Effect of sperm number and site/technique of insemination on pregnancy in mares

This survey reviews data currently published in literature with respect to the effects of sperm number and different artificial insemination (AI) techniques (e.g.: routine insemination into the uterine body, rectally or ultrasonographically guided deep intracornual insemination ipsilateral to the preovulatory follicle, hysteroscopic insemination onto the uterotubal junction ipsilateral to the preovulatory follicle) on the pregnancy rate in mares. It is concluded that a single insemination close to ovulation with reduced numbers of cooled [similar to25 - 100 x 10(6) progressive motile spermatozoa (pms) instead of similar to300-1.000 x 10(6) pms] or frozen/thawed [similar to25 - 100 x 10(6) pms postthaw instead of either a total of 800 x 106 frozen spermatozoa with a post thaw motility of 35% or 300 x 10(6) pms] semen from fertile stallions may achieve pregnancy rates similar to commercial doses (Minimum Standard requirements of WBFSH) under optimum conditions. However it remains questionable, whether or not this similarity can be attributed to the use of low dose insemination techniques. Sperm doses 25 x 10(6) pms post-thaw, when inseminated deep intrauterinely with the help of a videoendoscope, hold no advantage over conventional uterine body Al. When inseminating with < 10 x 10(6) pms post-thaw the mares should be inseminated hysteroscopically. This amount of spermatozoa is therefore considered to be close to the critical sperm number. When only very low numbers of spermatozoo (e.g. sex-sorted spermatozoo) are available the critical sperm dose seems to be close to 5 x 10(6) spermatozoo and should be transfered hysteroscopically rather than by rectally or ultrasonogrophically controlled deep intrauterine insemination to give the best results. Washing of spermatozoa preceeding insemination to remove damaged spermatozoo and debris seems to have no beneficial effect on pregnancy rates when low doses of spermatozoa are transferred hysteroscopically. Neither does preinsemination intrauterine treatment with prostaglandin E2. This hormone influences oviductal smooth muscle function and subsequent oviductal sperm colonization. Giving prostaglandin E2 prior to low dose transrectally-guided or hysteroscopic-guided deep intrauterine insemination was not seen to increase pregnancy rates in mares. Twin pregnancy rates may be lower after contralateral double ovulations when mares are inseminated deep into the uterine horn on the side of the major preovulatory follicle. Rectally guided deep intrauterine insemination technique may be more feasible for practitioners in the field when inseminating low doses of spermatozoo, but there may be a higher risk of uterine trauma compared to hysteroscopic Al. On the other hand, hysteroscopic Al is an expensive technique and requires skilled operators. In mares with an overlarge uterus or cases of excessive endometrial edema (fool heat) it could become impossible to locate the uterotubal junction (UTJ). Nevertheless, health risks associated with deep intracornual insemination warrant its further research before extensive use. Application of low dose insemination techniques for less fertile stallions is highlighted in the literature but its usefulness is debatable and further research is required into stallion sperm parameters that may or may not be compensable by the insemination technique.