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Nutrient requirements for horses

Proper nutrition is essential if a horse is to grow, reproduce and perform to its full potential. Feed cost is the major expense in most horse operations. The relatively large expenditure for feeds makes it important to pay considerable attention to the use of well-balanced rations in horse feeding. There is little point in spending large sums of money on fancy rugs, bridles, saddles, grooming equipment, coat gloss, etc if the nutrient requirements of the horse are neglected.

Most nutritionists now think in terms of nutrients and not just feed when balancing horse rations. A nutrient is any feed constituent, which aids in the support of animal life. The horse, like any other animal, needs the basic nutrients: water, energy, protein, minerals and vitamins. These nutrients need to be supplied in the proper proportion, level and form to provide adequate nourishment. The relative amounts of each vary with the size, age and use of the particular horse. For example young, growing horses have different requirements than older horses. The nutritional requirements of the horse in full work, be it racing, eventing or endurance, differ from those of the horse only ridden at weekends.

A good ration must also be palatable. Unless the horse consumes the ration readily, it is not successful regardless of how well balanced it may be. A variety of feeds may make a ration more palatable as well as improve the balance of the ration and thus prevent certain nutritional deficiencies. However, as the knowledge of nutrition increases, it is becoming easier to supplement more simplified rations with the nutrients they may lack.

Before discussing each of the basic nutrients required by the horse it must be emphasised that in addition to a well-balanced ration, regular parasite prevention and control, proper dental care, exercise and conditioning and preventative medicine (vaccinations etc) are also essential for the well being of the horse.

The horse must be fed and observed as an individual and no formula or specific amount of feed can be recommended in general for the need of the specific horse.

Energy and the horse

Energy is the fuel needed to power the many functions occurring in the horse, from maintaining body weight and controlling body temperature to propelling the horse over a racecourse or an endurance ride. Maintenance energy is the energy required to maintain body weight and body temperature of the horse at rest Energy supplied in excess of this requirement can be used for production i.e. growth, work or reproduction. A mature horse at rest will store excess energy as fat. A mature horse being exercised will use the energy for muscular work (any excess will again be stored as fat). Generally energy is equated to heat per unit of material and is usually expressed in units of kilojoules (W (1000 joules) or megajoules (Mi 1000 W).

A joule is the metric equivalent of a calorie if you are a dieter. 1 calorie = 4.184 joules).

The gross energy of a feed is the total energy yielded.

Digestible energy (D.E.) is the gross energy of the feed minus the energy not digested by the animal (energy lost in the faeces). This is basically the energy that available to the horse for maintenance and production (although there is a loss of energy in the urine, in heat produced during digestion and in gas production). Digestible energy is the unit of energy most commonly used in horse feeding. Other energy terms you may see are M.E. (metabolisable energy) used for cattle and poultry in particular, T.D.N. total digestible nutrients an out of date system, and S.E. (starch equivalents) an out-dated and inaccurate method.

Energy is obtained from these primary sources, carbohydrates, and proteins. Carbohydrates, sugars, starches and cellulose (fibre) are the primary energy sources for the horse. They can be expressed as high energy or concentrated energy sources such as grains, or low energy sources such as chaffs and hays. Generally, as the proportion of fibre in feed increases, the digestible energy content of the feed decreases. Thus, corn has a higher digestible energy content than oats and young pasture has a higher digestible energy content than mature pasture.

Idle or mature horses can be maintained on pasture and/or hay alone, provided that quality and quantity are adequate. Some roughage (hay, chaff or pasture) is required in the diet of all horses. Grain and supplements are added to the roughage ration, when it alone cannot meet the added energy requirements of growth, reproduction and work. As the nutritional demands of work, growth and reproduction are increased, the roughage is decreased or kept at a minimal level and the concentrate or energy content is increased.

Whereas fat provides approximately 40% of the kilojoules in the diet of the average Australian, it only provides approximately 8% of the kilojoules in the diet of the horse. Fats and oils are readily utilized energy sources. Most rations only contain 2-6% fat. The horse can utilize up to 20% fat in the diet and fat contains 2.25 times as much energy as an equal weight of carbohydrate or protein. The use of fats and oils in horse rations will be discussed in more detail in future articles.

Energy is obtained from these primary sources, carbohydrates, and proteins. Carbohydrates (sugars, starches and cellulose (fibre) are the primary energy sources for the horse. They can be expressed as high energy or concentrated energy sources such as grains, or low energy sources such as chaffs and hays. Generally, as the proportion of fibre in feed increases, the digestible energy content of the feed decreases. Thus, corn has a higher digestible energy content than oats and young pasture has a higher digestible energy content than mature pasture.

Idle, mature horses can be maintained on pasture and/or hay alone, provided that quality and quantity are adequate. Some roughage (hay, chaff or pasture) is required in the diet of all horses. Grain and supplements are added to the roughage ration, when it alone cannot meet the added energy requirements of growth, reproduction and work. As the nutritional demands of work, growth and reproduction are increased, the roughage is decreased or kept at a minimal level and the concentrate or energy content is increased.

Whereas fat provides approximately 40% of the kilojoules in the diet of the average Australian, it only provides approximately of the kilojoules in the diet of the horse. Fats and oils are readily utilized energy sources. Most rations only contain 2-6% fat. The horse can utilize up to 20% fat in the diet. Fat contains 2.25 times as much energy as an equal weight of carbohydrate or protein. The use of fats and oils in horse rations will be discussed in more detail in future articles.

Dietary protein in excess of the horses needs can also be converted into energy but as protein feeds are relatively more expensive than grains, this is a very expensive means of increasing the energy intake of the horse.

Table 1 shows the approximate weights of various common feedstuffs per a 2 litre ice cream container and the digestible energy contained therein. It can be seen from this table that a volume of corn contains twice as much digestible energy as an equal volume of oats. This highlights the need to weigh feeds and the dangers of substituting gains on a volume basis. The simplest method of weighing horse feeds is to weight the amount of feed that a commonly used scoop or container holds. This method makes it possible to estimate fairly accurately how many scoops of a feed equal so many kilograms of that feed. It is not essential to weigh every ingredient each time a feed is mixed, however, it is important that every time a new bag of feed is opened a new measurement is taken. Not only does this ensure that the horse is getting ting the amount it requires, but it also serves as a guide to the quality of the feed. Oats, for example, can vary greatly in weight per volume.

Protein

The horse continually uses protein either to build new tissues in growth and reproduction, or to repair worn out tissues.

Protein is the main constituent of the organs and tissues and about 20% of the bone matrix is composed of protein. Protein is required to develop muscling. Most body functions require protein in many different forms. Therefore, a continuous and sufficient supply of protein is needed. The importance of an adequate 
quantity of protein in horse nutrition is easily demonstrated.

Diets deficient in protein result in failure to grow normally and even result in death. It is important to note that animals engaged in heavy production (growth of foetus, milk etc.) have much higher protein needs than animals not in full production.

The protein needs of the horse are highest at birth and become progressively lower as the animal matures. At maturity, protein is required for maintaining tissues. The broodmare is an exception as she is also producing foals and milk.

The dietary requirements of animals are not only a matter of providing a sufficient quantity of protein, the quality of protein is very important. Proteins are made up of approximately 20 amino acids in varying combinations. Amino acids are often described as the building blocks of protein. Animals can only make about half of these amino acids and must therefore be supplied with the ones which they cannot make. These are called essential amino acids. The proportions of essential amino acids making up the protein determine protein quality. A horse will only utilise its dietary protein up to the lowest level of the limiting amino acids. A shortage of any one essential amino acid makes overall protein utilization by the horse less efficient.

Just as a pattern in a brick wall can only be built if there are sufficient of the different colours to make the pattern, so to does protein building rely on the amino acids being present in the correct proportions. The most limiting amino acid in feeds commonly fed to horses is lysine, with methionine also often deficient. These deficiencies are of greater importance to the growing horse than to the mature horse. Products such as Ranvet's Ranfac will overcome these deficiencies for the brood mare and growing horse, and Ranvet's Pro Mix will supply these amino acids for the mature horse.

The concept of protein and 'protein feeds' is sometimes difficult to grasp. In fact, most feeds used for horses contain some protein, the exceptions being molasses, fats and oils. Protein rich feeds such as soyabean meal, cottonseed meal and tickbeans are used to raise the overall percentage of protein in the diet. Just as most contain some protein, high protein feeds are also sources of energy.

Minerals

The horse requires at least 14 different minerals. Some of these termed macro or major minerals. Their requirement in the diet is usually expressed as a percentage of the diet, or grams per kilogram of the diet (g/kg) or grams per day (g/day). These are calcium, phosphorus, sodium, magnesium, potassium, chloride and sulphur. The three minerals of major concern in feeding horses calcium (Ca), phosphorus (P) and sodium (Na) usually in the form of salt (NaCl) Calcium and phosphorus nutrition is of utmost importance for proper bone development and maintenance in horses. While adequate amounts of calcium and phosphorus are needed, they must also be present in the right ratio. Horses are more apt to suffer from a lack of, or improper ratio of calcium and phosphorus than any other minerals. The calcium to phosphorus ratio should preferably lie between 1.1 to 1 and 2 to 1, although weanling foal can tolerate up to 3 to 1 and mature horses up to 6 to 1.

Feedstuffs such as grains, protein meals and grain by-products (bran and pollard) contain larger amounts of phosphorus than calcium, whereas legumes lucerne, clovers contain more calcium than phosphorus. Horses fed diets high in grain and/or bran and pollard will usually require supplementation with a high calcium 
source such as Folactin. The feedstuffs commonly fed to horses are naturally low in sodium in contrast to the average Australian diet that is high in sodium. Thus although humans have been advised to reduce their salt intake the same does not apply to the horse. Ten to sixty grams of salt should be added to the horse's ration daily, depending on the size of the horse, the type of ration and the amount of exercise.

Rations high in hays and chaffs do not require as much added salt as do high grain rations. Salt should also be freely available the horse, preferably as loose salt or a salt block. Rock salt often can be difficult for the horse to lick.

Electrolytes are those elements that in solution have an electrical charge; they include sodium, potassium, magnesium, calcium, chloride, bicarbonate and phosphate. The electrolytes are vital in maintaining the integrity of the physiological and biochemical processes within the cells. Alternations in their concentrations within the body can upset delicate balances and may impair performance or lead to clinical disease.

Sweating entails a loss of both water and electrolytes. The electrolytes of horse sweat consist principally of sodium and chloride, with lesser quantities of potassium and small proportion of calcium and magnesium. Extended work in hot dry weather by a 450-500kg horse may yield losses of as much as 80g sodium, 50g potassium and 150g of chloride. Therefore, it is recommended that where prolonged or pronounced sweating occurs due to exercise and/or hot conditions, an electrolyte preparation containing these electrolytes is included in the diet.

Trace elements or trace minerals are so described because they are required in minute quantities in the horse's diet; usually expressed as parts per million (ppm) or milligrams per kilogram (mg/kg) of diet. 1 milligram (1 thousandth of a gram) is very difficult visualise and even more difficult to weigh out. If you think of 1 millimetre in relation to 1 kilometre, then this is the same relationship as lmg to lkg. Requirements of trace minerals can range from 0.1 mg/kg feed for cobalt, selenium and iodine, up 50 mg/kg feed for iron.

Trace elements can be toxic if supplied in amounts exceeding the requirement, the actual magnitude of the toxic level depending on the individual element. Unfortunately, too many people have mistaken belief that if a little is good, more must be better, can have a detrimental effect on the horse. Often the manufacturer recommended dose is doubled or tripled. Alternatively, a number of different supplements from different manufacturers each containing the same nutrients are added to a horse's diet. It is very important to follow manufacturer's recommendations, use registered products and preferably use one manufacturers range of products so that problems are not encountered.

Vitamins

Vitamins are nutrients that horses require in very small quantities although the actual need for each differs considerably. Like mammals, horses require vitamins for normal bodily functions These requirements will be met by vitamins naturally present in feed, supplementary sources and in the case of B vitamins, can be partially supplied from microbial synthesis in the intestinal tract. Dietary requirements for specific vitamins are affected by circumstance. For example, horses kept indoors will have a higher requirement vitamin D. Rapidly growing foals and lactating mares may have a greater requirement for vitamins than non-productive adult horses. Similarly, the dietary requirements for certain B vitamins involved in energy metabolism are increased for horses involved in heavy work. Horses fed high oil rations have a higher requirement for vitamin E.

Vitamins can be separated into two major groups. The fat soluble vitamins (A, D, E and K) and the water soluble vitamins (vitamin C and the B Group vitamins). Water soluble vitamins are relatively non-toxic when supplied in excess of requirements but excesses of the fat soluble vitamins A and D can cause serious problems.

Because of the mistaken belief that "if a little is good, more must be better", horses are often over supplemented with vitamins. In the case of vitamins A and D, this is possibly worse than no supplement at all. If a vitamin supplement including vitamins A and D is being fed, avoid using cod-liver oil in the ration as this contains high levels of vitamins A and D.

Requirements for vitamins set by the National Research Council (1978) can really only be considered as bare minimums, as they do not contain any safety margins. Most of these were set at a level at which deficiency symptoms were not apparent.

Borderline deficiencies of vitamins may exist without the horse showing any observable symptoms. When this occurs, growth, reproduction and performance may be affected. Inclusion of vitamin supplements at reasonable levels would ensure an adequate supply of vitamins.

When choosing a vitamin supplement for your horse, check that it contains adequate levels of all vitamins, not simply large amounts of vitamins A and D and trace amounts of the other vitamins. In further articles, requirements for vitamins for various classes of horse will be discussed.

Water

Water is the cheapest nutrient and the one needed in the largest quantity, and yet is often neglected. A horse, or any other animal, will survive longer without feed than without water. A 450kg horse will drink 30-50 litres of water daily. Horses on lush pastures will drink less; working horses on dry feed will consume considerably more, especially in warm weather. Water must be kept clean and fresh. Frequent inspection of water supply and cleaning of troughs and buckets is essential. Horses should have free access to water, except immediately after exercise.

Our Thanks to Dr Percy Sykes for this article.