Suppressing unwanted hormonal behaviors in breeding stock

Article by Kate Dugher

The desire to suppress unwanted behavior in the horse can present for many different reasons. The behaviors that we are talking about can be anything from poor performance to hyper-excitability, distraction, discomfort on girthing up, not responding to the jockey, bucking, rearing, squealing, kicking or aggression.  

Is it hormonal? 

Often it is assumed that overt behaviors are hormonally driven; however, it can be easy to discount many other possible causes of these behaviors, especially those that are related to pain.  A full clinical examination by a veterinarian is always warranted when considering unwanted behavior in the horse in order to appropriately identify the cause and consider the most appropriate treatment options. 

Common causes of abnormal/unwanted behavior can include: 

• Musculoskeletal pain (lameness)

• Gastric ulceration

• Dental disease

• Poorly fitting tack

• Stress

• Hormonal influence

• Learned behavior 

There are also many reasons for normal and abnormal behaviors that can be associated with the reproductive system. Some of these could be identified as undesirable behaviors when associated with performance. 

The equine reproductive cycle

Horses are seasonal long day breeders and are influenced by daylight length. This means that the majority of mares have inactive ovaries in the winter and do not exhibit estrus behavior during this time. In comparison, in the summer months, they exhibit a reproductive cycle that lasts an average of 21 days. They spend, on average, 5-7 days in estrus, ‘in season’, and 14 days in diestrus, ‘not in season’. 

In the spring and autumn months the mare undergoes a transitional period. During this time, estrogen concentrations are variable, and estrus behavior can be seen irregularly. While stallions are also affected by seasonality, they still exhibit reproductive behavior all year round. The mare’s reproductive cycle can also be influenced by artificial light and therefore, it is worth considering that performance horses who are exposed to stable lights beyond the normal daylight hours in spring, autumn or winter may cycle for a longer period of the year or even throughout winter. 

Puberty

Timing of puberty in the horse is varied and affected by both genetic and environmental factors. Not only by age but also by time of year in which they were born, body condition and social cues. Puberty in fillies is usually at around 12-19 months compared to colts at around 10-24 months, however, there are wide variations from these reference ranges. 

Normal reproductive behavior in the mare

Normal estrus behavior occurs under high estrogen and low progesterone influence. Commonly associated behaviors include receptivity to stallions/geldings, vocalization, increased frequency of urination and presentation of hindquarters in a wide based stance.

Normal diestrus behavior under a dominant progesterone state includes repulsion to the stallion and can occasionally be associated with aggressive behavior to other horses. During pregnancy, the mare will also be under a dominant progesterone influence and is unlikely to exhibit estrus behavior particularly in the first trimester. Later in gestation a peak in testosterone and estrogen levels may be associated with changes in behavior. 

Abnormal reproductive behavior in the mare

Ovarian pain

Many mares will show an obvious reaction upon rectal palpation of the ovary when close to ovulation, suggesting that the dominant follicle/ovary can sometimes be tender at this time. Comparatively, humans often describe some ovarian pain around the time of ovulation. Therefore, it can be assumed that some mares could also experience discomfort around the time of ovulation. 

Other possible causes of ovarian pain that can occasionally occur in normal cyclicity include ovarian hematomas and haemorrhagic anovulatory follicles. It is also a consideration that external pressure placed onto the lumbar region close to the ovary around the time of ovulation could rarely elicit a painful response in some individuals.  

Vaginal pain

Vaginal pain has occasionally been associated with conditions such as vaginitis and pneumovagina. These conditions describe inflammation and/or air in the vagina. These are most commonly associated with poor perineal conformation and can be evident in some performance mares. 

If vaginal pain is suspected due to poor perineal conformation, then placement of a caslicks vulvoplasty may prove to be beneficial. If concurrent infection or urine pooling is suspected, then further intervention may be required. 

Reproductive tumors 

Reproductive cancer affecting the ovaries is one of the most common causes of cancer in the mare, the most common being the granulosa theca cell tumor (GTCT). These are generally locally invasive and are unlikely to cause any further health problems if the affected ovary is removed. They are often identified with a change in behavior. On rectal examination a common finding would be to identify one enlarged and one small ovary. 

Depending on which reproductive hormones the tumor secretes is likely to influence the associated behavior. This can include stallion-like behavior, aggression, persistent estrus behavior or complete absence of reproductive behavior. The severity of this often depends on the stage at which this condition is identified. Other types of ovarian tumors are less common but depending on if/which hormones are secreted will dictate which hormonal behaviors are associated. It is suspected that occasionally there could be ovarian pain associated with some of these cases particularly when the ovary is very large in size. 

Reproduction related treatment options

Mares

To have the most successful outcome in controlling reproductive hormonal behavior in the mare, it is important to understand whether the unwanted behavior is being exhibited all year round or just in the summer months and whether it is related to a particular stage of the estrus cycle. 

Whilst it is commonly assumed that most behavior problems are associated with the mare being in season, occasionally some mares can show unwanted aggressive behavior under the influence of progesterone – when they are not in season. 

Furthermore, it can be tricky to interpret this when trying to link hormonal behaviors to performance based unwanted behaviors and these signs can often be very individual. Keeping records of behavior versus stage of the reproductive cycle can help to try and decipher whether reproductive hormones are likely to be playing a part in the unwanted behavior. However, this does require careful monitoring and, most likely, multiple reproductive ultrasound examinations.

The other consideration is that unwanted behaviors are related to reproductive pain or abnormal hormone production due to pathological conditions of the reproductive tract as previously described. 

Ways to mimic the diestrus state and suppress estrogen related behavior

Progesterone/Progestins 

Progesterone is the dominant hormone produced by mares in diestrus. There are a multitude of systemic progestin (progesterone-like medications) available for use in horses in injectable and oral formulations. 

Altrenogest is a synthetic progestin commonly used to suppress estrus behavior by acting as a progesterone agonist. This means that the horse is likely to exhibit normal diestrus behavior for that individual whilst it is being administered. Altrenogest is molecularly very similar to the anabolic steroids trendione and trenbolone. Occasionally the product may contain trace levels of these anabolic steroids. Therefore, its use for horses in training is to be taken with extreme caution and withdrawal times adhered to. It is banned for use in racing thoroughbreds in some countries. 

There is also evidence to show that altrenogest can exhibit a reduced stress response and sedative-like effects in some horses, particularly mares. This effect may be beneficial in anxious individuals in training circumstances. However, arguably, dependent on the individual, a reduced stress response could have either a positive or negative effect on performance. 

Injectable progesterone applications have been used in racing thoroughbreds with appropriate clearance times before racing. These are often available in oil-based preparations which are commonly associated with injection site reactions and therefore, many trainers would avoid administering these within 3 days of racing. 

Upon cessation of progesterone supplementation, many mares will present with estrus signs 2-7 days after treatment, as this mimics normal luteolysis at the end of the diestrus phase. Therefore, the timing of administration and cessation of progesterone/progestin treatments is a crucial consideration when being used for the prevention of estrus behavior.

Intra-uterine devices (IUDs)

IUDs have been historically utilized to mimic early pregnancy in the mare with varying success. These require an ovulation to act upon to extend the life of the corpus luteum by blocking the hormonal release that normally brings them back into season. Therefore, they are only useful once the mare is already cycling. 

Glass marbles have been the most used IUD historically; however, these are no longer recommended due to multiple evidenced side effects including risk of glass fragmentation in the uterus. The use of PMMA spheres or magnetic devices such as the iUPOD would be a preferable and safer alternative if an IUD was going to be used.

Interestingly, in the author’s experience speaking with clinicians who have administered these devices, there is surprisingly positive client satisfaction despite the inconsistent and variable evidence of the success of these devices in the literature. 

Oxytocin

Administration of the hormone, oxytocin, at specific time points when the mare is in diestrus can extend diestrus by up to 60-90 days. This technique is evidenced by multiple studies. For optimal success, reproductive ultrasound would be used to identify ovulation and carefully plan the timing of injectable administration. 

However, some studies have also evidenced successful extension of the diestrus phase without known timing of ovulation. The major downside of this technique is the need for administration of multiple injections/multiple reproductive examinations to time ovulation. 

Immunological approach

Gonadotrophin releasing hormone (GnRH) is a hormone produced by the brain that is responsible for stimulating follicle growth in the ovaries and activation of a hormonal cascade to bring the mare into estrus. 

GnRH vaccinations generate an immune response against GnRH, suppressing the hormonal cascade and ovarian activity and therefore, estrus behavior. An equine licensed product has previously been available in Australia. However, this is no longer in production. We have the option of a swine formulation, Improvac®, which has commonly been used in equids off license. 

Major drawbacks for the use of this are common adverse injection site reactions, risk of anaphylaxis and concern over extended length of ovarian suppression. Therefore, this option would not be recommended in mares with a future breeding potential. 

Surgical approach

Ovariectomy is a treatment option for hormonal behavior in mares. The ovary is the only supply of progesterone in the horse but is not the only supply of estrogen. 

Ovariectomy has been associated with good client satisfaction in many cases to resolve unwanted hormonal behavior. However, in some mares, whilst removal of the ovaries would prevent cyclicity, it can occasionally result in persistent estrus behavior in the absence of progesterone produced by the ovaries. This is also a permanent option that will remove breeding potential.

The techniques discussed so far are not exhaustive and there are many other methods that have been used to affect cyclicity or hormonal behavior including pregnancy, induction of diestrus ovulation, GnRH analogue medication and infusion of intrauterine medical grade plant oils. 

Colts/stallions

There are a few medicated options for hormonal manipulation in males. Progestagen administration e.g. oral altrenogest administration can quieten stallion like behavior in males but is banned for use in racing and training. 

Immunization with off-license GnRH vaccines such as Improvac®, suppresses pituitary-gonadal hormone production aiming to cause a ‘chemical castration.’ However, results can be variable, particularly in mature stallions. As mentioned previously with mares, the downside of these vaccines are injection site reactions, risk of anaphylaxis and risk of prolonged sterility in future breeding animals.

Occasionally nutritional supplements have been used with effect in stallions such as L-tryptophan, a precursor of the neurotransmitter serotonin. This has induced calm and fatigue-like behavior in a number of species. 

Synthetic preparations of calming pheromones based on an equine appeasing pheromone produced in perimammary gland secretions of lactating females have also been used with such success. Of course, the use of these to calm behavior vs the desire to generate an athletic performance animal is a consideration and results are likely to have wide individual variation.

Nutrition - supporting the recovery process to improve performance - Train, Race, Recover, Repeat

Article by Dr Andy Richardson BVSc CertAVP(ESM) MRCVS

Introduction

Horses evolved as herd-living herbivores with a digestive tract designed to cope with a near continuous dietary input of forage in the form of a wide range of plant species. A large hindgut acts as a fermentation vessel where gut microbiota (predominantly a mix of bacteria, protozoa and fungi) exist in harmony with the horse in order to digest the fiber rich plant material.

Fiber is important to the horse for several reasons. The digestion of fiber releases energy and other key nutrients to the horse. Fiber also acts to provide bulk in the digestive tract, thus helping maintain the passage of fecal material through the system. Fiber also acts like a sponge to absorb water in the gut for release when required.

As horses became domesticated and used for work or sporting purposes, more energy-dense feeds in the form of cereal grains were introduced to their diet, as simple forage did not provide for all the caloric requirements. Cereal grains are rich in starch, which is an energy-dense form of nutrition. However, too much starch can cause problems to a digestive tract that remains designed for a pasture-based diet. The issues that can be caused by the trend away from a solely pasture-based diet can be digestive, behavioral or clinical.

Nonetheless, the combination of forage and cereal-based concentrates remains the mainstay approach for the majority of horses in training today, in order to maximize performance. A great deal of research and expertise are utilized by the major feed companies to ensure that modern racehorse concentrate feeds provide adequate provision of the major nutrients required and minimize unwanted effects of starch in the diet.

This article aims to discuss some scenarios where targeted or supplemented nutrition can act to help overcome some of the nutritional challenges faced by the modern horse in training, as they “Train, Race, Recover and Repeat.”

Equine Gastric Ulceration Syndrome (EGUS)

EGUS occurrence in racehorses is well documented, with prevalence shown to be over 80% in horses in training (Vatistas 1999). With a volume of approximately 2–4 gallons (7.53–15 liters), the stomach in horses is relatively small compared to their overall size due to its functional role in accommodating trickle feeding that occurs during their natural grazing behavior. 

As a horse chews, it produces saliva, which is a natural buffer for stomach acid. When the horse goes for a period of time without chewing, the production of saliva ceases, and stomach acid is not as effectively neutralized. The lower half of the stomach is better protected from acid due to its more resistant glandular surface. The upper, or squamous, region does not have such good protection, however, and this can be a problem during exercise when acid will physically splash upwards, potentially leading to gastric ulceration.

In practice, this can present a challenge for horses in training. Typically, they will be fed a concentrate-based feed in the early morning that stimulates a large influx of acid in order to help digest the starch. This may be followed by a period without ad-lib access to hay, thus reducing the amount of saliva subsequently produced to act as a buffer. When the horse is subsequently worked, there is a risk of acid damaging the upper squamous region of the stomach. There is some evidence to suggest that the provision of hay in advance of exercise may act like a sponge for the acid, as well as helping form a fibrous matt to minimize upward splash.

Gastric ulceration can go undetected in horses in training and may not lead to any obvious clinical signs. In other horses, it can lead to colic, poor appetite, dull coat and behavioral changes. In both scenarios, it is likely that the ulceration will have an impact on their performance, with decreased stride length, reduced stamina and inability to relax at speed all being possible consequences (Nieto 2009). Gastric ulceration can therefore have a significant impact on the ability of a horse to perform optimally day in day out in a training environment. This is exacerbated when ulceration leads to a reduction in appetite, with the obvious downside of a reduction in calorie intake leading to condition loss and further drop in performance.

This is an area where targeted nutrition has been clinically proven to play an important role. Ingredients such as pectin, lecithin, magnesium hydroxide, live yeast, calcium carbonate, zinc and liquorice have all been studied as having beneficial effects on gastric ulceration (Berger 2002, Loftin 2012, Sykes 2013). It is likely that a combination of the active ingredients will be most efficacious, with benefits noted when the supplement is added to the feed ration to help neutralize acid and form a gel-like protective coating on the stomach surface.

The daily administration of a targeted gastric supplement can be an important part of daily nutrition of the horse in training, alongside the use of pharmaceuticals such as omeprazole or esomeprazole when required.

Sweat loss

Horses have one of the highest rates of sweat loss of any animal, with sweat being comprised of both water and electrolyte ions such as sodium, potassium, chloride, magnesium and calcium. Therefore, it is not surprising that horses in training are at risk of unwanted issues should sweat loss not be replaced.

It is also worth noting that transportation can also lead to excessive sweat loss, with studies showing sweat rates of 5 liters per hour of travel on a warm day (van den berg 1998).

If the electrolytes lost in sweat are not adequately replaced, a drop in performance can result, as well as clinical issues such as thumps, dehydration and colic.

Electrolytes play key roles in the contraction of muscle fibers and transmission of nerve impulses. Horses without adequate electrolyte levels are at risk of early onset fatigue that may result in reduced stamina. It is also worth noting that horses that train on furosemide will have higher levels of key electrolyte losses, so will require targeted support to help maintain performance levels (Pagan 2014).

There is also evidence to suggest that pre-loading of electrolytes may be beneficial (Waller 2022). For horses in daily work, the addition of electrolytes to the evening feed will not only replace losses but also help optimize levels for the following day’s travel or race. The benefit of providing electrolytes with feed is that it will minimize the risk of the electrolyte salts irritating the stomach lining, which can occur if given immediately after exercise on an empty stomach. Feeding electrolytes when the horse is relaxed back in the stable will also allow them to drink freely, with the added benefit that electrolytes will stimulate the thirst reflex when they are relaxed, ensuring they are adequately hydrated for the following day.

Products should be chosen on the basis of adequate key electrolyte provision as not all products will provide meaningful levels of all the key electrolyte ions.

Muscle soreness

The process of muscle breakdown and repair is a normal adaptive response to training. This process can lead to inflammation and soreness or stiffness after exercise. In humans, there is a well-recognized condition called Delayed Onset Muscle Soreness (DOMS).

Further research is required to fully understand the impact of DOMS in horses. DOMS is the muscular pain that develops 24–72 hours after a period of intense exercise. There is no pain felt by the muscles at the time of exercise, in contrast to a ‘torn muscle’ or ‘tying-up’ for example.

In humans, DOMS is thought to be the result of tiny microscopic fractures in muscle cells. This happens when doing an activity that the muscles are not used to doing or have done it in a more strenuous way than they are used to.

The muscles quickly adapt to being able to handle new activities, thus avoiding further damage in the future; this is known as the “repeated-bout effect”. When this happens, the micro-fractures will not typically develop unless the activity has changed in some substantial way. As a general rule, as long as the change to the exercise is under what is normally done, DOMS are not experienced as a result of the activity.

In practice, avoiding any post-exercise muscle soreness in a training programme may be unavoidable, as exercise intensity and duration increases. Horses are far from being machines, so there is a fine balance between a programme that gets a horse fit for purpose without some post-exercise muscle discomfort. Physiotherapy, swimming and turnout will all likely benefit horses experiencing muscle discomfort. Whilst non-steroidal anti-inflammatories will always have their place for horses in training, one area of advancement is the use of plant-based phytochemicals to support the anti-inflammatory response (Pekacar 2021). These may have the benefit of not leading to unwanted gastrointestinal side effects and not having prolonged withdrawal times, although this should always be checked with any supplement particularly with the recent update regarding MSM.

Exercise will also lead to a process of muscle cell damage caused by oxidative stress. This is an inflammatory process and recovery from oxidative stress is key to allow for muscle cell repair and growth. Antioxidants are compounds that help recovery and repair of muscle cells following periods of intense exercise. The process of oxidative stress in muscle cells can lead to muscle fatigue and inflammation if left unsupported. Antioxidant supplementation in the form of Vitamin E or plant-based compounds can help protect against excessive oxidative stress and support muscle repair after exercise (Siciliano 1997).

Conclusion

Nutritional management of horses in training is a complex topic, not least as every horse is an individual and so often needs feeding accordingly. Whilst there is a lot of science available on the subject, the ‘art of feeding’ a racehorse—something that trainers and their staff often have in-depth knowledge of— remains an incredibly important aspect. Targeted nutritional supplements undoubtedly have their place, as discussed in, but not limited to, the scenarios above. 

Veterinarians, physiotherapists, other paraprofessionals and nutritionists all play a role in minimizing health issues and maximizing performance. In the quest for optimal performance on the track, nutritional support is one of the cornerstones of the ‘marginal gains’ theory that has long been adopted in elite human athletes. There is no doubt that racehorses themselves are supreme athletes that live by the mantra of Train, Race, Recover, and Repeat.

References

Berger, S. et al (2002). The effect of acid protection in therapy of peptic ulcer in trotting horses in active training. Pferdeheilkunde 27 (1), 26-30,

Loftin, P. et al (2012). Evaluating replacement of supplemental inorganic minerals with Zinpro Performance Minerals on prevention of gastric ulcers in horses. J.Vet. Int. Med. 26, 737-738

McCutcheon, L.J. and geor R.J. (1996). Sweat fluid and ion losses in horses during training and competition in cool vs. hot ambient conditions: implications for ion supplementation. Equine Veterinary Journal 28, Issue S22.

Nieto, J.E. et al (2009). Effect of gastric ulceration on physiologic responses to exercise in horses. Am. J. Vet. Res.70, 787-795.

Pagan, J.D. et al (2014). Furosemide administration affects mineral excretion in exercised Thoroughbreds. In: Proc. International Conference on Equine Exercise Physiology S46:4.

Pekacar, S. et al (2021). Anti-Inflammatory and Analgesic Effects of Rosehip in Inflammatory Musculoskeletal Disorders and Its Active Molecules. Curr Mol Pharmacol. 14(5), 731-745.

Rivero, J.-L.L. et al (2007). ‘Effects of intensity and duration of exercise on muscular responses to training of thoroughbred racehorses’. Journal of Applied Physiology 102(5), 1871–1882.

Siciliano, P.D. et al (1997). Effect of dietary vitamin E supplementation on the integrity of skeletal muscle in exercised horses. J Anim Sci.75(6), 1553-60.

Sykes, B. et al (2013). Efficacy of a combination of a unique, pectin-lecithin complex, live yeast, and magnesium hydroxide in the prevention of EGUS and faecal acidosis in thoroughbred racehorses: A randomised, blinded, placebo-controlled clinical trial. Equine Veterinary Journal, 45, 16.

van den Berg, J. et al (1998). Water and electrolyte intake and output in conditioned Thoroughbred horses transported by road. Equine Vet J. 30(4), 316-23.

Vatistas, N.J. et al (1999) Cross-sectional study of gastric ulcers of the squamous mucosa in thoroughbred racehorses. Equine Vet J Suppl. 29, 34–39.

Waller, A.P., and M.I. Lindinger. (2022). Tracing acid-base variables in exercising horses: Effects of pre-loading oral electrolytes. Animals (Basel) 13(1), 73.