A successful racing career is important for the potential value of a horse as a future breeding animal. The better the racing career, the higher the stud fee, so both are important for the equine industry to grow and improve. Horses may also race during the breeding season, as sometimes the interval between activities related to racing or breeding can be relatively short or they overlap. Mares may not cycle as predictably or conceive as readily and stallions may be tougher to collect or exhibit a decrease in semen quality.
Since the horse industry is largely based on how a horse performs, the effects of the training reflects directly to their future as a producer.
The association between racing and breeding arises through two channels: the environment and genetics. Racing, which usually takes place before breeding, can have favorable or unfavorable environmental effects on fertility. The environmental effects may be due to the stress induced by training and racing, general physical condition, or medical treatment.
It is also possible that racing and fertility traits are genetically connected. The genetic correlation means that genetic changes in racing traits, achieved through breeding, would influence fertility. A genotype–environment correlation may also play a role here: the best racehorses are destined to get the best treatment as broodmares and breeding stallions, because their foals are expected to be more valuable than those of mediocre horses. Racing may affect the fertility of horses for variable periods of time- from a few days to the majority of their lives.
The knowledge of possible effects of racing on fertility enables breeders to find the best possible combinations of racing and breeding careers for their animals.
Trials have been conducted on the effects of exercise on semen and embryo. Additionally, on demographic level, Langlois and Blouin (2004) have studied how the performance level measured by earnings affects fertility.
A study done in 2011 suggested that foaling outcomes were the lowest for the mares with the highest numbers of races during the year of mating and for the mares racing after the first mating, although the effects of exercise on hormone levels and fertility has only been studied in experimental conditions with small numbers of horses. Exercise and racing increase the body temperature of horses and change their hormone excretion. High body temperatures damage embryos in cattle and in horses. The increase in body temperature, due to exercise, was reported to decrease the number and quality of embryos recovered from embryo donor mares in Texas, and to prolong the time between luteolysis and ovulation. If it is less likely to get an embryo of good quality it should, in parallel, also be less likely to maintain a successful pregnancy.
Some results showed that mares that had raced during the mating year actually benefited in terms of fertility, as long as the number of races was relatively low (≤10) and the racing took place before the mating as opposed to after it. Even though excessive racing causes stress, all environmental effects caused by racing are not necessarily unfavorable to fertility. This could possibly explain the positive effect of moderate racing. If a horse has only raced in the beginning of the mating year, the acute stress is over by the time of the mating.
Because racehorses are exercised regularly, they are in a better physical condition than the horses used only in breeding. The low body condition score (BCS) of mares induced by restricted energy intake has been found to be associated with lower pregnancy rates, increased numbers of cycles per conception, and a delayed onset of ovulations after parturition, and a deeper anestrus in winter. According to Henneke et al. (1984), the loss of fertility was evident when the BCS was under 5.0, on a scale of one to nine. A mare in top physical condition would not tend to get a high BCS, because she does not have excess body fat.
On the other hand, a mare in racing condition is unlikely to be malnourished; on the contrary, the feed ration of racehorses is usually adequate for energy, protein, minerals and vitamins. Racehorses are often fed more conscientiously than broodmares. The feeding of racehorses and broodmares is different: racehorses have a greater energy demand, and more attention is focused on their diet. Pasture feeding is more natural, but also less precise. The quality and management of the grazing land can vary substantially, and insects and parasites can cause health problems. On the other hand, the high-concentrate diet associated with racing and training can expose horses to health concerns, such as ulcers. Horses that race regularly also risk contracting infectious diseases through a wide network of connections.
Even though a small number of races during the mating year had a positive effect on fertility, the mares with the most races still had worse foaling outcomes than mares with no races during the mating year. The causes of the better fertility of mares that have raced only a few times (possibly better feeding or general condition, as speculated) are probably outrun by the effects of more extreme racing stress.
Data collected on stallions racing during the mating year did not reduce the foaling outcomes, however, semen characteristics do change. Dinger et al. (1986) found no changes in stallion semen characteristics after light exercise. On the other hand, in a study by Janett et al. (2006), the plasma levels of cortisol, testosterone and lactate rose during exercise in stallions, and the exercise also had an unfavorable effect on the quality and freezability of the sperm produced by the stallions. Furthermore, Cayado et al. (2006) found that competing increased cortisol and ACTH excretion in sport horses, but less so in experienced horses. The changes in hormone levels in German Warmblood stallions caused by competing in show jumping and dressage did not lead to reduction in sperm quality: the stallions not used in competitions actually had the least motile sperm.
In addition to hormonal changes induced by exercise, high body temperature can damage sperm. The use of scrotal suspensories on stallions during training or racing leads to even higher scrotal surface temperatures, which may be harmful to sperm development. Stallion sperm are very temperature sensitive and normal spermatogenesis requires that the testis (site of sperm production) and epididymis (site of sperm maturation and storage) are maintained at about 3-4 degrees Celsius below body temperature, so a system evolved that allows for these special conditions to be maintained. If scrotal temperature ever rose to 105°F, millions of spermatozoa would die- a detrimental outcome since it takes about 65 days for spermatozoa production and maturation. The foaling rates of stallions for the first year in breeding have not been found to be inferior to those of the successive years: this indicates no semi-permanent stress or possible medication effects from the racing career. Physiological changes in hormone levels caused by racing should only be temporary, but the use of severe medication (possibly for doping purposes) could have more permanent effects.
When describing racing by best career time, we were measuring the level of the animal in racing, and not so much racing stress. The level of the animal in racing may have direct (genetic), or indirect (genotype–environment correlation) effects on its fertility. One study done on Fox trotters indicated that the mares with high earnings were more likely to produce a foal when bred than mares with small earnings; this also suggests that the best horses probably receive the best care. This kind of preferential treatment leads to genotype–environment correlation.
The possible negative effects that racing has on fertility should be weighed against the benefits of racing. Racing is the whole point and purpose of breeding. Racing produces financial benefits: the prize money won and the added value and reduced marketing costs of offspring or semen and the breeding animal itself. It is also a benefit that we can more accurately estimate the breeding value of the horse regarding racing traits if we have measurements of its own performance on the track. However, the increase in accuracy is marginal in situations when there already is an abundance of racing data available of the horse. Also the financial benefits would tend to plateau in the later years of the racing career, as younger horses enter the tracks to compete for the same money.
From a practical point of view, it is generally financially justifiable for professionally trained horses to continue racing as long as the prize money they win exceeds the training fees. However, mare owners should expect a reduction in the number of offspring that the mare is able to produce if racing is continued. The years that the mare spends in racing are taken away of the years she has to produce offspring. In addition, old mares are less fertile. When the best racing mares produce less offspring, they contribute less to the genetic gain in speed in the population. The question remains how long the racing career should last in order to optimize the benefits from both racing and breeding, and whether it is worth the risk to let a horse continue racing while being used in breeding.
When it comes to prevention of the reproductive issues caused by the strains of racing cooling the horse out effectively, managing the horses stress levels, staying up to date on general maintenance, paying close attention to how the horse reacts to certain environments, and more are all beneficial to support the horse on the track and to keep owner satisfied in the breeding followed. An active cool has been proven to be more effective than passive cool outs- trotting the horse after a race, allowing them free choice water, a cool bath, and take the horses temperature often to watch for overheating. Stress can be triggered by a change of location, not being able to see another horse, too much strenuous training or injury related pain.
Things like worming, keeping up on teeth work, feet, and regular soundness exams will assist in keeping the horse more at ease and reduce factors that may cause discomfort. A stallion that is expected to breed should be given six weeks for sperm to regenerate from potential stress, but if that not a possibility, light workouts would be suitable. A mare expected to breed and have a healthy foal should be given some light time around the time of insemination, though she can be ran until around the three month mark; after that is a risk for both the mare and the fetus.
Generally, it should not be profitable for any horse to avoid racing altogether, mainly because a horse with no performance record does not have much value as a breeding animal except under special circumstances. Studies also exposed, at least for mares, that refraining from racing do not benefit fertility. The question remains how long the racing career should last in order to optimize the benefits from both racing and breeding, and whether it is worth the risk to let a horse continue racing while being used to inbreeding. The effects of racing on fertility are different depending on the breed and gender of the breeding animal.
Racing during the mating year does not lower the fertility of stallions. For mares, a large number of races during the mating year posed a fertility risk, and particularly if the mare raced also after the first mating. The far better fertility of mares with the best career records could point to a preferential treatment of the most valuable mares, which is an example of a genotype–environment correlation. Based on the estimates of genetic correlations, there was no evidence of a negative genetic association between racing and fertility; in fact, the association was slightly beneficial. This means that genetic improvement in racing traits should not lead to impaired fertility and the industry can continue to grow and advance!