Reproduction

By W. Duane Mickelsen.

SHEEP

Effective management of reproduction is essential to profitability in the sheep business. Thus, knowledge in this area is very important.

Estrous Cycle: Most ewes are seasonal breeders, with peak fertility in the fall (September November). Duration of the breeding season varies with breed, geographical location, climate and nutrition. Average length of the estrous cycle is about 17 days and estrus (heat) about 36 hours. Ovulation occurs near the end of estrus.

Lambs born early in the spring usually will have their first cycle and conceive at 6 to 8 months. Those born late in the spring may not cycle until the next fall.

Gestation period in the ewe averages 150 days in length.

Birth: The fetus releases hormones from the brain which in turn cause the release hormones from the ewe that start uterine contractions. Normal second stage labor lasts for 30 to 60 minutes.

If no lambs are born after 1 1/2 hours of active labor, or no progress is made within 30 minutes after appearance of the water bag or a fetal extremity, veterinary assistance may be required. Single lambs, or lambs over 12 pounds at birth, are more likely to cause dystocia (abnormal birth).

Animal Selection: Select ram lambs with large testicles for breeding because they produce higher pregnancy rates and sire ewe lambs which mature at an early age and ovulate more ova. Ewe lambs that were born twins should be selected for the breeding flock.

Although it is often assumed that sheep are relatively fertile animals, according to USDA figures in 1982 there were 8.8 million ewes of reproductive age in the United States and yet only 8.5 million lambs were weaned. This amounts to .97 lambs per ewe exposed from a species that is capable of weaning at least 1.5 lambs per ewe.

Infertility in Ewes

Early and late (before October and after November) in the breeding season, fertilization failure is more common in sheep. Early embryonic death (EED) has a higher incidence in young ewes, with increased ovulation rates and with nutritional and environmental stress.

With high humidity and rapid growth, forages such as certain clovers, birdsfoot, trefoil or alfalfa cause depressed ovulation and cystic degeneration of the uterus which results in failure of the embryo to implant.

Many infectious diseases can cause abortion in ewes, such as vibriosis, chlamydiosis (enzootic ovine abortion), salmonellosis, brucellosis, leptospirosis, and toxoplasmosis.

The two most common causes are vibriosis and chlamydiosis. Both cause abortion during the last 4 to 6 weeks of gestation when the fetus is growing rapidly. The abortion rate may be as high as 60 percent when first introduced into a flock but is more commonly 5 to 10 percent. The fetus and placenta are both important for diagnosis and should be saved for your veterinarian. These two diseases can be prevented by vaccination.

Most infectious causes of abortion can be prevented by proper management, such as keeping feed off the ground and clean water sources.

Ram Infertility

Epididymitis is the most important reproductive disease in the ram. The primary cause is Brucella ovis, although other bacteria also have been shown to cause the disease. The incidence in the Western United States is 15 to 40 percent. The infection is spread by sodomy from older rams to young rams.

Diagnosis may be accomplished by feeling the testicles, isolating the organism from the semen, and serological testing. Control is by culling affected rams and keeping young rams separate from mature lambs or any ewes that the mature rams have mated.