An Update on Sheep A.I. (2004)

By Susan Schoenian

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.

Laparoscopic AI

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 AI

Laparoscopic 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.

Trans-cervical AI

The Gourley Scope

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

Either fresh or frozen semen can be used to inseminate ewes. In the past, the results obtained with frozen semen were unsatisfactory, especially when the semen was deposited outside the uterus, but through the use of improved diluents, pregnancy rates with frozen semen can be near equal to those achieved with fresh semen. While the freezing and thawing process kills sperm cells (up to 20-50 percent), freezing is necessary to prolong the life of semen and the period of time in which ewes can be bred. Without the use of frozen semen, it would be difficult to breed sheep at different geographic locations and impossible to utilize semen from other countries.

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.

Estrus (heat)

Sheep are seasonally, polyestrus. Their normal breeding season extends from about August to March, depending upon breed and geographic location. Ewes begin cycling in the fall when the day length becomes shorter. Their estrus cycle averages 17 days, with the luteal phase lasting 14-15 days and the follicular phase, 2-3 days. Day 0 of the cycle begins with ovulation, the release of a mature follicle, containing the egg. During the luteal phase, the follicle ruptures and transforms into a corpus luteum (CL or "yellow body"), which produces progesterone, the hormone that prepares the uterus for implantation and maintains early pregnancy in the ewe. If pregnancy does not occur, the CL regresses, due to a fatty acid (prostaglandin) produced by the non-pregnant uterus; and the cycle starts all over again.

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.

Estrus Synchronization

There are various methods of estrus synchronization which have been used successfully in sheep. Many require products which may not be available in the U.S. or are not FDA-approved. Several products which are available over the counter for cattle and hogs have been used extra-label to synchronize ewes. The extra-label use of any drug requires approval of a veterinarian in the context of a valid veterinary-patient-client relationship.

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.

Genetic progress

The primary advantage to AI is to make more rapid genetic progress. While the dairy industry has DHIA and the swine and beef industries have their own performance record-keeping systems, until recently the sheep industry had no means of identifying superior genetic stock. The National Sheep Improvement Program (NSIP) is a computerized performance record keeping program that gives the producer, through the use of genetic values or "EPD's" a objective system for identifying the genetically superior animals in a flock. Across-flock comparisons are also now possible in some breeds. While participation in NSIP is still limited, it does finally offer the sheep industry a system for improving economically important traits.

Practical uses of AI

From a producer's standpoint, AI must be economically justified if it is to be used. Up to this point, the primary use of AI in sheep has been in animals of high dollar value. In fact, the biggest user of AI technology is the seedstock industry. Rams with impressive show records are often collected and their semen is sold to other purebred breeders. The other common use of AI in sheep is to gain access to the gene pool of new or rare breeds. In recent years, several private companies and individuals have been able to import semen from Europe, giving U.S. producers access to breeds not previously available in the United States. Some of the superior sheep genetics in Europe, Australia, and New Zealand are available to U.S. producers via frozen semen.

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.

Conclusion

There are several U.S. companies and veterinarians which provide AI services, primarily LAI, to producers. LAI is performed at various locations throughout the U.S. Producers can have their ewes artificially inseminated by making arrangements with one of these companies. It is more economical to transport sheep to a central location for AI. It may also be possible to cooperate with a university to have ewes bred artificially. In the case of trans-cervical AI, a producer can receive specialized training and eventually be able to breed his own ewes.

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
Super Sire Ltd.
Heritage Genetics
Small Ruminant Genetics
A Comparison of Transcervical and Laparascopic Intrauterine Artificial Insemination Techniques on Reproductive Performance of Ewes

Did you know . . .

Artificial insemination is one of the oldest forms of applied reproductive technology. An 18th century Italian - Spalanzanni - is given credit for inventing artificial insemination. He is reported to be the first to successfully AI a domestic animal (a dog). Old Arab documents indicate the use of AI techniques, perhaps as early as 1322 A.D. The Russians pioneered AI research in birds, horses, cattle and sheep and began to make widespread use of AI in cattle in 1931. AI was first performed on cattle in the United States in 1937, and 1991 data indicate that 70 percent of the dairy cow population is bred artificially. Chilled semen is generally used in swine, and it is estimated that half of the U.S. sow herd is bred artificially.

©Copyright 2004.