The minimum insemination dose of 500 million progressively motile spermatozoa for fresh semen and double that for cooled semen, established by Colorado workers in the mid 70s has been widely accepted as the industry standard. However clinical impressions would suggest that mares can be impregnated with fewer sperm but that there is a profound stallion effect. It is not uncommon to have high levels of fertility when mares are inseminated with less than the 500 million sperm.

The increasing use of frozen semen and the popularity of some stallions has provided has triggered an incentive to reduce the number of spermatozoa used per insemination to maximize the efficiency of this procedure. In 1998 in separate studies Vasquez and Manning provided information that suggested that deposition of relatively low numbers of spermatozoa at the uterotubal papilla with the use of an endoscope resulted in pregnancy of mares Although pregnancy rates were low (22 to 30%) after insemination of 1 to 3.8 million PMS, these studies demonstrated the feasibility of dramatically lowering the number of sperm inseminated by direct deposition of sperm at the uterotubal junction (UTJ) via hysteroscopically guided techniques.

Rigby and coworkers (2000) showed that when semen is deposited in the body of problem or normal mares that the distribution of semen in the oviducts was approximately 50% in to each oviduct. However when semen was deposited deep in the uterine horn ipsilateral to the ovary containing the pre-ovulatory follicle, the distribution of sperm was skewed significantly and 80% of the sperm were found in the oviduct closer to where the sperm were deposited regardless of whether the mare was normal or a problem breeder.

Low-dose insemination

In 2002 Morris et al reported on the fertility of mares inseminated with different numbers of sperm. In that experiment this group showed that inseminations with 1,5 or 10 million sperm resulted in pregnancy rates ranging from 60-75%. However when mares were inseminated with 500,000, 100,000 or 1,000 sperm pregnancy rates dropped significantly.

The great majority of experiments have been directed at depositing the sperm at the UTJ either by rectally guided or by hysteroscopic techniques. However, Morris et al in one study indicated that low dose insemination (14x106 motile frozen-thawed sperm) placed into the uterine body or at the UTJ resulted in similar and acceptable fertility. However pregnancy rates significantly decreased when 3 million sperm were inseminated in the uterine body but not at the UTJ.

Endoscopic or rectally guided: Which one is better?

The initial reports dealing with low-dose insemination techniques in the mare were based upon endoscopically guided procedures to deposit the sperm on the UTJ, other workers have used a transrectally guided technique to direct a flexible insemination catheter to the tip of the uterine horn ipsilateral to the preovulatory follicle. In a report by Brinsko et al., pregnancy rates after insemination with 5 x 106 motile sperm at the tip of the uterine horn by either pipette or endoscopic procedures were similar. Samper et al reported a small difference (4%) in pregnancy rates in favor of the endoscopic technique when mares were bred at in a commercial setting. These studies suggest that low insemination doses of approximately 1/4 of a standard dose of 250 x 106 motile sperm can achieve acceptable pregnancy rates when deposited by deep-intrauterine techniques without the need for endoscopic equipment.

The typical volume of the inseminate when using endoscopic insemination ranges between 20 uls to 1,000 uls with volume having little to no effect on pregnancy rates. Volumes of 500 to 1,500 uls have been used successfully when using the deep horn insemination technique. However when deep horn insemination is used volumes larger than 5 ms seem to have a detrimental effect on fertility.

As a result of the success of these techniques, the obvious question that has been raised is if we can improve the fertility of sub-fertile stallions by changing the site of insemination. The answer is starting to be elucidated and it appears that it could be stallion dependent. For stallions that have non-compensatory defects such as head or chromatin defects it does not appear to be helpful regardless of the number of sperm that are deposited. On the other hand a small but consistent increase in pregnancy rates is observed with other stallions.