Fertility and infertility in male animals
EXAMINATION FOR BREEDING SOUNDNESS
Examinations of male animals are made for two main purposes: either to ascertain
whether normal fertility can be expected from the animal, or for the diagnosis of
infertility. In either situation, the requirements are a history of the animal, a general
examination, a detailed examination of the genital tract, observation of copulation,
and collection and evaluation of semen.
History-taking is an important part of the examination of a suspected infertile male
animal. Many of the causes of infertility do not manifest themselves until a
considerable period of time has elapsed from the original insult, so that careful
questioning of the owner, often over matters that may have been considered trivial at
the time of their occurrence, may be needed to elucidate such causes.
History-taking is also a useful way of assessing owners’ expectations of their animals,
for many cases of so-called ‘infertility’ result from no more than an unrealistic
expectation of a sire’s capabilities.
The history must establish whether or not the sire is likely to be the cause of the
infertility, the duration of infertility and the circumstances of its onset. The number of
females with which the sire’s infertility has been manifest must be determined, as
must the conditions under which mating has occurred. For example, it is not
uncommon for dogs to be presented for infertility examination after failure to achieve
pregnancy on no more than one or two occasions, with bitches that were scarcely in
oestrus. Clearly, under such circumstances, the probability of a pathological cause
of infertility is minimal. Amongst agricultural animals, the sizes of groups of females
and the system under which mating was taking place must be determined.
A common cause of apparent infertility in rams derives from no more than using
groups of too many ewes, especially if these have undergone synchronisation of
oestrus or are being used in out-of-season breeding regimens.
The time of year when the infertility was noticed may give helpful clues as to its
cause, and may help to determine whether female factors are likely to have been of
importance. Similarly, information regarding the previous achievements of the
animal is of great importance in differentiating between congenital and acquired
conditions, or between managemental and pathological causes.
The general examination of the sire must take into consideration its age and likely
sexual experience, body condition, the possibility of intercurrent illness and the
animal’s temperament.
Considerable importance can be attached to the body condition and general degree of
maturity of young animals; on one hand, puberty can be delayed in poorly grown
animals, while, on the other hand, animals that have achieved very high growth rates
during rearing may have a body conformation that belies their sexual immaturity. It is
also noticeable that young bulls of some later maturing breeds, notably the Charolais
and Holstein, may remain relatively subfertile for longer than their earlier-maturing
counterparts Thus, the assessment of young sires can present some difficulty, for
allowance has to be made for the maturity characteristics of the breed, yet the use of
young bulls in which puberty is excessively delayed is best avoided in view of the
evidence that the age of puberty in a sire is highly correlated with the age of puberty
in his daughters. This problem is further compounded by the pressure of time imposed
by the requirements of progeny testing,
particularly of dairy bulls. In a seasonally calving national herd, semen from young
bulls has to be available roughly 12 months after their birth, if progeny testing is to be
carried out successfully.
For Friesians, this target presented little difficulty, but a significant proportion of
young Holstein bulls do not produce consistently usable semen until beyond this age.
By the time they reach this age, the majority of cows in the national herd will already
be pregnant, so progeny testing is delayed until the following season.
When young sires are used in natural-mating regimens further difficulties may be
encountered. Firstly, it must be ascertained that they have learnt to mount and
successfully copulate. Similarly, it must be determined that copulation is feasible,
since, where young sires are running with fully mature females, copulation may not
be physically achievable. Furthermore, it is not uncommon for those young boars or
rams that are run with large, mature females to be bullied by these females, to achieve
no pregnancies and to lose a great deal of body condition. Secondly, young sires are
most unlikely to achieve high pregnancy rates amongst large groups of females and
are generally unsuitable for use with groups of oestrus-synchronised females.
Body condition is also important in adult males. Spermatogenesis tends to be limited
when body condition is poor, and can also be limited by specific micronutrient
deficiencies. In general, chronic and continuing deficiencies of protein and energy
are likely to be of greater overall importance than micronutrient deficiencies, although
the effects upon fertility can be severe when these occur simultaneously (Salisbury et
al., 1978). For most agricultural species, sires should be maintained in moderate
condition, although rams should start the breeding season in a high condition score,
due to the considerable weight loss they experience during the season. Conversely,
ruminants that are fed on very poor-quality roughage can develop such great rumen
fill that normal copulation can be physically difficult to achieve. Moreover, as excess
weight canlead to damage to females during mounting, it is important to determine
that the sizes of sire and females are compatible.
Whereas any systemic illness can affect reproductive performance, three groups of
conditions can be noted as of particular importance: namely, diseases of the
locomotor system, conditions causing pain in the caudal abdomen and conditions that
result in prolonged pyrexia. Specific conditions will be considered under the first two
headings later in the chapter but, in principle, it is important to note that hindleg,
hindfoot or back pain are incompatible with normal mating behaviour. Furthermore,
not only does locomotor pain limit mating directly, but also the stress of prolonged,
unresolved pain may cause corticosteroid-mediated impairment of spermatogenesis.
Systemic illness causing prolonged pyrexia can result in increased temperatures
within the testis, thereby causing temperature-limited impairment of spermatogenesis.
REPRODUCTIVE EXAMINATION
A complete examination of the reproductive system requires physical examination of
the genital system, observation of the response of the animal to an oestrous female,
observation of mating and the collection of semen. In practice, which of these
procedures are actually carried out and the order in which they are undertaken depend
upon the species and the nature of the owner’s complaint. For example, it is requently
desirable to observe mating or to collect semen before the animal has undergone the
stress of a physical examination. Thus, for a bull, collection of semen by an artificial
vagina (AV) is often better undertaken before palpation of the genitalia (especially
before examination of the internal genitalia per rectum), whereas collection by
electroejaculation is probably best left until the rest of the examination has been
completed. Also, observing mating (or collecting semen by an AV or with a
judiciously placed electroejaculator) is the easiest way of observing the penis of a
bull. Conversely, in the ram, it is generally best to examine the external genitalia first,
as this frequently obviates the need for causing the animal stress by collecting of
semen by electroejaculation.
Choosing the conditions for the observation of mating behaviour is important. Where
the inherent libido of the animal is high, such as in boars and dairy bulls, it will often
be willing to mount females that are not in oestrus, or even to mount other males,
castrates or dummy animals. Indeed, the willingness of a cow to be haltered and tied
is often a more important criterion for her use than whether or not she is in oestrus;
few bulls are willing even to attempt to mount a fractious cow that is fighting against
unfamiliar restraint! Rams and beef bulls, although usually of high inherent libido,
commonly refuse to mount an oestrous female in the presence of a human observer,
and considerable tenacity and patience are often required before mounting occurs.
Animals that are stressed by recent transport are also often unwilling to mount straight
away.Taken together, these many caveats mean that, although the results of
observations of mating are valuable and often provide diagnostic information in
infertility examinations, one should be most cautious about condemning an animal
that does not perform under observation. During examination of the genital tract, all
parts of the genitalia that are accessible externally should be palpated.When xamining
the contents of the scrotum, the temperature, size, texture, resilience and evenness of
the testes and epididymes should be determined. The testes should be freely movable
within the scrotum. It is generally possible to palpate the head and tail of the
epididymis, but the body is often difficult to feel, due to its medial site. The vasa
deferentia should be palpated throughout the scrotal neck and (particularly in rams)
the presence or absence of vasectomy scars confirmed. The spermatic cord should be
palpated up to the level of the inguinal ring for the presence of abdominal contents
(scrotal hernia) or abnormalities of spermatic vasculature.
Measurement of scrotal circumference is useful in animals with a pendulous scrotum
(Figure 30.2), while, in the stallion, measurement of the width of the testes by calipers
or ultrasonography is similarly valuable. Likewise, ultrasonography of the testes of
stallions and dogs to visualise fluid-filled structures within their substance is proving
to be a valuable additional examination
Scrotal circumference of yearling bulls should exceed 30 cm, while mature bulls
should be over 36 cm for British beef breeds and over 38 cm for most other breeds.
Scrotal circumference of mature rams depends upon body weight; values over 28 cm
are acceptable for smaller breeds, and over 34 cm for larger breeds.
Scrotal circumference of rams is highly dependent upon season, with a 25–35%
change in size occurring between the non-breeding and breeding seasons.The texture
of the testes and the turgor of the cauda epididymes undergo parallel changes.
It should also be noted that, although rams continue to produce sperm during the nonbreeding season, they often fail to respond to electroejaculation during that period.
Hence, much care must be exercised in interpretation of clinical findings in rams
during the non-breeding season.
After palpation of the preputial part of the penis, exteriorisation of its free part where
possible, palpation of the sigmoid flexures and palpation of the prepuce and preputial
orifice, such of the internal genitalia as are within reach should be palpated per
rectum. In the bull and stallion, all accessory glands can be palpated thus, but they
are generally out of reach to a digital examination of the ram. In large boars, rectal
examination is potentially feasible, but digital examination of smaller boars will only
reveal the bulbourethral glands. In smaller dogs, digital examination of the prostate is
possible, but radiography, which is essential when prostatic disease is suspected,
is required in any case for examination of the prostate in larger dogs.
Libido testing
Assessment of libido and serving ability is widely used in the examination of bulls for
breeding soundness (Chenowith, 1986, 1997). Observations of bulls’ mating ehaviour
(Blockey, 1976a, b) suggested that serving capacity tests might be a predictor of
bulls’ fertility (Blockey, 1978). Moreover, libido is considered to be highly heritable
in cattle, so early selection of animals for high libido is likely to result in an overall
increase in this aspect of reproductive performance (Boyd and Corah, 1988). A
number of tests have been devised that attempt to assess libido and serving capacity
(Chenowith, 1986), utilising either females in oestrogen-induced oestrus or nonoestrous females, and scoring the number and vigour of matings or mating attempts.
A number of reports have shown that high test scores are associated with good
reproductive performance (e.g. Makarechian and Farid, 1985; Blockey, 1989).
However, other reports have been more cautious, giving only qualified support for
libido testing (Chenowith et al., 1984) or suggesting that moderate numbers of
mounts were superior to high or low numbers (Coulter and Kozub, 1989).Yet other
workers have failed to demonstrate a relationship between test scores and fertility
(Crichton et al., 1987; Farin et al., 1989). Finally, the repeatability of individual
libido scores is not high, although rankings of libido score are much more consistent
(Chenowith, 1997). In consequence, the American Society of Theriogenologists
advocated clinical examination, rather than libido testing, in their recent guidelines
for breeding soundness examination of bulls (Hopkins and Spitzer, 1997).
Many of the factors that have been mentioned previously, such as age, sexual
experience and social dominance, can affect libido and so lead to unreliability of tests.
Nevertheless, when Chenowith (1986) discussed the reasons for the variability of the
relationship between libido and fertility, he noted that bulls with high libido are the
most efficient at detecting oestrous cows, and in multiple mating systems, bulls of
high social ranking mate more cows than low-rank animals.
However, even with bulls with a high intrinsic semen quality, an excessively high
number of services is likely to deplete sperm numbers, resulting in a reduced chance
of conception occurring to an individual service. A bull of high libido but with more
mediocre semen quality would undergo such a decline in conception rates more
readily.
Moreover, since clinical findings, especially scrotal circumference and the presence of
abnormal sperm (Coulter and Kozub, 1989), are poorly correlated with measures of
libido (Chenowith et al., 1988), it is quite feasible that a bull of high libido might have
inadequate semen quality to achieve an acceptable number of pregnancies.
Hence, Chenowith (1986, 1997) concluded that neither clinical examination nor libido
testing can be used in isolation for the evaluation of breeding soundness but, if the
findings from both examinations are taken into consideration, subfertile bulls
are most unlikely to avoid detection. Conversely, where range and beef bulls are
examined, a combination of clinical examination and libido testing gives good results.
Morris (1998), for example, suggests that a low serving capacity test score or a low
scrotal circumference should preclude a bull from use, but that, if minimum criteria
for both are met, the number of cows with which a bull can be run will be dictated by
a combination of both scrotal circumference and service capacity test
score.