Nutritional Requirements of Sheep
An adequate diet for optimal growth and production must include water, energy (carbohydrates and fats), proteins, minerals, and vitamins. Under field conditions of particular stress, additional nutrients may be needed. (For detailed nutrient requirements for sheep, refer to the most current Nutrient Requirements of Small Ruminants, published by the National Research Council [www.nap.edu].)
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, 1.5 gal./day for ewes nursing lambs, and 0.5 gal./day for finishing lambs. In many range areas, water is the limiting nutrient. 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.
Much of a sheep's diet depends on grass or other forages that can be either sparse or of poor quality, so the provision of adequate dietary 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 weeks 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 most efficient and accurate 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, nongrowing, nonlactating sheep. 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 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.
In the USA, except on certain alkaline areas of the western range and along the seacoast, sheep should be provided with ad lib salt (sodium chloride). Sheep need salt to remain thrifty, make economical gains, lactate, and reproduce. Mature sheep will consume ~0.02 lb (9 g) of salt daily, and lambs half this amount. Range operators commonly provide 0.5–0.75 lb (225–350 g) of salt/ewe/mo. Salt as 0.2%–0.5% of the dietary dry matter is usually adequate.
In plants, generally the leafy parts are relatively high in calcium and low in phosphorus, whereas the reverse is true of the seeds or grain. Legumes, in general, have a higher calcium content than grasses. As grasses mature, phosphorus is transferred to the seed (grain). Furthermore, the phosphorus content of the plant is influenced markedly by the availability of phosphorus in the soil. Therefore, low-quality pasture devoid of legumes and range plants tends to be naturally low in phosphorus, particularly as the forage matures and the seeds fall.
Sheep subsisting on mature, brown, summer forage and winter range sometimes develop a phosphorus deficiency. Sheep kept on such forages or fed low-quality hay with no grain should be provided a phosphorus supplement (ie, defluorinated rock phosphate) added to a salt-trace mineral mixture. Because most forages have a relatively high calcium content, particularly if there is a mixture of legumes, diets usually meet maintenance requirements for this element. However, when corn silage or other feeds from the cereal grains are fed exclusively, ground limestone should be fed daily at the rate of 0.02–0.03 lb (9–14 g).
Sheep seem to be able to tolerate wide calcium:phosphorus ratios as long as their diets contain more calcium than phosphorus. However, an excess of phosphorus may be conducive to development of urinary calculi or osteodystrophy. A calcium:phosphorus ratio of 1.5:1 is appropriate for feedlot lambs. For pregnant ewes, the diet should contain ≥0.18% and, for lactating ewes, ≥0.27%. A content of 0.2%–0.4% calcium is considered adequate, as long as the ratio is maintained between 1:1 and 2:1.
Occasionally, the iodine requirements of sheep are not met in the natural diet and thus iodine supplements must be fed. Goitrogenic substances are found in many types of plants (eg, Brassica spp) and interfere with the use of iodine by the thyroid. Regions naturally deficient are found throughout the western USA, in the Great Lakes area, and in other parts of the world. A deficiency of iodine (manifested as goiter in the adult and as lack of wool and/or goiter in lambs) can be prevented by feeding stabilized iodized salt to pregnant ewes. The young of iodine-deficient ewes may be aborted, stillborn, or born with goiters. Diets containing iodine at 0.2%–0.8% ppm are usually sufficient, depending on the animals’ level of production (maintenance/growth, lactation, etc).
Sheep require ~0.1 ppm of cobalt in their diet. Cobalt-deficient soils are found in North America but are relatively rare compared with other parts of the world. Normally, legumes have a higher content than grasses. Because cobalt levels of the feedstuffs are seldom known, a good practice is to feed trace mineralized salt that contains cobalt.
Pregnant ewes require ~5 mg of copper (Cu) daily, which is the amount provided when the forage contains ≥5 ppm. However, the amount of copper in the diet necessary to prevent copper deficiency is influenced by the intake of other dietary constituents, notably molybdenum (Mo), inorganic sulfate, and iron. High intake of molybdenum in the presence of adequate sulfate increases copper requirements. Because sheep are more susceptible than cattle to copper toxicity, care must be taken to avoid excessive copper intake. Toxicity may be produced in lambs being fed diets with 10–20 ppm of copper, particularly if the Cu:Mo ratio is >10:1. The Cu:Mo ratio should be maintained between 5:1 and 10:1.
Selenium is effective in at least partially controlling nutritional muscular dystrophy. Areas east of the Mississippi River and in the northwestern USA appear to be low in selenium. The dietary requirement is ~0.3 ppm. Providing selenium-containing mineral mixture may prevent selenium deficiency if animals are allowed free access. Levels of 7–10 ppm or higher may be toxic.
Growing lambs require ~30 ppm of zinc in the diet on a dry-matter basis. The requirement for normal testicular development is somewhat higher. Classic zinc deficiency (parakeratosis) is more common in other small ruminants (goats, llamas, alpacas), but is occasionally encountered in sheep, particularly if fed excessive quantities of dietary calcium (legumes).
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. Dietary modification should be done slowly and cautiously, to avoid vitamin D toxicity. Fast-growing lambs kept away from direct sunlight or maintained on green forages (high carotene) during the winter months (low irradiation) may show 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.