13 Practical considerations for feeding racehorses
To be successful in short duration, high-speed or strength-oriented activities such as racing, a horse needs to be able to perform at a high intensity and maintain that intensity longer than its competitors. There are many factors that can contribute to the ability to perform in this manner. Physical conditioning and soundness are perhaps the two primary factors that enable a horse to be in the position to perform successfully, but the nutritional program can also play a critical role in assisting a horse to achieve its athletic potential. A basic understanding of the typical racing and training programs associated with some of the more popular racing breeds, as well as some of the advantages and limitations of the various dietary constituents commonly fed, can aid in designing a feeding program which can support optimal performance.
Even for the high intensity racehorse, there can be tremendous differences in the distances raced and speeds reached. Standardbred racing in the US, for example, is typically over a distance of 1609 m at speeds of ~48 km/h, whereas Thoroughbreds generally race over 1006 m to 2414 m at faster speeds around 64 km/h, although this does depend on the race distance (Harkins & Kamerling 1993, Nunamaker et al 1989). Slower speeds typically are associated with the longer races and quicker speeds often are achieved in shorter races. The true equine sprinters, the racing Quarter Horse, routinely compete at distances of 402 m or less (Nielsen et al 2006). Interestingly over 402 m, the record times for both Quarter Horses and Thoroughbreds are similar, in the range of 20.7 s (Nielsen et al 2006). Granted, this similarity is due to Quarter Horse races being timed from a standing start when the starting gates open while Thoroughbred races are officially begun and timed from a point in front of the starting gates, when the horses are already running. Although Thoroughbreds typically try to maintain a consistent speed throughout a race, Quarter Horses are accelerating through the first half of the race and have been reported to reach speeds near 90 km/h (Nielsen et al 2006, Pratt 1991).
With such differences in racing distances and speeds, training programs also vary widely. Typically, Thoroughbreds train over relatively short distances at race pace while Standardbreds are trained over longer distances (Nunamaker et al 1989). Standardbreds may spend 60 minutes of their 140 minutes weekly exercise (~40%) being worked at a high intensity (Hamlin & Hopkins 2003). A small survey of racing Thoroughbreds in England reported that they were exercised over a mean distance of 10 km per week with fast work only being performed for 1.4 km suggesting that the horses on average performed at racing speed for less than 2 minutes per week (Verheyen et al 2009). Gallagher et al (1992a) reported that although Thoroughbreds and Standardbreds were exercised over similar daily distances (6.9 km/day vs 7.7 km/day, respectively), overall intensity of exercise was higher for the Standardbreds. Thoroughbreds walked 72% of the distance traveled as compared to only 58% for Standardbreds (Gallagher et al 1992a). The distance walked is not always considered as part of the exercise protocol for Thoroughbreds and Standardbreds and is used as a means of cooling a horse out after exercising or simply a means to get the horse to and from a racetrack for training. Nonetheless, walking can make a substantial contribution to the total exercise load as well as daily energy requirements. Pagan and Hintz (1986) estimated that walking requires 2.5 kcal digestible energy (DE)/kg/h above maintenance in riding horses and thus should be considered when evaluating energy requirements even of the racehorse.
The racing Quarter Horse is generally trained over the shortest distance – 5 km or less per week – and also does not undertake significant amounts of exercise at racing speeds. Though the distance can vary, it is common to work a horse at race speeds over a distance of 229 m only once every two or three weeks if a horse is not actively racing. If racing on a regular schedule, they may not be worked at speed at all in-between races. The duration of “all-out” galloping is often no more than 30 s during racing or training sessions. Compared to Thoroughbreds and Standardbreds, walking often constitutes a significant portion of the training program for racing Quarter Horses and, for many, walking is their only exercise for several days per week. In comparison, both Standardbreds and Thoroughbreds are often exercised at the jog, canter, or at a fast walk five to six days per week and would accumulate substantially greater distances trained monthly (Table 13-1).
Regardless of breed, for horses performing high intensity exercise the goal is to minimize excess weight whilst not sacrificing energy stores or their availability. Energy requirements for a mature horse in intense training are reported to be over twice that of a similar horse at maintenance (NRC 2007). Inadequate energy intakes, even of a short duration, potentially will result in a more rapid onset of fatigue and reduced performance. This highlights the importance of dietary energy, as a short-term inadequate intake of most nutrients usually does not have such a dramatic impact on performance.
Unlike in man, the relationship between body fatness, fat-free mass, and running performance has not been studied extensively in the horse. The optimal bodyweight of racehorses, however, appears to be highly correlated with performance and the optimal bodyweight of a racehorse only has a range of ±1.5% with performance appearing to be less affected by being overweight as compared to underweight (Lim 1981). Rider and equipment weight, in addition to the weight of the horse, also needs to be factored into the energy requirements needed to run so anything increasing the overall weight increases the energy cost of locomotion (Harris & Harris 2005). Thornton et al (1987) demonstrated that a 10% increase in weight load, increased oxygen consumption by 15% in horses exercising on a horizontal treadmill. By contrast, the acute reduction in bodyweight after administration of furosemide resulted in decreased anaerobic energy expenditure during brief high-intensity work and may be responsible for any improvement in performance (Hinchcliff et al 1996). Frape (1994) suggested Thoroughbreds of moderate fatness use depot fat as a source of energy more effectively than thin or fat horses. Webb et al (1990) reported horses in fleshy condition require more DE for maintenance (an additional 11.1 kcal/kg BW/day) than do horses in moderate condition and that the optimal body condition for the performance horse might vary according to ambient temperature and humidity at the time of performance.
Gaining or losing weight may alter lean mass as well as percentage body fat. As speed is linked, to a certain extent, to fat-free mass, this may alter performance as well (Kearns et al 2002). Therefore, during conditioning, a balance is needed to maximize lean body mass while minimizing excessive body fat, rather than just restricting calorie intake in an attempt to produce a very lean animal without the appropriate exercise and nutritional regimens.
To practically assess the adequacy of energy intake, a body condition score (BCS; Henneke et al 1983) should be determined on a regular basis (see Chapter 22). Even if the BCS remains unchanged, it does not guarantee that the animal is at the optimal BCS. Having a BCS that is either too high or too low can impair performance. For most athletic horses, the goal should be to have a BCS between 4 and 6 on a scale of 1 to 9 (Becvarova et al 2009) though it would be rare, and probably disadvantageous, to have a racehorse with a BCS as high as 6. When a horse is in a thin condition, glycogen stores may be lower than when they are in a moderate or fat condition (Jones et al 1992, Scott et al 1992) contributing to an earlier onset of fatigue. Given that glycogen is a preferred substrate for energy transduction during short-term, high-intensity predominantly anaerobic exercise such as racing, maximizing glycogen stores would seem advisable (see Chapter 26).
A small over-consumption of most nutrients may be wasteful but it is unlikely to impair performance. However, a horse in continual or frequent positive energy balance will store this excess energy as fat which can be detrimental to athletic performance. Glycogen storage does not appear to increase with BCS once over a certain level (Jones et al 1992) and there appears to be no performance benefit for an athletic horse having a BCS much higher than 5 (on a scale of 1 to 9). Indeed, a higher BCS may be detrimental to athletic performance. A Thoroughbred racehorse at BCS 6 and weighing 500 kg, for example, would be expected to weigh 16 to 20 kg more than a similarly built horse that has a BCS of 5 (NRC 2007) – “natural handicapping”. By being overweight, animals also put additional load on their limbs and, potentially, fat deposition in the thoracic cavity could reduce or limit lung expansion. Additionally, extra subcutaneous fat serves as insulation increasing the challenge for the horse to dissipate heat (Morgan 1997). All of these factors may prevent the horse from performing optimally. Trainers will often attempt to determine the optimal racing weight for an individual and then try to maintain this weight. A weighbridge, calibrated appropriately, is the ideal way to determine the weight of racehorses as weight tapes (heart girth measurement) tend to have reduced accuracy in very fit lean animals (personal observation). Even if weighbridges are used, it is still recommended that racehorses are scored for body condition as that will aid in determining whether the weight is appropriate or if weight needs to be gained or lost. Use of weight tapes in combination with body condition scoring is another approach to monitoring.
Due to the anabolic changes in muscle associated with the start of physical conditioning and the need to repair any resultant muscle damage as well as enable muscle hypertrophy, protein requirements of athletic horses are increased compared to sedentary horses (Custalow 1991; see Chapter 6). Quantifying this additional demand has been challenging, in part because variation in conditioning programs likely results in different protein needs. Furthermore, while requirements are established as a minimum to replace losses and prevent health problems, the amount needed for optimal performance is likely to be greater. As a result, many trainers feed protein greatly in excess of published requirements (Gallagher et al 1992a, b). This, however, may not be desirable, as discussed below.
Although horses in exercise have a higher protein requirement, as evidenced by an increase in nitrogen retention in association with greater exercise load (Freeman et al 1988), the increase is not as dramatic as many would believe. Even without raising the actual concentration of protein in the feed, any increase in the protein requirements is often met simply by the enhanced dry matter (DM) intake (NRC 2007). Recently, however, with the increased practice of switching to energy-dense, oil-supplemented diets at the start of training, this increase in DM intake may not occur. Consequently, it is important to estimate how much protein an animal is receiving in order to determine whether its protein requirements are being met. High quality hay (particularly legume hay) can provide substantial amounts of protein, often equaling or exceeding the amount provided by the grain portion of the diet (Connysson et al 2006). Under these circumstances it is often unnecessary to provide a high protein complementary/concentrate feed. A complete ration evaluation is therefore recommended. It is, however, important to take into account that published requirements for protein, as well as other nutrients, as established by the NRC (2007) are minimum requirements, as opposed to optimal amounts, and were established for a population of horses. Individual requirements may vary greatly and extra allowances to account for individual variation may be warranted (Becvarova et al 2009).
Providing excess dietary protein may be undesirable because of the potential adverse effects on heat production, acid-base balance (especially at maximal exercise), and possibly respiratory health due to ammonia accumulation under confinement housing conditions (Graham-Thiers et al 2000). Excess protein intake has been shown to result in increased urine production and possible evaporative fluid losses that could result in an unnecessary challenge for horses during prolonged exercise (Connysson et al 2006). However, potential benefits have been shown by Essen-Gustavsson et al (2010) who reported that trotters in training fed a high CP (16.6%) forage-only diet had greater glycogen and free leucine concentrations in the muscle than when fed a forage-only diet containing CP close to the 1989 NRC-recommended concentrations (12.5%). It was suggested that the higher muscle glycogen concentrations were due to either greater synthesis or lower degradation and that possibly the additional amino acids provided via the diet were used for oxidation in the muscle, thus sparing the glycogen.
Additionally, there is some evidence that substantial amounts of nitrogen may be lost through sweat (Freeman et al 1986), possibly in the range of 1 to 1.5 g of N/kg of sweat and estimates of sweat losses have been as high as 5 kg/100 kg of BW (Meyer 1987). Nitrogen losses of that magnitude would equate to losses of over 200 g of crude protein. This demonstrates the potential for protein requirements to be influenced by the ambient environmental conditions and not just the duration and intensity of the exercise.
Finally, as discussed in Chapter 6 (Protein and Amino Acids), the quality of the dietary protein (the amino acid profile) is important. Unfortunately, there is minimal published research on this subject (Antilley et al 2007) especially in exercising horses. Hence, knowledge regarding the true amino acid requirements for exercise is limited but research has suggested that exercising horses can successfully be fed lower concentrations of crude protein if their diets are fortified with limiting amino acids (Graham-Thiers et al 2001). Some commercial concentrates are now including lysine and threonine in their formulations.
In both humans and horses, protein is needed in the repair and recovery process so an adequate intake of quality protein is necessary. Both arginine and ornithine have been reported to stimulate growth hormone release and promote increased lean tissue (Clarkson 1998). The effect is small and, despite several amino acids being sold as “anabolic agents” for humans, it is doubtful that amino-acid supplements will promote gains in muscle mass (Clarkson 1998) unless the core diet is deficient or marginal in amino acid composition (O’Connor et al 2002). Still, it is not known whether supplementation of certain amino acids could cause subtle long-term benefits on muscle structure, including repair and recovery from exercise. For instance, L-arginine supplementation in rats appears to reduce oxidative damage and inflammatory response to the myocardium after exhaustive exercise (Lin et al 2006).
Branched-chain amino acid (BCAA) supplementation had no influence on the metabolic response to exercise in Standardbreds (Casini et al 2000, Stefanon et al 2000), though any potential role in recovery nor other types of exercise was not explored. Additionally, timing of ingestion may play a role in the response of exercise to nutritional supplements. In humans, net muscle protein synthesis was greater when an essential amino acid-carbohydrate supplement was given before, rather than after, resistance exercise (Tipton et al 2001). Recent pilot work in the horse, however, suggests that post- rather than pre-exercise amino acid supplementation results in higher plasma amino acid concentrations (Graham-Thiers & Bowen, 2011) which, in turn, may results in increased protein synthesis and decreased protein degradation (Matsui et al., 2006). More work is needed in this area.
Mineral requirements of athletic horses vary considerably according to a number of factors including the management of the animals during training, as well as the stage and intensity of the training (Nielsen et al 1997).
Calcium is usually considered first when discussing mineral nutrition in racing animals due to its importance in bone health. There is an added dimension in high-intensity exercising animals due to the dynamic nature of bone, which tends to respond fairly rapidly to the forces that are applied to it. In essence the skeleton tries to achieve a balance between bone that is strong enough to withstand frequently encountered strain loads and maintaining an energetically efficient skeleton without storing excess mineral. In response to sprint exercise (whether forced or free-choice), bone becomes stronger or, if already strong, maintains its strength. By comparison, when no high-speed exercise is performed, there is no signal to increase, or even maintain, bone mineral content and skeletal strength is lost. For racehorses competing at high speeds, maintaining skeletal strength is critical.
During periods of training when high-speed exercise is being introduced, the demand for calcium is increased. In contrast, when the forces placed upon the skeleton are decreased (i.e. when horses are stall-rested without a requirement for high-speed exercise or when the training intensity is decreased), the demand for calcium is decreased (Nielsen et al 1998). Even when fed dietary Ca at twice the recommended levels, bone loss decreased when conditioned horses were stall-rested for 12 weeks (Porr et al 1998). These variations in mineral requirements associated with stage of training or management of horses makes it difficult to provide firm guidelines as to the specific calcium requirement of an individual horse. This probably also applies with respect to the requirements for phosphorus (Elmore-Smith et al 1999) and magnesium (Stephens et al 2001). Again, it is important to note that the guidelines established by the NRC (2007) attempt to ensure that minimum requirements for minerals are met, regardless of the stage of exercise and the type of management used.
As with the protein content of the diet, full ration analysis is required when evaluating mineral intake vs. requirements. Generally, if a commercial fortified complementary feed/concentrate is being fed at the recommended levels, the risk of deficiency is low.
In comparison to the other macrominerals, hard working horses have a substantially increased requirement for the electrolytes sodium, chloride, and potassium due to the need to replace the electrolytes lost in sweat (Coenen 2005). The NRC (2007) provides estimates based on perceived intensity of exercise performed by the horse (see also Chapter 10 and Chapter 14). This estimate is, however, quite subjective and contains an inherent flaw because ambient environmental conditions can greatly influence sweat rates. Additionally, though many would consider all racehorses to be in heavy work, if work load is based on distance trained weekly, Standardbreds are in fact likely to be subjected to a greater work load than most Thoroughbreds or Quarter Horses and thus in theory would have greater electrolyte requirements. It also is important to recognize the potential importance of sweat loss during any transportation to a race.
Fortunately, if allowed free access to salt (sodium chloride), fed a commercial fortified concentrate grain mix that contains about 1% salt, and provided with sufficient access to good-quality forage, most horses will consume enough electrolytes to meet their needs. This is particularly true for the horse exercising at a high-intensity as the length of their exercise bout tends to be shorter than, for instance, an endurance horse. However, even with free access to salt, there is no guarantee that adequate quantities will be consumed and, particularly with some extremely hard-working horses in hot climates, additional electrolyte supplementation may be justified. Some horses can consume substantially more salt than they need if allowed free access. Thus, it is critical that adequate water is available. Electrolyte deficiency or imbalance can arise when forage intake is restricted, especially when haylage replaces hay on a weight to weight basis as potassium intake may become inadequate.
Development of a vitamin deficiency is unlikely provided horses are fed green, high quality hay that has been relatively recently harvested or allowed access to good pasture, and are exposed to sunlight (Saastamoinen & Harris 2008; see also Chapter 9). Additionally, commercial feeds used in racehorse diets are usually highly fortified with vitamins, further decreasing risk of a vitamin deficiency. However, inadequate intakes of some vitamins, especially the fat-soluble vitamins, may occur if horses are fed a diet consisting of unfortified grains and conserved forage for an extended period of time (Saastamoinen & Harris 2008). Additionally, inappetence may occur in some hard working horses and it has been speculated that in these circumstances supplementation with B-vitamins, thiamin in particular, may help restore appetite and improve cellular energy (Carroll et al 1949, Wolter 1987). Although B-vitamins are normally synthesized in sufficient amounts by the microflora in the hindgut, given their role as co-factors involved in energy metabolism, such supplementation may be justified especially when DM intake is depressed. While some vitamins can be stored by the body for extended periods, storage is much shorter for others, e.g. thiamin reserves may last only 1–2 weeks (Saastamoinen & Harris 2008). This provides perhaps additional justification for supplementation if it is believed that endogenous synthesis is not keeping up with the demand in the intensely exercising horse.
The important antioxidant role that certain vitamins play, especially vitamin E, is obviously important to the exercising horse. There is, however, little published work showing specific benefits to the intensely exercising horses of enhanced supplementation and it is unclear if additional supplementation to a horse fed a balanced diet is warranted (see also Chapter 9).