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February 2014
How can AI (artificial insemination) further parasite resistance in sheep?



Joan Burke, USDA ARS
Stephan Wildeus, Virginia State University
Roxanne Newtwon, Hound River Farm
Kathy Bielek, Misty Oaks Farm

The only growing sector of the sheep industry has been smaller forage-based farm flocks east of the Mississippi, particularly in the south (NASS, 2011). This increase is primarily due to the introduction of Katahdin hair sheep since they are hardy, well adapted to forage-based systems, and have greater resistance to parasites compared with traditional wool breeds.

Semen collection

To ensure future growth and sustainability, small farmers must have access to sires with superior genetics for economically important traits such as milk production and growth. Equally important is access to sires with known resistance to health related problems such as hoof rot and parasitism, both heritable traits. The use of Estimated Breeding Values (EBVs) (see allows for the selection of these superior animals, but a method is needed to share these genetics across more animals and/or flocks.

It is often difficult, especially for small flocks, to justify the purchase price of a superior ram, yet the success of these flocks, and the sheep industry as a whole, requires this advancement. The widespread use of artificial insemination (AI) in the cattle, swine, and poultry industries has resulted in accelerated genetic progress for economically important traits, while minimizing exposure to animal diseases. The use of AI would also improve biosecurity, alleviate transportation costs associated with moving live animals, and reduce the risk of heat-stress related sterility of rams that often occurs during transport in late summer, particularly in the south. However, there is limited use in the small ruminant industry due to cumbersome AI procedures.


Historically, AI in the U.S. has been limited in sheep due to the complexity of the ewe’s cervix and because of difficulties in storing and thawing frozen semen. Laparoscopic or surgicalAI has been used successfully to bypass the cervix, but is aveterinary procedure that is cost-prohibitive and not accessibleto most small farmers. Vaginal AI is a low technology procedure that can be accomplished using fresh semen with reasonable pregnancy rates. There is potential to train farmers to use this technique. Moving sheep production into the future on small and mid-sized farms will depend on the development of a farmer friendly AI program using superior genetics and is also cost-effective for farmers.


A vaginal procedure was previously developed using prolific sheep breeds in Norway (Paulenz et al., 2002, 2005) and at Virginia State University (VSU, Wildeus, 2012).  In the Norway research, conception rates were 63% using liquid semen with 150 × 106 spermatozoa on 52 farms (2002), and lambing rates ranged from 56 to 83% among 10 farms and 58 to 78% among 6 rams using 200 × 106 spermatozoa (2005).

Results at VSU indicated that pregnancy rate of yearling ewes was 75%, and 59% using fresh or 39% using 12-hour stored semen in mature ewes (Wildeus, 2012). Mook et al. (2008) at VSU indicated that fertility after fresh and 72-hour stored liquid semen extended with Tris-egg yolk using the vaginal AI procedure was similar to intrauterine AI. The Norway group and Wildeus (2012) used a skim milk/egg yolk extender; this was compared with the Tris-egg yolk extender, which reduced fertility (Paulenz et al., 2003). Optimizing the semen extender may increase sperm viability and conception rates. Preliminary results of a SARE on-farm study in Ohio using vaginal AI in Katahdin ewes indicated conception rate of 50% (Bielek, personal communication).


Prolific breeds such as the Katahdin or St. Croix should have higher success with vaginal AI than less prolific breeds. There were 8,131 registered Katahdins, the second highest breed rank in 2012, and 42 Katahdin flocks enrolled in NSIP ( NSIP creates an annual elite sire report including updated EBVs of more than 120 mature rams and more than 1300 yearling rams (September 2013 report).

ARS has a working relationship with many of these flocks and will be able to obtain semen from elite rams from this report. The Dorper, Dorset, and Southdown breeds are prolific breeds included in the top six registered breeds, and the Polypay and Dorset have several breeders included in NSIP which could be considered for future AI studies. There is considerable opportunity to conduct research on AI in these breeds.

In summary, technology that includes AI by farmers who use select sires will be able to accelerate progress on heritable traits and minimize risk of disease transmission. The acquisition of sheep that are uniquely suited to the southeastern U.S. will provide increased marketing opportunities for farmers, even cattle producers adding diversity to their farm.

Vaginal AI

Literature cited
Mook, J.L., Collins, J.R. and Wildeus, S. 2008. Retention of sperm motility, viability and fertility in ram semen after liquid storage at 4°C for up to 96 hours. J. Anim. Sci. 86 (Suppl. 2):496-497.

NASS (National Agricultural Statistics Service). 2011. Sheep and Goat Report. , Agricultural Statistics Board, USDA, Washington, D.C.

Paulenz, H., Ådnøy, Fossen, O., Söderquist, L., T., Berg, K.A., 2002. Effect of deposition site ans sperm number on the fertility of sheep inseminated with liquid semen. Vet. Rec. 150:299-302.

Paulenz, H., Söderquist, L., Ådnøy, T., Fossen, O., Andersen Berg, K., 2003. Effect of milk and TRIS-based extenders on the fertility of sheep inseminated vaginally once or twice with liquid semen. Theriogenology 60:759-766.

Paulenz, H., Söderquist, L., Ådnøy, T., Nordstoga, A.B., Andersen Berg, K., 2005. Effect of vaginal and cervical deposition of semen on the fertility of sheep inseminated with frozen-thawed semen. Vet. Rec. 156:372-375.

Wildeus, S., 2012. Pregnancy rates in hair sheep ewes following vaginal insemination with fresh and short-term stored liquid semen. Reprod. Domest. Anim. 47 (Suppl. 4): 422.

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