The use of conventional and unconventional supplements in the thoroughbred horse.

The conventional nutrition of the racehorse over the centuries has centered around grain (oats or maize) bran, hay and water, with the addition of various other compounds to provide variety, palatability and improve the general health of the horse. These additions have included herbs, honey, molasses, Guinness, eggs and extracts of garlic.

Recent years have seen the addition of more specific compounds to the diet of performing horses in the search for an extra advantage that may lead to optimum or even enhanced performance.

The use of such supplements can be divided into two categories: (1) those aimed at the prevention of depletion states which could lead to a lowering in general health and loss of athletic performance, such as vitamins, minerals, and specific amino acids, and (2) the growing number of supplements being introduced to enhance performance, such as carbohydrate and bicarbonate, as well as such substances as dimethylglycine.

Racehorse trainers often use more than one supplement, and sometimes several of a similar composition. In addition, most of the pelleted or complete feeds available today have added vitamins, electrolytes and trace elements. Due to the general lack of scientific evidence, as in man, evidence for the beneficial effects of many of these compounds is largely anecdotal. From time to time certain supplements or single substances become fashionable, depending on the success of the trainers using them. An obvious difficulty is that carrying out the large-scale controlled studies necessary to show either improved racing times of the order of 1-5% (which can mean the difference between first or last) or improved health would be extremely expensive.

A further difficulty in evaluating the effectiveness of supplements is that those racing yards that are using the most supplements tend to have better overall management, as well as a higher class of animal. Often the use of supplements is directed by economics, and therefore, in a yard with wealthy owners the extra cost of supplements can be justified, even if they are beneficial in only one in every ten or twenty horses. At the other end of the spectrum, supplements will often have to be excluded to prune costs.

In the present paper the following supplements will be considered; electrolytes, trace elements, vitamins and compounds intended to improve the efficiency of energy supply in the muscles.

Today the necessity for the addition of extra sodium chloride to the diet of the working horse is generally accepted. This is because equine sweat is hypertonic, with concentrations of sodium, potassium and chloride five or more times greater than those in human sweat. This can result in large amounts of Na and K being rapidly lost during exercise (Kerr & Snow, 1982). The major problem is with Na, since the normal horse diet is low in this electrolyte, and a deficiency in Na may rapidly occur.

In horses receiving no Na supplementation only very low amounts of Na are excretedin the urine, and in a deficiency state this is reduced almost to zero. For the workinghorse, Meyer (1987) recommended intakes ranging from 20 mglkg per d for maintenance to as high as 250 mg/kg per d for a horse in heavy work. Requirements are increased in hot climates where sweating is more pronounced.

Recently a common muscle problem, rhabdomyolysis (azoturia, tying-up, setfast), has been shown sometimes to be associated with a Na imbalance (Harris & Colles, 1988). In several cases, further episodes have been prevented by increasing the NaCl in the diet.

Supplemental K is only necessary when marked sweating occurs for several hours each day, especially on a high-grain diet. Calcium is included in many supplements, especially for the growing horse, where daily requirements are stated to be approximately 95 mg/kg, whilst maintenance requirements for the mature horse are 50 mg/kg (National Research Council, 1978). In a study carried out in racehorses in a number of stables in Australia, Cable er al. (1982) concluded that 40% of horses received inadequate Ca nutrition whilst being maintained on high-grain diets. Furthermore, studies in Hong Kong attributed a high percentage of lameness and fractures to Ca deficiency, with an improvement in the situation when Ca supplementation was introduced (Mason et al. 1988). On the other hand, Krook & Maylin (1988) have suggested that racehorses in America are being given excessive Ca,which could contribute to the increased incidence of osteochondrosis and osteopetrosis due to hypercalcitoninism. Furthermore, they questioned both the National Research

Council (1978) recommendations, and the view that even six times the recommended level of Ca ingestion was of no consequence.

Some cases of rhabdomyolysis have been shown to have low Ca excretion, and to respond to additional Ca in the diet. Preliminary studies indicate that the absorption of calcium carbonate (limestone) in the horse varies greatly between individuals (P. Harris, J. Gray and D. H. Snow, unpublished results). Obviously, much more detailed research into Ca requirements, and possible interactions with phosphorus and other minerals, is required in both the growing and mature working horse.

The various fat- and water-soluble vitamins are usually found in most pelleted feeds and complete supplements. Unfortunately there has been very little work carried out on the requirements of these in the sedentary, let alone the working horse. As with the human athlete, some very successful trainers are using megadoses of some vitamins.