Discussion

Introduction:
Evaluation of the reproductive system in a stallion should assess whether he has the physical and mental faculties necessary to deliver semen containing viable spermatozoa and no infectious disease to the mare's reproductive tract at the proper time to ensure establishment of pregnancy in a reasonable number of mares mated per season. The evaluation needs to assess libido, mating ability, any congenital defects that may be transmissible to offspring as well as quality of semen ejaculated. The results of the examination should be recorded in a systematic and accurate manner.

Identification:
Accurate and detailed identification of any stallion which is the subject of a fertility examination is vital.

History:
As detailed a history as possible should be collated.

Physical examination:
Evaluation of the general body systems is an important part of the examination.

External genitalia:
The urethra, penis and prepuce should be routinely examined thoroughly by direct manual palpation and any palpable or visual lesions recorded. Where problems are suspected, ultrasound and endoscopy may also be useful. The scrotum and its contents (testes and epididymes) should be palpated in their entirety. Measurements of the length, width and height of each testis are made along with total scrotal width, using callipers or ultrasonography. Total scrotal width of mature, fertile stallions should be greater than 8 cm. Testicular size is highly correlated with daily sperm output (DSO). If the number of sperm collected does not approximate with the calculated number of sperm produced by the testes of a given size, this may suggest nonrepresentative semen samples or testicular degeneration.

Internal genitalia:
Examination of the internal genitalia is difficult unless suitable examination facilities are available and the veterinary surgeon is experienced in what to palpate. The internal genitalia can be examined by manual palpation and/or ultrasonography per rectum. The size and consistency of the internal genital organs should be noted.

Bacteriological swabbing:
Several bacteria can be transmitted by sexual contact. In many cases the stallion shows no signs of infection and is not affected in any way, i.e. is purely a carrier..

Assessment of libido:
Sexual behaviour should be evaluated by bringing the stallion into contact with a mare in oestrus and the reaction of the stallion to the mare should be noted. The intensity of the reaction also known as libido is dependant on mating experience, management and in some cases on season. Before a stallion can be considered a satisfactory breeding prospect he must be able to copulate normally, i.e. he must develop an erection, mount without hesitation, insert the penis, provide intravaginal thrusts, and ejaculate.

Collection of semen:
Most stallions can be easily collected artificially regardless of their age or their previous breeding method. The most convenient method for semen collection is the artificial vagina (AV). It must be remembered, particularly if the stallion has been sexually rested for some time, that unless semen quality is adequately stabilised, it is easy to overestimate or underestimate the quality of semen ejaculated.

Evaluation of semen:
To ensure that the semen sample obtained is representative of the stallion's sperm producing capability, it may require 2 - 10 days of daily collection before the output has stabilised. the key to determining the sperm production of a stallion is to determine his daily sperm output (DSO). The DSO is defined as the total number of morphologically normal progressive motile spermatozoa (TNM) produced in an ejaculate on a daily basis. DSO can be determined by collecting semen once per day over a period of 10 days and expressed as TNM per day. When a stallion has not been collected or mated for a considerable period, the total number of spermatozoa in the ejaculate will be high and so will the TNM. If he is collected every day, the total number of spermatozoa ejaculated per day will usually reach a constant level during the second week of collection. At that point, you will know how many spermatozoa the stallion produces per day and, therefore, what you can rely on when calculating insemination doses likely to be possible for that particular stallion.
In general, to be classified as a satisfactory breeding prospect stallion must be free of undesirable potentially heritable defects, behavioural disorders and transmissible diseases; possess no physical traits that would interfere with mating ability, semen quality and spermatozoa output. There should be at least 1.1 x109 live, morphologically normal spermatozoa for the stallion to be considered as having adequate fertility. Most stallions should be considerably in excess of this. The number of spermatozoa in a second ejaculate from a sexually rested stallion will usually approximate 50% of the number obtained in the first ejaculate.



Specific causes of subfertility in the stallion:

Behavioural dysfunction in the stallion:
Several factors affect stallion behaviour:
- Season and hormones
- Stimulus mare
- Sociosexual environment
- Management and training

Poor libido
This is the commonest behavioural disorder in stallions. It may be straightforward to remedy such as prolonged teasing under conditions that give maximal (safe) arousal. This is particularly important for slow-starting novice stallions. Management solutions as discussed above offer the best approach (prolonged teasing, natural oestrus mares, minimal distractions and established routine in the breeding shed). Poor libido can be associated with low circulating levels of testosterone and/or luteinising hormone. Stallions with low concentrations of these hormones can have good libido. Some therapies have been suggested, but their use is largely empirical.

Failure to attain or maintain an erection:

Ejaculatory dysfunction:
This can involve a failure to mount and/or thrust properly. Causes can be neurological, vascular or orthopaedic disease. Painful hindlimb joints or musculoskeletal back pain are the commonest causes in the author's experience. Management can include weight loss, good footing in the covering shed, reducing the height of the dummy mare (for stallions in an AI programme) or even semen collection on the ground.
Blocked ampullae can also cause ejaculatory failure in the absence of any obvious libido problems. This condition was first described by Varner et al. (2000) where certain stallions have abnormal retention of sperm within the duct system.

Urospermia:


Haemospermia:


Poor semen quality:
Possible factors include:
- Drug administration
- Pyrexia
- Overuse
- Underuse
- Nutritional factors

Management changes may be applicable. Attempts to treat subfertile stallions with hormonal methods have been largely unsuccessful. GnRH, hCG and steroids have been used, but evidence for any benefit is equivocal.