Originally published in the January 2016 issue of Veterinary Practice News. Did you enjoy this article? Then subscribe today! It’s a new year, and for horse breeders it’s exciting. New foals are being born, and it’s almost time to start breeding mares. It’s also going to be time to identify and treat breeding problems. For mare owners, it’s soon going to be time to culture and ultrasound and biopsy, especially when breeding issues come up. Of course, it takes two to tango, and for stallion owners, it’s going to be time to identify and treat stallion problems; otherwise stated, it’s not always the mare’s fault when a mare doesn’t get in foal. Stallion problems fall into some broad categories. Here are some things to think about if you’re worried that a stallion might be the problem. Lack of Libido A lack of sexual desire is a frustratingly common problem in stallions. As natural as it may seem for a stallion to want to breed, many circumstances can conspire to make a stallion a reluctant breeder. These include: How old is the stallion? Young stallions new to their assigned task often take their time, especially early in their breeding careers. Fortunately, through patient handling most young horses overcome shyness within a couple of weeks. The breeding season has a significant and important effect on libido. Typically, early in the season the number of mounts per service is increased. In stallions of all ages, reaction time is increased and libido is generally decreased. A lack of libido may develop from preferences of the stallion. Some may prefer mares of a certain color, for example. Stallions asked to compete in riding competitions as well as breed can display confusion and decreased libido. In competitions, stallions are generally discouraged from exhibiting their natural excitement at seeing other horses. If stallions are reprimanded for rearing, extending the penis or showing interest in mares during competition, they may become loathe to exhibit normal stallion behavior in the breeding shed. Stallions that compete should be managed so that breeding and competition events are separated. The horse should be trained to act in ways that are appropriate for each situation. Popular stallions get the opportunity to breed many mares. Booking too many mares can result in a stallion’s lack of libido. A lack of libido may develop from preferences of the stallion. Some may prefer mares of a certain color, for example. There may be a significant handler effect. That is, some horses work better with their preferred stallion groom. Certain breeds may be less driven sexually than others. For example, some professionals feel that standardbred and thoroughbred horses have poor libido in comparison with warm-bloods. For example, while a standardbred or thoroughbred stallion may breed two to three mares a day (30 to 45 mares per season), some warmbloods can breed up to 10 mares a day and up to 200 per season by natural cover. Of course, there is normal variation between individuals in any particular breed. Physical problems can cause stallions to lose interest in breeding. For example, stallions with hind limb or back problems can experience pain during covering or semen collection. Such stallions quickly learn to associate breeding/collection with pain, and this is reflected in a loss of libido. Low circulating testosterone or luteinizing hormone may be associated with a lack of libido. This is more likely to occur at the onset of the breeding season. Mounting and Intromission Failure Some stallions may show interest in breeding but fail to mount or successfully achieve intromission. Reasons for this may include: Orthopedic pain, especially from the hind limbs, or from back pain. Penile deviations, especially in older stallions that have been collected artificially throughout their breeding careers. It has been hypothesized that repeated bending of the penis to the left to facilitate entry into the artificial vagina can damage the corpus cavernosum on the right side, causing filling defects that result in deviation. Penile injury will cause some stallions to avoid mounting or intromission. Penile abrasions are common. Particular attention should be pain to breeding phantoms to make sure they are safe. Mares’ tails should be wrapped to prevent penile lacerations. Sometimes a mare is simply too tall for a stallion to mount comfortably. Erection Failure Of course, horses that lack libido may fail to achieve erection, but in horses with good libido the most common cause of erection failure is injury at breeding. For instance: Stallions that have been kicked during breeding attempts may become reluctant breeders who fail to achieve erection. Paraphimosis due to preputial trauma will inevitably lead to edema, and, depending on severity, vascular compromise and ultimately ischemia. Severe injuries can result in a ruptured penile suspensory ligament. Equipment used to prevent masturbation, such as stallion rings, can injure the penis and occlude the vascular supply. Particular care should be used in administering phenothiazine tranquilizers to stallions. Phenothiazine-derived penile prolapse or priapism can be caused by drugs such as acepromazine maleate. Low circulating luteinizing hormone, testosterone and estradiol 17B have been implicated by some investigators as causes of erection failure, but there is no consensus as to whether these hormones play a significant role. Neurological deficits may cause failure of contraction or asynchronous contraction of vertebral smooth muscle. Psychogenic reasons for erection failure include excessive teasing, overuse during the breeding season, Poor handling or management, and fear or pain resulting from encounters with difficult or violent mares or handlers. Ejaculatory Dysfunction Many causes of ejaculatory dysfunction are psychogenic. These include: Rough handling Injury or mating accidents Pain Overbooked/too many mares Strange surroundings/handlers There are, of course, organic causes of ejaculatory dysfunction, as well. These can include: Lack of stimulation of afferent pathways, e.g. due to pneumovagina Damage to the dorsal nerve of the penis Spermiostasis. This condition occurs in about 5 percent of sexually rested stallions. Blocked ampullae may result in degenerative changes to semen, such as the separation of sperm heads, may cause sperm to be packed into “plugs,” or may result in dramatic decreases in viable sperm count (azoospermia or oligospermia). Infertility Of course, stallions that exhibit normal sexual behavior may still be infertile for many reasons. These include age, disease, drugs and infection. Veterinary examinations to assess stallion health and semen quality are essential in identifying and resolving such problems.
