coat, and hoof abnormalities are the result of excess selenium.
Grazing horses derive their vitamin A from the carotenoid pigments present in herbage. Horses that graze for 4 to 6 weeks build up a 3 to 6 month supply of vitamin A in the liver.
Vitamin E functions as a cellular antioxidant in conjunction with Vitamin A and is required for normal immune function. Fresh green forage and the germ of cereal grains are rich sources of vitamin E. Although deficiencies are rare, equine degenerative myeloencephalopathy and equine motor neuron disease have been recognized to involve α-tocopherol status.
Types of feed
Roughages include grasses and forage legumes cut for hay. Most common species of grass are suitable, but rye grasses, fescues, timothy, and cocksfoot are preferred. Species found in permanent pastures are satisfactory as well and these include meadow grasses, brome, bent grass, and foxtails. Legumes utilized are red, white, alsike, crimson clovers and trefoils, as well as lucerne and sainfoin.
Roughages are considered to be the foundation to an equine feeding program. Legumes and non-legume grasses that are well managed and fertilized provide > 10% crude protein as opposed to carbonaceous grasses which provide < 10% crude protein. Veterinary technicians can educate clients on the quality of roughage using the following guidelines:
• Free of mold
• Soft and pliable to touch
• Leafy with fine stems (2/3 energy, ¾ protein)
• Pleasant, fragrant aroma
• Bright green, not brown or yellow
Ms. Burns added that owners need to understand that excess handling of roughages can result in a partial loss of leaves, energy and protein, and β-carotene.
Concentrates are typically a cereal grain that may or may not have supplemented protein, minerals, and vitamins. They are high in energy and are less than 18% crude fibre. Often, concentrates are used as a supplement if forage is insufficient in nutrients – especially energy and protein. Concentrates are needed more often in certain lifestages such as gestation, lactation, growth, and in work horses. It is important for healthcare team members and owners alike to recognize that excess concentrate may lead to laminitis, rhabdomyolysis, developmental orthopedic disease, and obesity.
Complete feeds are typically a mixture of roughage and concentrate – usually 80% roughage to 20% concentrate mixture. Because the complete feed is pelleted or wafered, care must be given to the potential risks associated with inadequate particle size, including colic, choking risk, wood chewing, and coprophagy.
Summary
Ms. Burns concluded by stressing the importance of having clients understand the importance of having fresh water available at all times. She said that while energy for maintenance can be met entirely with quality hay, it may be supplemented with concentrate if necessary. Good quality green roughage should supply adequate amounts of vitamins A and E. Nutrition, she said, is one area of equine veterinary medicine that affects every horse and should be discussed on every visit, every time. CVT
