Chapter 7 BREEDING MANAGEMENT OF THE THOROUGHBRED STALLION
The stallion is perhaps the most important asset of a breeding operation. Therefore, the primary concern of the stallion manager should be to maintain the stallion’s health and to maximize the stallion’s reproductive capacity. To use the stallion to his maximum capability, the stallion manager must meet the basic needs of the stallion and must understand his behavioral patterns as well as his reproductive limitations. Having one mare who is a poor reproductive performer is unfortunate, but one poorly managed stallion can have a disastrous effect on a breeding program. Because of the impact that the stallions can have on the productivity of a farm, they are judged more critically and culled more extensively than the brood mares.1
There are basically two criteria for selecting a stallion.1 The first is that a stallion prospect be a horse that is recognizable. Trying to promote an unknown stallion in the modern Thoroughbred market is difficult. A horse without a pedigree or a good race record is not a stallion prospect.1 In addition, a stallion prospect must have shown ability on the racetrack because the object is to produce future racehorses. Horses selected as stallions should be able to potentially improve the quality of their offspring in a breeding program. If the stallion passes on desirable traits to a large percentage of his offspring, he is fulfilling an important genetic role. The thoughtful selection of a stallion for a breeding program must take into consideration the goals that the breeding program is intended to achieve.
The selection criteria for a breeding stallion are based mainly on performance, conformation, and pedigree. Although the main purpose of the stallion is to breed and impregnate mares, in general, his reproductive potential is not considered.2 Therefore, the stallion manager must implement procedures to increase the reproductive efficiency of any stallion regardless of his inherent fertility.2 The long-term effects on the reproductive performance of the offspring of subfertile stallions are potentially catastrophic.
A successful breeding program requires a sound feeding program. Feeding is still considered an art, and although a great deal of scientific knowledge has been gained in recent years, the stallion’s body condition should be evaluated on a regular basis and feeding adjustments made as needed. In general, the nutritional needs of a stallion during the breeding season do not appear to be different from his needs during maintenance.3 A slight increase in energy intake may be necessary or beneficial during the height of the breeding season; however, it is easy to overestimate the nutrients needed by a stallion at this time.3 Overfed and obese stallions are more common and of greater concern than underfed ones. A maintenance ration consists of enough balanced nutrients to support normal, basic bodily functions.4 Adequate pasture or good-quality hay can usually meet these requirements, and free access to trace mineralized salt and fresh water is also necessary. Grain as an energy supplement in cold weather or under certain stressful conditions may also be warranted. The stallion’s size, condition, activity, and temperament all play a role in his nutritional needs. Therefore, diets should be adjusted for individual stallions.
The healthy stallion consumes 2%–3% of his body weight daily. At least 50% of this should be in the form of roughage. Vitamin A plays an important part in reproduction.3 A severe deficiency of vitamin A can result in a decrease or cessation of sperm production.3 Leafy green forages generally supply adequate amounts of vitamin A. Contrary to popular belief, supplementation with vitamin A and E over National Research Council requirements does not improve reproductive performance of stallions. Stallions generally require 10% protein in their feed; younger stallions require 12%–14%. Obesity may adversely affect libido and mating ability. Therefore, the only dietary requirement for efficient sperm production and good breeding performance is a balanced diet that maintains the stallion at his optimum weight. There is no conclusive evidence of any nutrient that is able to increase sperm numbers or quality.
Horses naturally are roaming and grazing animals. Therefore, exercise for stallions is an integral part of their management that affects their mental and reproductive well-being. This basic need for exercise is often ignored or underestimated, and nutrition and exercise go hand-in-hand. A stallion needs exercise to remain in good physical condition and to maintain a sharp mental attitude. Exercise can be provided in one of the following ways or in various combinations: (1) turn-out in paddock, (2) riding, (3) lunging, (4) treadmill, (5) swimming, or (6) mechanical walker.
In determining the type of exercise program, the stallion manager must consider the physical condition of the stallion as well as his temperament. Turn-out in a small, 1- to 2-acre paddock should be the least that is done to provide exercise for the stallion. If the horse is not very active outside or seems to be gaining weight, a more forced type of exercise may have to be provided, such as riding or lunging, to keep the horse fit and happy. In addition, lack of exercise may lead to vices such as weaving, stall walking, and cribbing.
The amount of exercise time must be tailored to the stallion’s personality. Free exercise in a paddock can last as long as 24 hours a day for some horses depending on the weather conditions. Others may do well with just 1–4 hours a day. Riding or driving in a jog cart should be at a slow canter or a jog for 1–2 miles a day. This exercise should be done 6 days a week depending on how easily the horse maintains his condition. The goal is to keep a stallion fit for the breeding shed, not the racetrack, so that he has a good attitude toward his daily duty of covering mares.
The personnel responsible for exercising stallions must be good horsemen or horsewomen so that overwork and injuries can be avoided. If soreness, lameness, or an attitude problem develops, the person responsible should be capable of detecting the problem. The exercise program must be discontinued and the stallion must be completely evaluated before continuing with any exercise.
Regardless of the type of exercise, the horse must have a positive attitude toward the routine. The form of exercise must be safe and minimize the chances of injury to the breeding stallion. Horses that have to go to the breeding shed several times a day must be kept as sound as possible because this may influence their attitude toward covering mares. For the exercise program to be effective, it must complement other aspects of the overall management of the horse (e.g., amount of turn-out, physical condition, and mental attitude).
An area of recent study is the effect on testosterone levels and seminal parameters by the grouping or housing of stallions. Intermale effects may be involved in behavior-related subfertility seen in some domestic breeding stallions or in the generally lower behavioral vigor and apparent fertility of stabled stallions compared with pasture-bred stallions.4 Pasture-bred horses generally exhibit high levels of fertility and greater sexual behavior endurance than stabled, hand-bred stallions.4 In general, stallions that have access to outside and get exercise have increased testosterone levels.
Preventive medicine should cover several areas of care for the overall well-being of the stallion. Foot care, dental care, vaccinations, and deworming should be performed in unison with the other management programs.
A good parasite control program should incorporate the use of deworming agents and pasture management. Although there are few studies to evaluate the effect of antiparasitic drugs on stallion fertility, most drugs are considered safe unless stated to the contrary by the manufacturer. Regular pasture rotation and harrowing are also part of a parasite control program. Periodic fecal examination for parasite eggs is a good way to monitor the effectiveness of the control measures used. Stallions are generally dewormed every 60 days, with rotation between ivermectin, pyrantel pamoate, and fenbendazole.
Most stallions are isolated for 6 months of the year, but during the breeding season, which lasts approximately 5.5 months (150 days), they are exposed to mares from different farms and countries. Stallions used for dual-hemisphere breeding are at greater risk of contracting contagious diseases. Therefore, it is important that an organized vaccination program be considered. At breeding farms, mares come from several farms and different areas, as well as countries, into a central location for breeding. Not only does this put the stallions at risk, but any exposed mare is at risk of spreading disease to many boarding farms.
Breeding stallions should be vaccinated approximately 60 days before the breeding season. In that way, any fever that may occur does not adversely affect semen quality. Generally, most stallions in Kentucky are vaccinated in December against rabies, tetanus, influenza, rhinopneumonitis, botulism, and West Nile virus. Rhinopneumonitis and influenza vaccines are repeated every 2 months. In addition, equine viral arteritis vaccine is required for Thoroughbred stallions in Kentucky with a subsequent 28-day isolation period following vaccination. If appropriate, eastern and western encephalomyelitis vaccine and Potomac horse fever vaccine are generally given in the spring.
Hoof care should be tailored to the individual stallion. Daily turn-out in a grassy area is conducive to good hoof quality and is another reason to keep stallions out in the paddock as much as possible. Hooves tend to become dryer and more brittle when stallions are kept in a stall, especially when stabled on wood shavings or sawdust.
In general, hooves are trimmed every 6–8 weeks. Some stallions with specific hoof problems may have to be shod. Whenever possible, stallions should not wear shoes because shoes tend to constrict and damage the hoof wall.
Dental examination is conducted routinely once or twice a year or whenever a dental or oral problem is suspected. Routine dental floating is carried out as dictated by the oral examinations. This ensures the proper use of feeds so that body condition is maintained and digestive upsets can be kept to a minimum.
Horses are considered to be long-day breeders. Reproductive function in the stallion is not arrested during the winter months as it is in most mares. However, certain seminal and hormonal characteristics and many aspects of sexual behavior are affected by day length. Testicular size and weight, daily sperm production, semen volume, hormonal concentrations, and libido are increased during the natural breeding season compared with the non-breeding season.5
The breeding season of brood mares is accelerated with an artificial lighting program. Providing stallions with the same artificial lighting program as for mares (16 hours of total light beginning December 1) results in increased testicular size and increased sperm output early in the year (i.e., February).6 Thoroughbred stallions show distinct seasonal and age-related changes in most of the reproductive parameters studied, and the exposure of such stallions to increased photoperiod produced significant alterations in these changes.6 In non-lighted stallions in central Kentucky, testicular diameters increased between February and June in young and middle-age stallions. In lighted stallions, testicular diameters were smaller in June than in February or April. Changes in testicular diameter were seen only in young or middle-age stallions, not older stallions, when under light treatment.
Similarly, hormone concentrations are affected by artificial lighting programs. Increased artificial light exposure results in elevated testosterone in February in both young and middle-age stallions, but by March, testosterone decreases and is similar to December levels. This increase is generally short-lived because stallions become refractory to this light stimulation. If the majority of mares are to be bred in February and March (early breeding season), exposing stallions to light may be beneficial. This is not the usual case, however, and a lighting program may not be suitable because the largest number of mares in a stallion’s book are generally presented in April and May in the northern hemisphere. Under the stimulation of lights, as the breeding season progresses, testicular size regresses to near that of stallions exposed to only natural day length. Therefore, increased day length by means of an artificial lighting program causes stallions to “peak” earlier than they would have on their own.