ReviewEvaluation of fertility and infertility in natural service bulls
Introduction
Since artificial insemination (AI) was introduced to the dairy industry in the 1950s, it has progressively displaced natural service as the preferred method of breeding in most developed countries of the world. Hence, in dairy herds, natural service sires have largely been relegated to rather more `secondary' duties, such as serving cows that have failed to conceive to AI, serving cows of low genetic merit (i.e. as crossing sires), or serving maiden heifers. With the consequent decrease in both numbers and importance of natural service sires, bulls have become the `poor relations' of dairy herd reproductive management compared to the attention that is focused upon female fertility.
Conversely, AI has been less widely adopted by the beef industry; for, whilst intensively managed beef animals are relatively amenable to breeding by artificial insemination, logistical difficulties have limited its uptake in extensively managed beef herds. Natural service has therefore remained the predominant method of breeding in such herds. Management and the fertility of bulls is thus of paramount importance to the owners of beef herds, and there is a reasonable amount of research literature underpinning recommendations that are made to farmers on these subjects. However, as most of the available information on bull management has been generated from beef herds, recommendations about the management of bulls in dairy herds are also largely derived from this source, despite the many differences between beef and dairy herds.
Nonetheless, the role of the bull in the fertility of both beef and dairy cows also has to be understood in terms of the overall constraints to fertility in cattle. Domestic cattle are not a highly fertile species, as maximal calving rates to each service are no better than 50–60%; a figure that is hard to increase since it is capped by the ability of embryos to survive gestation (see Rowlands and Weir, 1984). The consequence of this mediocre per-service calving rate of domestic cattle is that they have to be managed in ways which prevent further loss of potential fertility and which maximise the number of opportunities that each animal has to conceive. In other words, the management of fertility in domestic cattle is, in reality, management to minimise the risk of infertility (Lean, 2000; Parkinson and Vermunt, 2001). In this context, the management of bulls can be understood as a way of minimising the risks associated with male infertility.
Bull fertility, bull management and cow management are, of course, highly interrelated. For example, bull:cow ratios can be underestimated if conception rates to AI were worse than expected, resulting in over-use of bulls. This situation is not uncommon amongst dairy cattle in Australia and New Zealand, where the AI period is short and the breeding season highly compact (Macmillan, 1998). Infertility, due to overuse of bulls, supervenes, despite the prognostication that the bull(s) should have been able to perform adequately with the expected number of cows (Parkinson and Vermunt, 2000). Likewise, the apparent fertility of bulls can be affected by the length of time that they are running with cows: a bull which might achieve an acceptable final pregnancy rate with a small group of cows, or over a long period of time, might perform disastrously with a large group of cows or in an intensive mating period. Likewise, a sub-fertile bull might achieve an acceptable final pregnancy rate at the end of the mating period, even though this may be at the cost of an unacceptably spread-out calving pattern. In consequence, despite the expenditure of a great deal of effort upon trying to identify characteristics of bulls that are indicative of their fertility, difficulties remain for both veterinarian and farmer about the interpretation of clinical findings in the context of each individual herd's particular circumstances.
In essence, the breeding potential of a bull can be considered to depend upon both its ability to mate and its ability to fertilise. Field assessments can be made of a bull's ability to mate, its libido and of its physical capability to mount, achieve intromission and ejaculate. Assessments can also be made of the quality of the semen that the bull produces, which is, in turn, related to physical characteristics of its genitalia. Yet whilst it is relatively easy to assess such traits in the field, their value as predictors of bulls' fertility unfortunately remains the subject of considerable debate.
Section snippets
Libido
Subjective observation of the mating behaviour of bulls at pasture is not, of itself, a very good indicator of bulls' mating ability. Much better indicators are the objective assessments provided by libido and service capacity tests (Blockey, 1976a, Blockey, 1976b, Blockey, 1978; Chenoweth et al., 1979). There are many permutations of `libido tests' and `service capacity tests' (Barth, 1997) and the two terms are often used loosely in the literature. However, a good working definition of the
Physical (breeding soundness) examination
Physical examination of the genitalia has long been used as a means of assessing the potential fertility of bulls (e.g. Carroll et al., 1963). Detailed instructions for the conduct of physical and semen examinations of bulls have been developed by the American Society of Theriogenology (Hopkins and Spitzer, 1997) and are widely used as the basis for such examinations (Higdon et al., 2000). Physical examination involves palpation of the penis (including sigmoid flexure), prepuce, the accessory
Semen quality
Semen quality standards for natural service sires (e.g. Hopkins and Spitzer, 1997) reflect generally accepted ideas about minima that have to be attained if a bull is to have acceptable fertility. Minima of 30% motility, 70% normal sperm and age-dependent minima for scrotal circumference probably represent a consensus of opinion over the standards that are acceptable to veterinary andrologists. Using such criteria, bulls that fail breeding soundness generally have more sperm abnormalities than
Hormones as indicators of fertility
All aspects of the male reproductive system are under the control of the endocrine system. Gonadotrophins regulate the synthesis of reproductive steroids, which themselves control libido, spermatogenesis and the actions of the epididymis and accessory sex glands. Gonadotrophins also directly regulate spermatogenesis. Potentially, therefore, measuring circulating hormone concentrations might provide a straightforward way of assessing either libido or sperm output.
Circulating concentrations of
Relationship between libido and physical examination scores
The main criticism that has been levelled at L/SC testing is that it only considers one of many factors that contribute to the fertility of a bull, and ignores other factors related to fertilising capacity that are likely to be independent of (or only tenuously related to) service ability. Thus, even when service testing takes into account the entire process of mounting, intromission and ejaculation (i.e. thereby excluding the presence of abnormalities of the locomotor system, penis, prepuce,
Conclusion
In the assessment of the potential fertility of bulls, established tests, such as physical examination, semen examination, and assessment of libido are used to eliminate low fertility bulls and to estimate the bull:cow ratios that should be used. In terms of managing the risk of infertility, however, assessment of the individual bull is but one of the strategies that are used to minimise the chances of poor conception rates. It is not the purpose of this review to examine the management
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