The GST-AI was developed in the late 1980's by researchers at the University of Guelph and the Ontario Ministry of Agriculture, Food and Rural Affairs. It was the first system that demonstrated, under practical conditions, that the cervix of most ewes could be penetrated successfully, and that pregnancies could be established after depositing reasonable doses of frozen semen. Since that time, much has been learned about the benefits and limitations of the system. It has been used in most sheep producing countries on many breeds and in a variety of conditions.
The cervix of the ewe is longer and more complex than other domesticated ruminants (cattle, goats, farmed deer). It is approximately 8 to 10 cm in length and contains 6 or 7 offset rings that make passage of AI equipment extremely difficult and even dangerous. An extensive study of the cervix was part of a MSc project of Dr. Gary Halbert, University of Guelph. The study demonstrated the complexity and the anatomy of the average canal from ewes from a variety of breeds and reproductive stages. The top photo shows the cervix cut open to demonstrate the rings. The smaller photo (left) of a latex mold made inside the cervical canal and shows the mixture of tight narrows and the open cul-de-sacs that can be found around each opening. Using the information from the study, instrumentation was designed to assist with cervical passage. The most satisfactory instrument was narrow, reasonably rigid and had an offset end, to assist with passage into the offset rings. That became the design for the insemination tip seen below and attached to the modified Cassou AI gun, designed for handling semen in straws.
The most challenging part of the technique proved to be the entry into the first cervical ring. If that could be accomplished, then passage could be made in most cases. By grasping the tissues around the opening with surgical forceps, it was found that penetration could be greatly assisted. We soon learned that an experienced inseminator could learn where to grasp the tissues to increase the chances of success. Typical os presentations, as seen in the photo at right, suggest where to grasp to the experienced inseminator.
Of great importance was the positioning of the ewe. Our studies found that positioning on their back with the head tilted down and the hind legs drawn forward greatly improved the success of cervical grasping and penetration. A Poldenvale Foot Trimming cradle proved to be ideal. The Sydell system also works well.
Results from Breeding trials
Results from trials using the GST-AI are reasonably consistent. Overall, an inseminator with some experience, can successfully penetrate the cervix of about 75 % of ewes presented. That penetration rate is affected by may factors:
Some ewes are easy, some are difficult. Inseminators that are overly aggressive can puncture the cervix of the difficult ewe. By trying to penetrate all ewes, the risk is great that some punctures will occur. Generally, nothing bad happens, but the ewe doesn't get pregnant. A study at the Scottish Agricultural College, in Edinburgh, showed infection occurred in some ewes and that they failed to rebreed, likely as a result of infected equipment or previously infected ewes. Another group, laparoscopically inseminated, also had failed pregnancies. No other studies have shown any negative affects on future fertility. It is not recommended to persist in attempts to penetrate the cervix of the difficult ewe beyond a few minutes.
Pregnancy and lambing results tend to be similar to those experienced in the cattle and goat industries when semen is deposited into the body of the uterus. About 50% of those successfully penetrated and inseminated will lamb in well managed programs. In other words, about half of the 75% attempted or between 30 and 40 % of the entire group. There are many factors that have been shown to affect the lambing rate: