Fertility parameters, subfertility

Measuring stallion fertility is not an exact science and all of the indices used have obvious shortcomings. Many factors such as management of the stallion (nutrition, housing, semen collection, semen processing and storage) and his mares (optimal time of insemination, reproductive status and conditions of the mares) may have a large impact on the success of insemination and overall fertility of the stallion. The fertility of stallions is most accurately defined based on pregnancy of mated/inseminated

mares. For objective assessment of fertility, however sufficient number of mares is required. Reproductive performance of the stallion cannot be estimated if only a few mares are mated/inseminated and the mares haven’t been pregnant for many years (Juhász and Nagy 2003). The fertility of stallions can be characterized by three parameters: foaling rate, pregnancy rate per season and per cycle pregnancy rate.

The per season and per cycle foaling rates (the number of foals produced as a percentage of the mares mated) are often considered the ultimate measures of stallion fertility, they are influenced strongly by non-stallion factors such as the age and reproductive status of mated/inseminated mares and the intensity and quality of veterinary management (Sullivan et al. 1975, van Buiten et al. 1998, Morris and Allen 2002, Colenbrander et al. 2003). For the precise determination of foaling percentage data of the given stallion from several breeding seasons are taken into account (in the case of 25 mares per year inseminated approx. ten years). To respect the complicating elements above this parameter is less suitable for rapid examination and monitoring changes of stallion fertility (Juhász and Nagy 2003). Using the first cycle foaling rate instead of the all cycles or seasonal rates, will remove some of the factors attributable to mare subfertility, but the foaling rate remains a retrospective measure for which data are only available in the year after the stallion has begun to mate (Colenbrander et al. 2003). According to data from France the foaling rate is around 63% in stallions with average fertility (Juhász and Nagy 2003).

Per season pregnancy rate (end of season pregnancy rate; seasonal pregnancy rate) is calculated of the pregnant mares at the end of the season divided the total number of inseminated mares. The value is strongly influenced by the reproductive status, veterinary and housing management of mated/inseminated mares (Juhász and Nagy 2003). Seasonal pregnancy rates should be above 85% in well - managed herds.

Stallion which has this parameter below 70 % is considered subfertile, however the notable exception is the Thoroughbred breed where the seasonal pregnancy rate is around 50-55% (Card 2010).

A quicker and most practical way of assessing fertility is to examine the per cycle pregnancy rate (number of cycles resulted in pregnancy/total number of cycles in which insemination obtained). Detected pregnancy by ultrasound or later by rectal palpation, healthy foal born, early embryonic loss and abortion are also counted as a positive pregnancy result in this system. If a mare is inseminated in several cycles, data of each cycle are recorded separately (Juhász and Nagy 2003). This parameter has the advantages that the result is available in a shorter interval and gives 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 stallion 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 unusual, for a stallion to be a major source of pregnancy loss because of sperm chromosome or chromatin abnormalities (Kenney et al. 1991, Colenbrander et al. 2003). This parameter is less influenced by breeding management of the mares and suitable for monitoring changes of stallion fertility (Juhász and Nagy 2003). Ideally to avoid a larger impact of subfertile mares on a stallion’s apparent fertility it is preferable to include only 1st cycle pregnancy rates. Nevertheless because the average stallion breeds relatively few mares (10 or less) a per cycle pregnancy rate is a reasonable compromise (Card 2010). The non-return rate is another parameter often used as an early index of fertility in farm animal species in which pregnancy examination can only be performed reliably after the time of the next expected oestrus, and in horses when mares are not routinely returned for early pregnancy diagnosis (eg. Shetland ponies, van Buiten et al. 1999). The non-return values suffer from the obvious disadvantage that mares may not be returned for mating for reasons other than becoming pregnant. Otherwise the non-return rate gives similar information to the pregnancy rate. Per cycle pregnancy rate (fertilization rate) for stallions is lower (43–60%; Woods et al. 1987; Brück et al. 1993; Morris and Allen 2002) than for domestic livestock species e.g. rams (80–90%; Menzies 1999) and boars (85–90%;

Colenbrander et al. 1993) because at the latters, reproductive performance is an integral part of productivity, selection of males for good fertility is much more rigorous. This parameter is considerably more variable between individual stallions (35–90%; Morris and Allen 2002). And while experimental studies demonstrate that fertilization rates can reach high levels in mares mated at the appropriate time with semen from a stallion of proven fertility (>90%; Ball et al. 1989), the discrepancy between experimental fertilization rates and pregnancy rates in the field appears to be explained primarily by deficiencies in stallion fertility and management, and only partially by pregnancy loss (Jasko 1992, Colenbrander et al. 2003). In the case of insemination with fresh stallion semen the per cycle pregnancy rate is in average 45-55% (Card 2010) or 50% ((Juhász and Nagy 2003) in a non-selected population of mares. Using chilled or frozen semen the value of 40-45% is satisfactory (Juhász and Nagy 2003).

Comparing frozen stallion sperm parameters to seasonal pregnancy rate is less reliable than per cycle fertility, especially when field results are being utilized, since seasonal fertility is affected by more variables. For instance, multiple inseminations using increased numbers of spermatozoa may have been necessary to achieve acceptable seasonal fertility for some ‘‘lower fertility’’ stallions (Kirk et al. 2005).

In general, an end of season pregnancy rate of 80% and a foaling rate of 70–80% are considered satisfactory, and can be achieved within four cycles per mare by a stallion with a per cycle pregnancy rate as low as 35%, assuming normal rates of pregnancy loss (Colenbrander et al. 2003).

Reduced fertility (subfertility) of the stallion is stated if per cycle pregnancy rate was significantly lower than 50% using fresh semen for inseminating mares. Significance of the differences is influenced by the number of cycles used for insemination. The stallion which inseminated mares in 25 cycles is only considered safely as subfertile if the per cycle pregnancy rate was lower than 30%. Result of < 40 % per cycle pregnancy rate based on 100 cycles is significantly different from the average 50%.

Ratio of subfertile stallions is approximately 4.5-7.5% among breeding stallions depending on horse type; in coldblood horses this rate is higher than warmbloods (Juhász and Nagy 2003). Most stallions loose commercial viability if they have <30%

per cycle pregnancy rate. Stallions are generally considered not suitable for commercial use if their per cycle pregnancy rate (PR) falls below 30%. The reason for this is that mares are re-bred many more times and most mare owners are reluctantly to breed their mare more than twice or 3 times, because she will foal later in the subsequent year. If a stallion was breeding at 30% per cycle PR after 3 breedings he would leave 35 out of a 100 mares non-pregnant. In consideration of the long gestation length of the mare, and the fact that there is pressure to get the mares pregnant early so they do not foal later and later every year, the time lapsed due to lower per cycle pregnancy rate becomes a significant problem. Stallion which has 0-5

% PR and < 10% seasonal pregnancy rate, is considered infertile (Card 2010).

For comparing fertility of individual stallions, or groups of stallions it is need to take into account the followings: When 60% versus 70% per cycle pregnancy rates are compared, the difference is not significant even when based on 100 mares for each stallion. Important rule is that if the difference in 1st cycle pregnancy rate is ≤10 percentage units, it will not be statistically significant unless the two values each are based on >190 mares. Comparing 45% versus 60%, only if each 1st cycle average was based on >95 mares is the difference significant. Moreover, calculations based on cumulative pregnancy rates over four cycles tend to mask a possible difference between two stallions if both stallions have a cumulative pregnancy rate >60%

(Amann 2005).