Variation between stallions has been shown to influence equine reproductive rates in the northern hemisphere. Although poor reproduction (approximately 50% live foals) is apparent in the Australian Breeding Industry, there has not been any attempt to determine the causes of variation in stallion semen characteristics or their relationships with fertility under Australian environmental conditions. Therefore, it was decided to examine these aspects of stallion performance on commercial studs where sufficiently large numbers of stallions were available for study.
Ejaculates were collected from 222 stallions, which comprised 13 breed or colour types, located at 107 commercial studs in southern Queensland and northern New South Wales. A total of 648 attempts were made to collect semen, using an artificial vagina, over 4 consecutive breeding seasons (1974/75 to 1977/78). Of all collection attempts, 621 (96%) were successful, with at least one processable semen sample being collected from each of 216 stallions (97%). No significant relationships were observed between collection failures and breeding season, time of year or age and breed of stallion. Time of year (seasons and months) was the only factor having a significant relationship with collection failures; highest failure rates occurring in autumn and winter.
The number of mounts per ejaculation was significantly affected by breed and age of stallion, time since last service and volume of water used in the AV. Season of year, weather conditions at the time of collection and time since last service had significant effects on the number of urethral pulsations per ejaculation into the AV. The results indicate that most stallions should only be used for two services per day if their ejaculates are to contain sufficient numbers of spermatozoa to ensure normal fertility.
Overall means of semen characteristics of the 536 ejaculates from 168 stallions, for which complete data were available are generally similar to the values reported in the literature. Significant variation in semen characteristics was associated with differences in breed and age of stallion, season of year, weather conditions at the time of collection, time since last service and volume of water used in the AV.
Overall means of spermatozoal characteristics of the 531 semen samples from 168 stallions for which complete data were available are basically similar to the results of other workers. Significant sources of variation in the spermatozoal characteristics were associated with differences in breed and age of stallion, season of year, time since last service and volume of water used in the AV.
Significant between stallion within breed variation existed in most characteristics. Based on this variation, repeatability estimates of the semen and spermatozoal characteristics were low to moderate and indicated that at least 2 ejaculates should be examined to obtain a reliable assessment of stallion semen.
Percentage pregnancies per service appears to provide a better measure of fertility of stallions at stud than percentage pregnant mares, as high pregnancy rates can be achieved by sub-fertile stallions if sufficient repeat services occur. For this measurement of fertility significant differences were related to differences in breed and age of stallion and breeding season. Six stallions, examined because their fertility was "suspect" had significantly lower fertility than stallions considered by their owners to be "normal".
Semen characteristics that have meaningful associations with fertility and threshold levels that can be used as criteria for breeding soundness examinations of stallions are total volume of the ejaculate (35 ml); gel-free volume (25 ml); spermatozoal concentration (20 million per ml); total number of spermatozoa per ejaculate (1300 million) and total number of live spermatozoa per ejaculate (1100 million). Evaluation of these characteristics for breeding soundness examinations and artificial insemination can be readily performed at the site of collection.