If management of a flock could be distilled down to a primary principle, it would be nutrition. Sheep that are properly and adequately fed grow better, produce more offspring, and suffer fewer health issues, including internal parasites. Feed is often the largest expenditure for any producer, particularly for flocks that are confined during part of the year, so decisions regarding nutrition have important financial repercussions. In resource-limited countries, lack of adequate nutrition is a common management challenge. In resource-rich countries, overfeeding more often contributes to disease and production loss.
Nutrition is influenced heavily by local conditions. Therefore, the specifics of any feeding plan should be tailored to the environment immediate to the flock. In general, all sheep should have daily access to water, good-quality forage, and supplemental vitamins and minerals.
Water is a critical nutrient but is often overlooked. A 50-kg sheep can be expected to consume 2–4 gallons of water per day under maintenance conditions. Intake fluctuates with changes in the environment (eg, heat and cold), activity level, and physiological state (eg, growth, pregnancy, and lactation). Clean, fresh water must be easily accessible at all times. Water can contain high concentrations of minerals and should be evaluated as part of a complete feeding program.
Because they are ruminants, sheep should be fed a diet consisting primarily of good-quality forage. Forage can be hay (grass or legume), silage, or pasture/range. Forage is the best substrate for the microorganisms that live in the rumen and that provide a majority of the protein required by a sheep for energy. Forage should be clean, free of mold, and moderately mature. Maturity influences the amount of neutral detergent fiber (NDF) in the forage. Forage NDF can be an intake-limiting factor. Studies show that most sheep ingest approximately 1.2% of their body weight per day in NDF. Therefore, the higher the amount of NDF in forage, the less of it a sheep will consume.
On average, sheep consume 1.8%–2.0% of their body weight in dry matter a day. For maintenance, sheep should consume forage with a crude protein concentration of 7%–9% and a total digestible nutrition (TDN) value of 50%. These values increase during different physiological states and under greater production pressures. Late-gestation ewes, lactating ewes, and growing lambs need a crude protein concentration and a TDN of up to 16% and 70%, respectively.
Although forages should be the foundation of any sheep nutritional plan, additional energy sources may be required to meet nutritional demands during different physiological states. At these times, fermentable fiber supplements (eg, beet pulp or soy hulls) or starches (cereal grains or pelleted concentrates) should be fed. The amount and frequency depend on the quality of the overall forage and the production state of the sheep. For example, a ewe in the final weeks of pregnancy on a moderate-quality forage may need 1–2 pounds of cereal grains per day to meet her energy requirements. A pet ewe on moderate-quality forage will likely need no cereal grains at all. Sugars and starches should never be fed to replace poor-quality forage. Overfeeding or improper feeding of starches is common in small hobby or pet farms and is a frequent cause of disease.
Minerals are classified into two categories: macrominerals and trace minerals. Macrominerals are derived primarily from forages and water, all of which should be evaluated when a feeding program is being formulated. Such evaluation can be challenging for producers with smaller flocks who purchase small amounts of forages frequently from multiple sources. Concentrations of calcium, phosphorus, potassium, and magnesium should be assessed, and macromineral supplements should be adjusted to try to achieve a 2:1 calcium-to-phosphorus ratio and a 4:1 potassium-to-magnesium ratio.
Trace minerals, including copper, selenium, zinc, molybdenum, and cobalt, are highly influenced by local geography. Trace minerals compete with each other for absorption, so ratios of one to another are often as important as absolute concentrations. Trace minerals are often fed as a commercial supplement. The aim in selecting such supplements is to pick the one best suited for the local needs of the flock. Trace minerals can be mixed into a concentrate; in many small flocks, however, they are offered as a free-choice product. Loose minerals and salt are preferred over blocks. Free-choice mineral salts are mixed with sodium chloride to regulate consumption. If additional sources of salt are placed near the minerals, consumption of the mineral mix may be decreased. Sheep are extremely sensitive to copper toxicity, and any trace-mineral mix fed to sheep should contain no more than 30 ppm copper. The practice of administering copper boluses, a component of some parasite control programs, should be conducted only with extreme caution.
Wool growth, unlike gestation, lactation, or body growth, is not a physiological state that requires additional nutrition. Maintenance energy requirements are considered sufficient for wool growth. However, the quality of wool produced is influenced by a sheep’s nutritional plane. A high nutritional plane results in more wool production, albeit with slightly coarser fibers. A low nutritional plane produces less wool, with fibers so thin that breaks occur, markedly decreasing the worth of the fleece. A moderate but adequate nutritional plane should produce fine, strong fibers in ample quantity.
Nutrition-Related Diseases of Sheep
Diseases related to nutrition are some of the most common and most preventable in a sheep flock:
Pregnancy toxemia, a condition of late-gestation ewes, is a combination of hypoglycemia and ketosis that is brought on by a negative energy balance. Toxemia often occurs in ewes carrying multiple fetuses, but it can occur in any pregnant animal, thin or fat, that is fed poor-quality or insufficient feed, has had its access to feed restricted, or has been through a period of stress that has limited its feed intake. Early clinical signs of toxemia may include limping, reluctance to stand, and decreased appetite. In many ewes the condition is not noticed until they are recumbent and anorectic. When clinical signs progress to this point, ewes and lambs rarely survive, even with the best veterinary care.
Hypocalcemia occurs in late-gestation or early-lactation ewes that have been fed a diet that is too low in calcium and thus cannot meet the calcium demands of fetal bone growth and milk production. Clinical signs, which include lethargy and an inability to stand, can mimic the signs of pregnancy toxemia. However, hypocalcemia can be easily treated with the administration of calcium. Both ewes and fetuses often survive if this condition is identified and treated early.
Lactic acidosis is due to large, abrupt increases in dietary sugar and starches that cause starch-digesting microorganisms in the rumen to overwhelm the rumen environment, suppressing fiber fermentation and cytolytic activity. Rumen pH declines quickly, and lactic acid accumulates, further damaging the lining of the GI tract. Clinical signs range from isolation to bloating to diarrhea and then to recumbency. The severity of this disease is directly related to the nature and quantity of the concentrated foodstuff ingested. The prognosis is directly related to disease severity and may be poor even with intensive care.
Clostridium perfringens type C infection is due to an overgrowth of C perfringens type C in very young lambs that is due to the ingestion of large milk meals resulting from improper bottle-feeding, high milk production by the ewe, or greater access to milk because of the loss of a sibling. These lambs become lethargic, are reluctant to eat, and often die. The infection can be prevented by vaccinating ewes before parturition with a C perfringens type C and D vaccine, ensuring that lambs consume adequate high-quality colostrum, and adhering to good bottle-feeding practices. Treatment should include injections of C perfringens type C and D antiserum.
Clostridium perfingens type D infection (enterotoxemia) is due to an overgrowth of C perfringens type D and can lead to sudden death in older, fast-growing lambs fed a high sugar/starch diet or moved to rich pastures. Lambs may be found alive, especially if they were previously vaccinated, but they are difficult to save. Injections of C perfringens type C and D antiserum can be administered as part of a treatment plan. C perfringens type C and D vaccine administered to both the late-pregnant ewe and the growing lamb can aid prevention.
Urolithiasis, the formation of urinary calculi, can be a fatal disease in male ruminants. Calculi obstructing the urinary tract can lead to painful bladder distention, bladder rupture, and uremic poisoning within days. Although the roles of early castration and genetic predisposition to the formation of calculi are often discussed as causes of this disease, diet is the most important management tool that can be used to minimize risk. Rams, ram lambs, and wethers should be fed a diet low in magnesium that has a calcium-to-phosphorus ratio of ~2:1, to decrease the risk of calculus formation. Grain consumption should be minimized to keep phosphorus concentrations as low as possible, and to keep the patient's urine diluted. Feeding ammonium chloride to acidify the urine may be an appropriate treatment for some types of calculi; not all types of calculi, though, will dissolve. Removal of the urethral process may be a successful treatment for a single calculus; for severe obstructions, however, surgical procedures such as perineal urethrostomy and tube cystotomy are often required.
Polioencephalomalacia is a CNS disease that can manifest as blindness, incoordination, ataxia, or atypical vocalization. The exact pathogenesis is unknown; in some instances, however, the disease has been linked to high sulfur in the diet. Metabolites formed during GI disturbances may also play a role. This condition can occur after subclinical and clinical incidents of lactic acidosis. Administration of thiamine and flunixin meglumine, along with supportive care, is often successful.
White muscle disease is a degenerative muscle disease due to a deficiency of selenium and/or vitamin E. It affects both skeletal and cardiac muscle, often in newborn and fast-growing lambs, which may be weak and unable to rise or stiff, especially in the hind limbs. Sudden death due to acute cardiac failure may occur. Prevention by providing adequate mineral and vitamin supplementation is preferred to treatment with selenium and vitamin E, because treatment may not completely resolve clinical signs.