Breeding, Parturition, and Common Lamb Diseases of Sheep

Before the breeding season, both ewes and rams should undergo thorough physical exams. Body condition scores should be evaluated to ensure that all animals have enough reserves to meet the demands of breeding and, in the case of ewes, parturition and lactation. On average, a body condition score of 3.5/5 is desired. Both thin and obese animals should be identified for diet modification and treatment if needed. Feet and teeth should be examined, as should ease of movement. Animals that have difficulty walking or eating are not ideal candidates for breeding. Conjunctival membranes should be examined for clinical signs of anemia and potential complications from GI parasites.

Ewes should have their udders examined, and rams their scrotums. Both should be palpated for consistency, asymmetry, inflammation, or other clinical signs of disease or injury. Scrotums should be measured because scrotal circumference correlates directly with reproductive health and capacity. Sheaths should also be examined, particularly in heavily fleeced males on high-protein diets. Urine trapped in the wool can cause irritation and scabbing at the preputial orifice, leading to pain that can interfere with breeding. Also at this time, flocks can be tested for communicable diseases such as paratuberculosis and ovine progressive pneumonia, and infected animals can be culled.

Selection of breeding stock is an often overlooked aspect of flock management; producers should select only the best of their flock to breed. Animals with conformational faults, poor feet, an unthrifty phenotype, udder or scrotal defects, or histories of vaginal or rectal prolapses should be removed from the breeding pool. Animals with correct conformation and strong production characteristics, such as parasite resistance or good maternal skills, should be retained. It might seem counterintuitive to breed fewer animals; however, taking time to select the best stock for reproduction can make the flock much healthier and more productive in the future.

Vaccines that prevent reproductive diseases such as Chlamydia and Campylobacter infections, if needed, should be administered per the manufacturer’s directions, typically 60–90 days before ram introduction. Flushing, the process of increasing ovulation rate by feeding ewes a high-energy diet, should begin 2–4 weeks before breeding. A New Zealand study demonstrated a 25% increase in ovulation rates when ewes were offered an additional 5 kg of dry matter per head per day.1 This dry matter can be grain or forage, provided it has appreciably higher protein than the maintenance diet has. Flushing works best for animals that are thin or on a low-protein diet. Changes in ovulation rates may not be noticeable in ewes that already have an above-average body condition score on a medium to high nutritional plane.

Sheep are seasonally polyestrous, short-day breeders, with differences in seasonality between breeds. Geographic location can also affect breeding, because breeding seasons are longer near the equator. The sheep estrous cycle averages 17 days, with estrus lasting on average 30 hours. Ewes reach puberty at 6–9 months old but should weigh 60%–65% of their mature body weight (or their dam’s mature body weight) before breeding.

Ewes show very subtle clinical signs of estrus—namely, a slight enlargement of the vulva and a mild increase in mucus secretion. Because it is difficult to detect whether a ewe is in heat, the most common breeding strategy that producers use is to commingle rams with ewes for multiple estrous cycles. On average, a ram-to-ewe ratio of 1:40 is sufficient for adequate coverage in a compact time frame. Inexperienced ram lambs may need a lower ratio; more experienced rams may do well with a higher ratio. In very large grazing flocks, ratios of 1:150 have been reported as successful. Visual guides such as marking harnesses are useful for determining breeding date, sire (if more than one ram is being used), and ewes that are not settling. Ewes can become naturally synchronized through the “ram effect” if rams and ewes are kept distant from one another before commingling. Techniques for artificial manipulation of the estrous cycle and artificial insemination are well developed for sheep and frequently used in intensively managed flocks.

Pregnancy can be confirmed by serum tests and transabdominal ultrasonography. Both methods can be used as early as 30 days after breeding. Ultrasonography enables the counting of fetuses, which can be useful for more efficient dietary management in late gestation. The best window for counting fetuses is 40–70 days after breeding.

Gestation in sheep lasts an average of 147 days. Pregnant ewes need exercise and good-quality nutrition. Ewes carrying multiple fetuses need proportionally higher energy intake than do nonpregnant ewes or ewes carrying a single fetus. Both obesity and emaciation should be avoided. A pregnant ewe’s plane of nutrition should increase during the last trimester of pregnancy to meet the demands of rapid fetal growth. Failure to feed ewes adequately during this time can lead to pregnancy toxemia and hypocalcemia. Ewes that require concentrates during late pregnancy should be carefully managed to ensure that sugars and starches are gradually introduced and not abruptly overfed, which can lead to lactic acidosis. Ewes should be vaccinated 4–6 weeks before parturition as a means of increasing the concentration of antibodies in the colostrum. Clostridial vaccines, such as CDT (the vaccine against C perfringens types C and D) as well as C tetani, are the most common vaccines administered at this stage of gestation. Ewes may be shorn completely or just around their hindquarters and udders (crutching) during this time to give lambs easy access to the udder soon after birth.

The onset of parturition may be indicated by udder development, behavior changes in the ewe (isolation, vocalization, nesting), and vulvar discharge. Ewes should be separated from the flock and placed in a clean, dry, warm, draft-free area for lambing. Ewes can suppress labor when stressed, so they should be monitored with minimal disturbance. After the onset of strong abdominal contractions, the first lamb should be born within an hour. Clean hands, gloves, or sleeves, along with copious amounts of obstetric lubricant, should be used when correcting a malpositioned lamb. The lamb should be gently manipulated, with no excessive pulling. After all lambs have been delivered, the placenta should be expelled over the next 12 hours.

Newborn lambs should have their navels dipped in 7% strong iodine to decrease the risk of navel ill (described in detail below). The navel should be quickly dried off and examined for entropion, umbilical hernias, cryptorchidism, and other congenital defects. Hypothermia is a leading killer of neonates, especially in colder climates, so the simple acts of drying and warming lambs can prevent many deaths.

Lambs should stand and nurse within 30 minutes of birth. Ewes and lambs should bond by themselves for 2–3 days before being introduced into the larger flock. Individual pens (jugs) can be used, but they should be large enough (1.5 square meters) to prevent crushing of the lambs when the ewe reclines. Recording a lamb’s birth weight provides a tool to measure productivity goals and monitor overall health. Any lambs that lose weight or fail to gain weight during the first weeks of life should be further evaluated for illness.

Lambs that are rejected or require additional care should be bottle-fed a minimum of 10% of their body weight in high-quality colostrum during their first 24 hours. Lambs that do not receive adequate colostrum are at higher risk for infection and death. After the initial colostrum feeding, milk or milk replacer should be fed at a rate of 10% of lambs' body weight per day, distributed over three feedings. The source of milk for bottle-fed lambs should vary as little as possible. Frequent changes can disrupt the GI environment and predispose lambs to disease. Any fresh colostrum or milk should be heat-treated or pasteurized, or derived from disease-free animals to prevent the transmission of milk-borne diseases to lambs. Young lambs can be exposed to hay and grain-based creep feed within days of birth. Lambs should be weaned when solid food composes a majority of their diet. Generally, lambs are not ready for weaning before 6 weeks old. Weaned lambs should be fed a high-quality diet to provide sufficient energy for growth.

If ewes were not vaccinated before parturition, a clostridial vaccine should be administered to lambs at birth and then according to the vaccine manufacturer's directions. Castration and tail docking should be performed within the first 7–10 days of age, after the ewe-lamb bond is established. These procedures can be performed on older lambs, but for older lambs anesthetic use is recommended. Tails should be docked no shorter than the distal ends of the caudal tail folds.

Diseases related to breeding and parturition include nutritionally related diseases and common lamb diseases, such as:

• Navel ill is an ascending infection of the umbilicus that is common in lambs born in dirty, wet, crowded conditions. A swollen umbilicus, swollen joints, a reluctance to move, and failure to gain weight are all clinical signs of this disease. Drying and sealing the umbilicus by dipping it in 7% iodine immediately after birth can decrease the risk of navel ill; however, a clean birthing and jugging area is essential. Erysipelas arthritis can look similar to this disease. Antimicrobial treatment is essential.

• Coccidiosis is due to coccidia, intestinal protozoa that are a common environmental contaminant, carried and shed in the feces of subclinical adults. Lambs that are thin, have diarrhea, or die suddenly at ~3 weeks old should be tested for coccidia. Coccidiosis is of particular concern in overcrowded flocks where feces are allowed to accumulate and are impossible to avoid. Lambs ingest coccidian oocysts at an early age both from the ground and from their dam’s contaminated teats. Diseased lambs show poor weight gain and can succumb to dehydration. Clean housing is the key to controlling this disease. Treatment may include the administration of drugs such as sulfadimethoxine, amprolium, and toltrazuril. Coccidiostats such as decoquinate can be given to lambs in milk or feed to control the disease, provided feed intake is sufficient to achieve therapeutic drug doses.

• Hypothermia and starvation are two management-dependent diseases that cause a large number of lamb deaths yearly. Housing young lambs in a warm, bedded, draft-free area protected from precipitation is key to keeping them alive in cold, wet environments. Coats can be beneficial as well. Lambs that are shivering, thin, or hunched should always be checked for starvation. Mismothering, undiagnosed mastitis, or sibling dominance at the teat can all be causes of starvation.

• Contagious ecthyma (sore mouth) is a skin disease due to the orf virus, which forms painful lesions around the mouths of lambs and on the teats of ewes. The virus itself is not fatal; however, the lesions can make feeding difficult, leading to anorexia and starvation. The orf virus is transmitted by both direct contact and fomites. Cases usually spontaneously resolve in 1–2 months; however, the virus can live in the soil for years. The vaccine for sore mouth is a live vaccine, so it should not be administered unless the disease already exists on the premises.

• Abortion storms have multiple causes in sheep flocks. Campylobacter, Chlamydia, Toxoplasma, and Listeria are some common pathogens. Once a storm starts, it can be hard to slow or stop. Treatment and prevention measures should be based on an accurate, definite diagnosis. Placenta is often the best tissue to submit for diagnostic testing, superior even to fetal tissue.