The Predictive Value of Semen Analysis in the Evaluation of Stallion Fertility B Colenbrander 1 , BM Gadella 2 and TAE Stout 1 1 Departments of Equine Sciences and 2 Biochemistry and Cell Biology, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands Contents Pregnancy rates in managed horse populations depend on the innate fertility of the mares and stallions involved and on the quality of breeding management. Of course, because a single stallion usually mates many mares, stallion fertility is a critical factor in the overall success of a breeding program. Unfortu- nately, accurate evaluation of stallion fertility per se requires a large number of normal mares to be mated and is necessarily retrospective. Rather, the ideal is to predict fertility in advance of the stallion’s breeding career, and this is currently attempted by way of a thorough physical examination and a routine analysis of semen quality. However, while such a ‘breeding soundness examination’ identifies stallions that clearly lack the capacity for adequate fertility, it is of limited use for predicting the level of fertility and fails to identify some seriously sub- fertile animals. Similarly, while various sperm function tests (e.g., sperm head morphometry, the hypoosmotic swelling test, glass wool-sephadex filtration, progesterone receptor expo- sure) have been shown to correlate fairly well with fertility in the field, most examine only a single or a narrow range of the attributes that a sperm must possess if it is to fertilize an oocyte in vivo, and are thus more useful for identifying specific causes of sub-fertility than for predicting the level of fertility. On the other hand, combining the results of the various sperm function tests does improve the reliability of fertility estimation and current research is therefore concentrated on identifying a range of tests that covers as many important sperm attributes as possible but that can be performed rapidly and cheaply. In this respect, flow-cytometry has proven to be an ideal tool because it allows the objective, rapid and simultaneous analysis of a number of properties in a large number of sperm. Moreover, stains are available for an increasing range of sperm characteristics including viability, capacitation and acrosome status, mitochondrial activity and chromatin integ- rity. Flow-cytometric analysis of sperm with appropriate probes thus offers considerable promise for the prediction of stallion fertility. Introduction Evaluating the fertility or ‘fertility potential’ of a stallion is an important part of sire selection and of breeding management. In addition, knowledge of past fertility and semen quality can be invaluable in the investigation of problems, or suspected problems, with fertility. Of course, the true indices of fertility are the pregnancy and foaling rates, however both are retrospective and are influenced dramatically by factors extrinsic to the stallion, such as mare quality and breeding management (Sullivan et al., 1975; van Buiten et al., 1998; Morris and Allen, 2002). Furthermore, in many circumstances, a prospective test is desired so that likely (sub)-fertility can be identified before a stallion embarks on his breeding career. Within this context of selecting stallions for ‘breeding soundness’, it has become accepted that a combination of a thorough physical examination and conventional semen evaluation provides a useful alter- native to actual fertility data (Kenney et al., 1983; British Equine Veterinary Association, 1991). However, while poor semen quality is a good indicator of sub- fertility, good semen quality (in terms of sperm number, motility and morphological normality) is no guarantee of acceptable fertility. For this reason, considerable effort is being invested in identifying markers for functional sperm capacity that can more accurately predict a stallion’s fertility. This is no easy task given that any one test is likely to measure only one, or a limited range, of the many attributes that a sperm must possess if it is to fertilize an oocyte. Nevertheless, many recently developed tests offer promise in identifying abnormalities inconsistent with ‘normal’ fertility, and a combination of the available tests should identify most subfertile stallions, and may eventually be fine-tuned to give a true prospective index of fertility (Graham, 2001). This review aims to summarize the most appropriate indices of stallion fertility and to examine the utility of standard or newly developed semen quality tests for identifying subfertility or for predicting the level of fertility in stallions. Measuring Stallion Fertility Measuring stallion fertility is not an exact science and all of the indices used have obvious shortcomings. For example, while the per season and per cycle foaling rates (i.e. the number of foals produced as a percentage of the mares mated) are often considered the ultimate meas- ures of stallion fertility, they are influenced strongly by non-stallion factors such as the age and reproductive status of mated mares and the intensity and quality of veterinary management (Sullivan et al., 1975; van Bui- ten et al., 1998; Morris and Allen, 2002). Using the first cycle foaling rate instead of the all cycles or seasonal rates, will remove some of the bias attributable to mare subfertility, but the foaling rate remains a retrospective measure for which data are not available until a year after the stallion has begun to mate. A quicker way of assessing fertility is to examine the pregnancy rate (first cycle or per cycle), which has the advantages of a shorter interval until figures are available and of giving a ‘rolling’ indication of fertility during the breeding season; this can be a useful early warning system for a developing fertility problem. However, if the figures are to give an accurate indication of fertility, data must be collected from a relatively large number of normal young mares. The day 14–18 pregnancy rate will also exclude later pregnancy losses where it is possible, if Reprod Dom Anim 38, 305–311 (2003) Ó 2003 Blackwell Verlag, Berlin ISSN 0936-6768 U.S. Copyright Clearance Centre Code Statement: 0936–6768/2003/3804–0305$15.000/0 www.blackwell.de/synergy