Nutritional Requirements of Sheep

A clean, fresh, easily accessible source of water should be available at all times. As a minimum requirement in temperate environments, the usual recommendations are ~1 gal. (3.8 L) of water/day for ewes on dry feed in winter, 1½ gal./day for ewes nursing lambs, and ½ gal./day for finishing lambs. In many range areas, water is the limiting nutrient; even when present, it may be unpotable because of filth or high mineral content. For best production, all sheep should have their water availability monitored daily during all weather conditions. However, the cost of supplying water often makes it economical to water range sheep every other day. When soft snow is available, range sheep do not need additional water except when dry feeds such as alfalfa hay and pellets are fed. If the snow is crusted with ice, the crust should be broken to allow access. Still, when possible, sheep should have unlimited access to fresh, clean water.

Because so much of the diet can depend on grass and forage that is either sparse or of poor quality, the provision of adequate energy is important. Poor-quality forage, even in abundance, may not provide sufficient available energy for maintenance and production. The energy requirement of ewes is greatest during the first 8–10 wk of lactation. Because milk production declines after this period and the lambs have begun foraging, the requirement of the ewe is then reduced to prelambing levels. The easiest way to assess energy adequacy in sheep is to perform and record body condition using an objective 1–5 scoring system, with 1 being extremely thin and 5 being extremely obese. The body condition score is determined by palpating the amount of fat covering on the spinous processes and transverse processes in the lumbar region. Most healthy productive ewes will have a score of 2–3.5. Sheep with a score of 1–2 should be examined and fed to attain a higher score, whereas those with a score >3.5 should be fed less. Dietary changes should be done slowly, and abrupt reduction in total energy intake should always be avoided, particularly in middle to late gestation.

Good-quality forage and pasture generally provide adequate protein for mature sheep. However, sheep do not digest poor-quality protein as efficiently as do cattle, and there are instances when a protein supplement should be fed with mature grass and hay, or when on winter range. Therefore, a minimum of 7% dietary crude protein is needed for maintenance in most sheep. Protein requirements depend on the stage of production (growth, gestation, lactation, etc) and the presence of certain diseases (internal nematode parasites, dental disease, etc). If available forages are unable to supply adequate dietary crude protein, protein supplements, such as oilseed meals (cottonseed meal, soybean meal) or commercially blended supplements should be fed to meet nutrient requirements. Protein should be fed to meet, but not exceed, requirements. Excess protein feeding can be beneficial in cases of excessive internal parasite burdens but result in increased production costs and may result in higher incidences of diseases (eg, heat stress, pizzle rot).

Sheep can convert nonprotein nitrogen (such as urea, ammonium phosphate, and biuret) into protein in the rumen but possibly less efficiently than beef cattle. This source of nitrogen can provide at least a part of the necessary supplemental nitrogen in high-energy diets with a nitrogen:sulfur ratio of 10:1. In lamb-finishing diets, the inclusion of alfalfa, approved growth stimulants, and a source of fermentable carbohydrates (eg, ground corn, ground milo) enhance nitrogen utilization.

Sheep require the major minerals sodium, chlorine, calcium, phosphorus, magnesium, sulfur, potassium, and trace minerals, including cobalt, copper, iodine, iron, manganese, molybdenum, zinc, and selenium. Trace mineralized salt provides an economical way to prevent deficiencies of sodium, chlorine, iodine, manganese, cobalt, copper, iron, and zinc. Selenium should be included in rations, mineral mixtures, or other supplements in deficient areas. Sheep diets usually contain sufficient potassium, iron, magnesium, sulfur, and manganese. Of the trace minerals, iodine, cobalt, and copper status in ewes are best assessed via analysis of liver biopsy tissue. Zinc adequacy can be assessed from the careful collection of nonhemolyzed blood placed in trace element–free collection tubes. Selenium status is easily assessed by collection of whole, preferably heparinized, blood.

Sheep diets usually contain an ample supply of vitamins A (provitamin A), D, and E. Under certain circumstances, however, supplements may be needed. The B vitamins and vitamin K are synthesized by the rumen microorganisms and, under practical conditions, supplements are unnecessary. However, polioencephalomalacia can be seen and is due to aberrations in ruminal thiamine metabolism, secondary to altered ruminal pH and/or microflora content. Vitamin C is synthesized in the tissues of sheep. On diets rich in carotene, such as high-quality pasture or green hays, sheep can store large quantities of vitamin A in the liver, often sufficient to meet their requirements for as long as 6 mo.

Vitamin D₂ is derived from sun-cured forage, and vitamin D₃ from exposure of the skin to ultraviolet light. When exposure of the skin to sunshine is reduced by prolonged cloudy weather or confinement rearing, and when the vitamin D₂ content of the diet is low, the amount supplied may be inadequate. The requirement for vitamin D is increased when the amounts of either calcium or phosphorus in the diet are low or when the ratio between them is wide. But such dietary modification should be done cautiously, because vitamin D toxicity is a severe syndrome. Fast-growing lambs kept in sheds away from direct sunlight or maintained on green feeds (high carotene) during the winter months (low irradiation) may have impaired bone formation and show other signs of vitamin D deficiency. Normally, sheep on pasture seldom need vitamin D supplements.

The major sources of vitamin E in the natural diet of sheep are green feeds and the germ of seeds. Because vitamin E is poorly stored in the body, a daily intake is needed. When ewes are being fed poor-quality hay or forage, supplemental vitamin E may result in improved production, lamb weaning weights, and colostrum quality. Vitamin E deficiency in young lambs may contribute to nutritional muscular dystrophy if selenium intake is low.