In boars, both testes should be fully descended and present in the scrotum at birth. Absence of one or both testicles in the scrotum is indicative of cryptorchidism, a heritable condition that should preclude further consideration of the boar for future use in a breeding programme.

Sperm production is directly correlated with numbers of Sertoli cells in the testes. The proliferation of Sertoli cells in the boar occurs postnatally, in two distinct phases. The initial proliferation phase starts at birth and continues through to 1 month of age, with the second and final phase occurring at 3–4 months of age (França et al., 2000).

Puberty is a temporal phase in a boar’s sexual maturation in which distinct anatomical and physiological changes occur. These changes are directly under the influence of steroid (i.e. androgen) production, which originates primarily from the Leydig cells of the testis. In the prepubertal boar, there is an ill-defined penile sigmoid flexure. This results from the presence of a fibrous band known as the frenulum, which attaches the glans penis to the internal lamina of the prepuce and aids retention of the penis within the prepuce in the young boar. As androgen levels increase, penile growth and development of the sigmoid flexure ensue. Behaviourally, physical mounting activity and penile thrusting commence, and these promote the breakdown of the frenulum. Testicular synthesis of androgens increases from 70 to 170 days of age, with blood concentrations markedly increasing after 200 days of age (França et al., 2000). Puberty culminates with the presence of all sperm progenitor cells in the testis (Plate 44) and of spermatozoa in the ejaculate, which may occur in domesticated large breeds as young as 4.5 to 6 months of age (Cameron, 1987).

Genotype, environment and management all contribute to boar development and puberty. Crossbred boars tend to reach sexual maturity earlier than purebred boars. Group rather than individual housing has been found to be beneficial to the boar’s sexual development (Hacker et al., 1994). In swine, the sexes exhibit differences in growth rates, feed utilization and nutrient needs. As such, split sex feeding is a successful management strategy that is used to optimize development of the young boar and gilt. Split sex feeding has its best benefit when applied to boars after they have reached 45–50 kg body weight.

Assessment

Physical and behavioural examination

The boar should first be visually examined while in an enclosed, secure area. Boars are normally inquisitive and will thoroughly investigate their surroundings. At this time, the boar should be assessed for body condition, symmetry, balance and skeletal conformation as it ambulates freely in the pen. Particular attention should be paid at this time to current or potential musculoskeletal conditions that may interfere with the boar’s ability to approach and mount a collection dummy or female. Visual acuity in each eye is next determined by the investigator extending a hand from a perpendicular position towards the eye to elicit a blink reflex, or to elicit animal movement towards or away from the extended hand.

As AI is widely used in the global swine industry, most boars will be trained for semen collection using a dummy sow. Therefore, boars should demonstrate interest in investigating and mounting a stationary dummy sow. Field observations suggest that the majority of boars successfully trained for semen collection will have mounted the dummy sow within the first four training sessions. If a boar is to be used in a natural mating programme (e.g. pen breeding or hand mating), he should be exposed to several oestrous females in order to assess his ability to mount and perform intromission. In both cases, assessments can be made of both libido and of how well the boar ‘works’ the female/dummy sow.

Semen collection

Because of the widespread use of AI in swine, a large proportion of boars have been trained for semen collection using a collection dummy. The boar is introduced into a semen collection pen and allowed to freely approach the dummy. Within a few minutes, the boar will mount the collection dummy, and this is soon followed by pelvic thrusting. Good footing at the base of the collection dummy is needed at this point so that the boar maintains thrusting and exteriorizes the distal portion of the glans penis.

Ejaculation then commences with the initial pre-sperm fraction, which is usually clear and free of sperm. The pre-sperm fraction is not collected. Sperm-rich and sperm-poor fractions follow, both of which are collected into the receptacle. A gel fraction, originating from the bulbourethral gland, is normally emitted in conjunction with the sperm-poor fraction. As this gel fraction is of minimal value, it is intentionally separated from the fluid portions of the ejaculate by placing a filter or gauze over the opening of the semen collection container. Once ejaculation is completed, the penis is visually examined for any pathology before it is released. The neat semen is taken to the laboratory for analysis, making sure that it is protected from chemical (e.g. water, detergents, alcohol, etc.), temperature (hot, cold) and ultraviolet light insults.

Although not commonly used, electro-ejaculation (EE) can be performed in boars when it is not possible to obtain an ejaculate using the gloved hand technique and a collection dummy. Boars selected for EE should be presented with a history of known libido and mating ability. The animals are initially placed under general anaesthesia and then positioned in lateral recumbency. Using a gloved hand, the rectum is evacuated and a boar-specific rectal probe inserted (Plate 45). The penis is exteriorized from within the prepuce using atraumatic dressing forceps (see Plate 45). Once exposed, gauze is wrapped around the penile shaft to maintain exposure during the process and to facilitate visual examination. Using an electroejaculator, a pulsating current of progressive amplitude and frequency is applied through the rectal probe until ejaculation commences. The ejaculate is collected into a sterile, warmed receptacle for protection and transport to the laboratory for further examination.

Semen evaluation

Evaluation of a boar ejaculate should include assessment of both its physical and cellular characteristics. Laboratories should be cognizant of the appropriate handling conditions in order to avoid temperature, osmotic and/or pH shock to the neat semen. Neat semen should be held and manipulated at body temperature (~38°C) during evaluation, and all equipment and materials that come into contact with the neat semen should be at a similar temperature.

Physical assessment

The ejaculate should be examined for colour, turbidity, odour and viscosity. Normal boar ejaculates will be whitish grey to white in colour. Samples will be opalescent when held up to light. With gentle agitation, mass swirling of suspended cells can be easily visualized by the naked eye. Neat semen will have a very fluid viscosity, similar to aqueous solutions. Fresh blood in the boar ejaculate (haemospermia) will colour the semen pink to bright red. A yellow or brownish colour to the ejaculate may be due to a pathological problem (i.e. infection, oxidized blood) of the reproductive tract, or to contamination with preputial fluid or urine. A characteristic odour will confirm the latter. Ejaculate volume is determined by weighing the ejaculate (1 g weight = 1 ml volume), or by using an isothermal volume measuring device. Ejaculate volume can be multiplied by sperm concentration to obtain the total sperm number in an ejaculate.

Cellular assessment

Estimations of sample motility should be made as soon as possible after semen collection. Accurate assessment of motility is performed by a skilled examiner using a high-quality microscope with, preferably, a heated (38°C) stage. Given the subjective nature of this assessment, accuracy is highly dependent upon sample preparation, microscope quality, technician experience and natural ability. To minimize the effects of systematic and experimenter error, sample preparation and microscopic assessment must be standardized. The most common technique employed in laboratories is to place a 7–10 μl sample of semen on a warmed (38°C) microscope slide, overlaid with a coverslip. The loaded slide is focused under a microscope at 200× magnification. If appropriately prepared, the sample should appear as a monolayer of cells, within which the motility of individual sperm can be easily ascertained. If the sample is too concentrated, a subsample can be diluted with equal (v/v) parts of semen and a thermo-equivalent isotonic diluent, before placement on the slide with a coverslip. At 200× or 400× magnification, motility should be estimated to the nearest 5% by viewing sperm activity in at least four different fields on the slide, and taking the average of these readings for the final motility estimate. From this examination, the presence of any spermatogenic progenitor cells, leucocytes, erythrocytes and bacteria should also be noted.