Artificial insemination (AI) can be a powerful tool to the livestock producer, providing the means to make rapid genetic progress. AI is widely used in the U.S. dairy cattle industry, having contributed to the near linear increase in milk production. It is used to a somewhat lesser extent in swine, beef and goats, but in sheep has seen only limited use, due to several problems which are gradually being overcome.
There are three methods of artificially inseminating sheep: 1) vaginal; 2) laparoscopic (LAI); and 3) trans-cervical (TAI). Vaginal AI is by far the simplest method, requiring the least amount of equipment and skill. Results from this method are also the least reliable, as semen is deposited at the opening of the cervix in the vagina, where the sperm cells have the smallest chance of fertilizing eggs. If enough sperm cells are deposited (~300 million), pregnancy may result, but generally results are not consistent enough to justify the expense of semen, labor and estrus synchronization programs.
When cows are artificially bred, an insemination pipette is inserted through the vagina and threaded through the rings of the cervix. The operator inserts his hand into the cow's rectum and uses his thumb and fingers to help guide the pipette through the cervical canal. Semen is deposited into the body of the uterus. Conception rates of 60% are typical. In small ruminants, especially sheep, this procedure is much more difficult, due to the smaller body size and in the sheep's case, a more complicated anatomy.
Sheep have a longer and more complex cervix than other ruminant livestock. It is approximately 12 centimeters in length and has 6 or 7 offset rings which make passage of AI equipment very difficult, if not dangerous to the ewe. There is also a flap of tissue at the opening of the cervix which makes entry through the first cervical ring especially difficult.
Laparoscopic AILaparoscopic AI (LAI) by-passes the cervix and deposits semen directly into the uterine horns. It is a minimally-invasive, minor surgical procedure. Ewes are held off food and water and given a mild sedative before the procedure. They are placed in a breeding cradle, which is elevated to an angle of approximately 30 degrees. The ewe's abdomen is sheared and scrubbed and a local anesthetic is injected under the skin. Two small incisions are made with a surgical blade. Trocars and trocar sleeves (surgical instruments) are inserted through the incisions and pushed through the body wall into the peritoneal cavity of the ewe. The trocars are replaced with a laparoscope (light source) and manipulating probe. The operator looks through the laparoscope to locate the ewe's reproductive tract. The body cavity is inflated with CO2 to allow the uterus to be observed. Once the tract is manipulated into place with the probe, the probe is replaced with an insemination pipette and semen is injected into the lumen of each uterine horn. After insemination, the equipment is removed and the ewe is released and allowed to walk to a recovery pen. If there is no bleeding, it is not necessary to close the incision sites. The ewe is given an antibiotic injection to prevent possible infection.
The LAI procedure takes less than 5 minutes per ewe when performed by a skilled operator and ewes generally recover with no ill effects. In fact, they can be inseminated in subsequent years using the same procedure. Pregnancy rates of 70-85% have been reported with LAI, however, due to the specialized equipment, skills and drugs needed, it is a procedure that must be performed by a veterinarian or other trained professional. It is also a costly procedure.
The Univeristy of Guelph has developed transcervial artificial insemination technique. Trans-cervical AI is done with a specialized fiber optic endoscope that can transverse the cervix of the sheep. It can be done with the female standing or in a breeding cradle. The first step in TAI is to insert a lubricated speculum into the ewe's vagina. Different size speculums are available for sheep and goats. A light source is affixed to the speculum so that the operator can visualize the opening of the cervix. Surgical forceps are used to tie off the tissue at the opening of the cervix. The scope is inserted into the opening of the cervix. The operator looks through the eye piece to navigate the scope through the rings of the cervix. Once the final cervical ring is penetrated, semen is deposited in the body of the uterus. Higher doses of sperm (at least 100 million sperm cells) are required for TAI as compared to LAI, because the semen has farther to travel to the point of fertilization. Fertilization occurs in the oviducts (fallopian tubes).
It is generally recommended that maiden ewes not be inseminated trans-cervically due to the smaller size of their reproductive organs. They are also less predictable in their responses to hormonal manipulation of the reproductive cycle.
Semen is generally collected using an artificial vagina (AV), after the ram has mounted a ewe in heat or been electro-stimulated to produce an ejaculate. A rectal probe is be used to induce an ejaculation in the ram. Some rams will produce an ejaculate after internal massaging, while others will require pulses of electro-stimulation.
The semen is observed under a microscope and evaluated for ejaculate volume, sperm concentration, motility, morphology and abnormalities. Fresh semen is generally extended (diluted) so that a larger number of females can be serviced by a single ejaculate. The typical ram ejaculate is 1 to 2 ml and contains 1 to 5 billion sperm cells. A typical AI dose is .25 to .5 ml of semen, with 25 to 300 million sperm cells, depending upon the location of semen deposition. Extenders are usually egg-yolk based and contain nutrients, buffers to protect from cooling damage and antibiotics to prevent spread of disease.
The life of fresh-extended semen is about 6 to 8 hours. Frozen semen can be stored indefinitely in liquid nitrogen (-240 degrees F). Extreme care must be practiced when handling semen. It is very sensitive to changes in temperature. Water is lethal to sperm cells. Ram semen should be thawed according to the protocol provided by those freezing the semen.
Female domestic livestock are far from promiscuous. They display only short and isolated periods of sexual activity (receptivity to the male) which occur near the time of ovulation when conception is possible. The ewe is receptive to the ram for only 24-36 hours.
One of the keys to a successful AI program is knowing when to inseminate the female. In nearly all mammalian species, it is critical to inseminate just prior to ovulation (egg release), so that sperm can capacitate in the female tract while it waits for the arrival of the egg(s). The ewe ovulates at approximately 48 hours after the onset of heat. Unfortunately, it is very difficult to detect estrus (heat) in the ewe. The visual signs of heat, such as mucous discharges and behavior changes, which are common with other livestock, are largely absent in ewes. A teaser (vasectomized) ram, fitted with a marking harness, can be used to detect estrus, but it is often more practical to use hormonal therapy to manipulate the ewe's reproductive cycle so that all ewes will come into heat at a predictable and convenient time and a timed insemination can be done. A teaser ram can be used in combination with estrus synchronization programs. A teaser can also be used to induce heat in non-cycling ewes.
Synchromate-B® is an ear implant used in the cattle industry to suppress estrus in feedlot heifers. MGA is a feed additive used for the same purpose. Both products contain a source of progesterone, the hormone which maintains early pregnancy and suppresses ovulation. Vaginal pessaries (sponges) are another method of supplying progesterone to ewes. They are inserted into the ewe's vagina using a special applicator. CIDRs, developed in New Zealand, are a similar devices for estrus synchronization.
In estrus synchronization programs, ewes are provided a source of progesterone for 10 to 14 days. They will come into heat 36 to 48 hours upon removal of the progesterone source and are inseminated 48 to 56 hours after removal of the implant. Removal of progesterone is usually accompanied by a source of FSH (follicle stimulating hormone) which causes ovarian follicles to develop and produce estrogen, the hormone needed for ewes to come into heat. An injection of PG-600 or PMSG given following the removal of progesterone, has been shown to improve pregnancy rates, especially when breeding outside the normal breeding season. PG-600 contains 400 IU of of serum gonadotropin (PMSG) and 200 IU of chorionic gonadotropin (HCG)) and is an injectable product that is used to induce puberty in pre-pubertal gilts. In research trials, PG-600 has given better results than PMSG (Pregnant mare serum gonadotropin) alone. The negative aspect of PG-600 is its high cost - approximately $4.50 per 5 ml dose.
Prostaglandins can be used to synchronize ewes during their normal breeding season. Prostaglandin causes regression of the Cl and starts the ewe's cycle all over again. Treatment consists of two injections of PGF2.
It is important to note that all of the estrous synchronization methods have costs associated with them. In addition to the financial outlay, there may be reductions in fertility and increased reproductive losses. Not all females respond to hormonal manipulation. In goats, repeated use of PMSG in conjunction with progestogen treatment has resulted in reduced fertility in subsequent years.
There are numerous other advantages to AI in sheep. The number of diseases that can be transmitted via the semen is less than the number of diseases that can be transmitted from the live animal. For example, there is no indication that scrapie is transmitted through the semen. With AI, fewer rams need to be maintained. While it is generally cheaper to maintain a ram than purchase the semen necessary to breed many ewes, AI can offer genetic diversity to single sire flocks. It can eliminate the need to maintain both a terminal sire and sire for replacement females. Finally, it is possible to breed ewes and rams of different physical sizes and at different geographic locations.
AI is still in its infancy in the U.S. sheep industry. Semen prices are relatively high, and the industry still doesn't do the best job of identifying genetical superior stock. But there are a lot of positive developments that should help to make AI a more viable technology as we move into the 21st century. To learn more about sheep AI, be sure to check out these web sites:Useful Links