Health-management Interaction: Sheep: Introduction

Summer Grazing with Winter Confinement and Feeding

Management Practices and Predisposition to Disease

The major products from sheep are meat, wool, and hides; sheep milk is important in some areas of the world. Sheep are raised in many different environments, with great variation in efficiency. The type of production system in any area depends on many factors, primarily the availability and cost of pasture, the climate, and the interaction with other livestock and cropping systems.

Different areas of the world use different production systems. Extensive year-round grazing, with large flocks (>2,000 sheep) and minimal sheep handling, is the typical system of sheep management in the major wool-producing nations of Australia, New Zealand, South Africa, and parts of South America and the USA. Confinement and intensive feeding during the winter months, with access to pasture for the rest of the year, is the common system of sheep management in Europe and most of the USA. Close confinement in feedlots in the final growth stage of lambs for meat is virtually restricted to North America. Shepherding small flocks of sheep and goats along roadsides and common grazing areas is a typical management system in the Middle East and in Asia.

Extensive Grazing Systems:

Economics are an essential consideration. In addition to animal welfare and public health considerations, veterinary services should aim to increase net farm income rather than just control disease. Although the diagnosis, treatment, and control of disease, including the investigation of outbreaks to solve a disease problem, are important on most farms, outbreaks of clinical disease are of minor importance to longterm profitability. Advice that is technically sound from a disease control perspective may be detrimental to the overall economic well-being of the farm. For example, a reduced stocking rate may mean better nutrition for the flock, fewer lamb mortalities, and lower burdens of GI parasites, but a significantly lower net farm income. Net income per hectare is generally very sensitive to changes in stocking rate.

A better approach involves a “flock health” program. These are designed to anticipate health and production problems and, when it is economic to do so, to implement appropriate prevention measures. A flock health program must be tailored to the needs of the specific farm. For most farms, key areas of flock health programs include: 1) controlling internal and external parasites; 2) preventing diseases for which there is a cost-effective vaccination program; 3) preventing the introduction of contagious diseases, such as footrot, brucellosis, and external parasites; and 4) improving the number of lambs weaned per ewe bred, with enhanced ewe and ram fertility and reduced lamb mortalities.

When wool is the major source of income, the economic outcome from an outbreak of disease or nutritional problem is not always obvious. For example, a moderate parasite burden may cause ill-thrift and a reduction in kilograms of wool cut per head, but a reduction in fiber diameter as well, which results in a more valuable fleece. A ewe that does not conceive or aborts in the first 3 mo of pregnancy may produce more wool than a ewe that rears a lamb, because a ewe rearing a lamb has a higher nutritional requirement and produces less wool than a dry ewe. Fiber and meat prices should be considered when providing advice.

Beyond a flock health program, veterinarians can also develop a comprehensive flock management advisory service. These programs adopt a whole farm approach that considers the physical and financial resources of the farm and the interaction of livestock production with other activities such as cropping and pasture production. The stocking rate, type of stock run, timing of husbandry procedures, marketing strategies, and risk management should be reviewed as part of the program. A financial analysis of the farm as a business and preparation of farm budgets and gross margin analyses is a key part of most programs.

Summer Grazing with Winter Confinement and Feeding:

Veterinary services in these systems include clinical and preventive medicine and management recommendations. Clinical medicine becomes more cost-effective as the value of the individual sheep increases. Wool production is usually a minor concern, and lambs marketed per ewe joined is the major determinant of economic return. The greatest potential loss is caused by neonatal lamb mortality resulting from mismothering, starvation, cold stress, and hypothermia. Intensive management at lambing reduces this loss but is accompanied by increased risk of mortality from infectious neonatal disease. Labor-intensive lambing systems; intensive care of young lambs; and diagnosis, treatment, and surgery of individual sheep may be justified by the value of the animals. Preventive medicine programs should be developed to prevent the numerous husbandry-related diseases associated with pastoral practices in summer and close stocking during confinement in winter. Feed and labor are the largest production costs for housed or intensively fed sheep. Therefore, nutritional management can have a major impact on farm profitability. Management issues involve minimizing flock energy requirements during the winter months, using body fat reserves without incurring undue decreases in production, and using least-cost rations (including fodder produced on farm). Feed imbalances and close confinement with sheep in feedlots are major determinants of diseases such as pneumonia ( Respiratory Diseases of Sheep and Goats: Introduction), urolithiasis ( Urolithiasis: Overview), pulpy kidney disease ( Type D Enterotoxemia), and polioencephalomalacia ( Polioencephalomalacia: Introduction).

Shepherding:

Veterinary services to the small, shepherded sheep and goat flocks in the Middle East and Asia are primarily concerned with controlling clinical diseases and improving the survival rate of young lambs and kids. The restricted availability and low nutritional value of feed limit productivity. Sheep and goats are kept primarily as a source of meat and milk for the owner’s family and for sale as a source of cash income. The value of animals in the flock is often high relative to the income of the owner, and funds available to invest in veterinary services are limited. The death or severe ill-health of a few animals can have a major impact on both the productivity of the flock and the well-being of the owner.

In Asia, the system of land use is often complex, with sheep and goats integrated with, but grazing around the fringes of, a more productive cropping or plantation enterprise. This may also be true in the Middle East, or sheep and goats may graze poorly productive arid areas that will support little else. In either case, opportunities for major changes to the management system are limited.

Management Practices and Predisposition to Disease:

Management practices can be the primary determinant of cases or outbreaks of infectious or metabolic disease in all flocks of sheep.

Pregnancy toxemia ( Pregnancy Toxemia in Ewes) may be seen in late-pregnant ewes subjected to a falling plane of nutrition, especially ewes with twins or triplets. It is also associated with starvation, ewes that are too fat in early pregnancy, ewes that are fat in late pregnancy and voluntarily reduce feed intake, and ewes subjected to stress in late pregnancy, such as transport or other environmental changes. Hypocalcemia ( Parturient Paresis in Sheep and Goats) is seen in pregnant ewes or ewes in early lactation subjected to a period of temporary starvation, especially ewes with twins or triplets; as a result of decreased food intake in late pregnancy; and in weaner sheep on a grain-based ration during drought conditions. Hypomagnesemia ( Hypomagnesemic Tetany in Adult Cattle and Sheep) may be seen during a period of temporary starvation in late pregnancy or early lactation, after movement of lactating ewes to lush growing pasture (especially green cereal crops), or among lactating ewes on rapidly growing pastures (eg, spring growth). Dermatophilosis ( Dermatophilosis: Introduction) is associated with poor shearing practices leading to shearing cuts, dipping immediately after shearing with contaminated dip, and sheep in long wool at times of high rainfall. Some sheep are also genetically predisposed. Caseous lymphadenitis ( Caseous Lymphadenitis of Sheep and Goats) may be associated with not shearing according to ascending age groups, not separating infected or discharging sheep before shearing, close confinement (nose to fleece) of sheep after shearing and dipping, and contaminated dip. Pulpy kidney disease ( Clostridium perfringens type D infection, Type D Enterotoxemia) is seen in sheep on a rising plane of nutrition, such as those moved to better pasture, following a “flush” in pasture growth, or in sheep fed grain. C perfringens type C infection is seen in artificially reared “bummer” lambs. Malignant edema ( Malignant Edema) and blackleg ( Blackleg) may follow wounds (eg, improper shearing, vaccination). Tetanus ( Enterotoxemia Caused by Clostridium perfringens Types B and C) may also be seen after a wound associated with procedures, such as castration, docking, improper shearing, or vaccination, performed in contaminated permanent yards. Black disease ( Infectious Necrotic Hepatitis) is seen among grazing sheep on pastures that support the intermediate host of Fasciola hepatica . Erysipelas arthritis ( Erysipelas: Introduction) is associated with contaminated dip, and poor hygiene at docking, castration, and lambing. Ovine posthitis is seen in merino wethers on high protein pasture. Squamous cell carcinoma is seen on the vulva of short tail docked or mulesed sheep. Actinobacillosis ( Actinobacillosis: Introduction) is seen in sheep grazing on abrasive, thorny pasture.

For disease risks associated with pasture or with specific plants (eg, the risk of bloat, polioencephalomalacia, hemolytic anemia, esophageal obstruction, and enterotoxemia in sheep, and goiter in the lambs of pregnant ewes grazing Brassica spp), see plants poisonous to animals, Plants Poisonous to Animals. For the risk for nutritional deficiency or toxic disease associated with formulated feeds, see nutrition:sheep, Nutrition: Sheep: Introduction.