As with people, sheep do not require specific feeds. They require energy, protein, vitamins, minerals, fibre, and water. Many different feedstuffs can meet their nutritional requirements.
Energy makes up the largest portion of the diet and is usually the most limiting nutrient in sheep diets. Carbohydrates, fat, and excess protein in the diet all contribute towards fulfilling the energy requirements of sheep. Carbohydrates are the major sources of energy. Concentrates (grain) contain starch, which is a rich source of energy. Forages contain fibre or cellulose, which is not as rich in energy as starch. The major sources of energy in a sheep’s diet are pasture and browse, hay, silage, and grains.
Meeting energy requirements without over or underfeeding animals is one of the farmer’s biggest challenges. An energy deficiency is the most common nutritional deficiency in sheep. An energy deficiency will manifest itself in many ways. In growing animals, an early sign of energy deficiency is reduced growth, then weight loss, and ultimately death. In reproducing females, early signs of an energy deficiency are reduced conception rates, fewer multiple births, and reduced milk production.
With restricted energy consumption, wool growth slows, fibre diameter is reduced, and weak spots (breaks) develop in the wool fibre. An energy deficiency reduces the functioning of the immune system. Undernourished sheep are more susceptible to diseases, especially gastro-intestinal worms.
Excess energy consumption can cause problems in sheep, too. Extra energy is stored as fat (adipose tissue). Gross excesses in adipose tissue impair reproductive functioning in rams and ewes. During late gestation, fat ewes are more prone to ketosis (pregnancy toxaemia) and dystocia (difficult birth). Fat lambs do not gain muscle efficiently, and they are undesirable to most consumers.
Energy in the ration is quantified in many ways. The simplest measure is TDN, which is the acronym for “total digestible nutrients”. Metabolisable energy (ME) and net energy (NE) values are more accurate measures of energy in a sheep’s diet. TDN is usually used to formulate rations for breeding animals, while the net energy system is often used to calculate diets for growing lambs.
Protein is usually the most expensive part of the diet. Since the rumen manufactures protein from amino acids, the quantity of protein is more important than the quality of protein in a sheep’s diet. Protein requirements are highest for young, growing lambs who are building muscle, and lactating ewes who are producing milk proteins.
The most common protein supplement for sheep is soya bean meal. Other less common sources include sunflower meal, cottonseed meal, whole cottonseed, whole soya beans, peanut meal, canola (rapeseed) meal, fish meal, and alfalfa pellets. In most countries, it is illegal to feed meat and bone meal derived from other ruminants. Legume hays, when they are harvested in the early to mid-bloom stage are intermediate sources of protein.
Though levels vary, grains are usually low in protein. Urea is the most inexpensive source of protein or dietary nitrogen. Urea is converted to protein in the rumen. It has an equivalent crude protein value of 280 percent. It needs to be carefully incorporated into sheep rations and should not be included in creep rations.
Protein blocks are the most expensive way to provide supplemental protein to pastured animals, but they save labour. The hardness of the block regulates intake by the sheep. Excess protein is an expensive and inefficient source of energy. It can have a detrimental effect on animal health, as excess protein is converted to urea and ammonia. Animals overfed with protein excrete more nitrogen in their urine and faeces.
Sixteen (16) minerals have been classified as nutritionally essential in sheep diets. Macrominerals are required in large quantities. They include sodium (Na), chloride (Cl), calcium (Ca), phosphorus (P), magnesium (Mg), potassium (K), and sulphur (S). Microminerals (also called trace minerals) are required in small quantities. They include iodine (I), copper (Cu), iron (Fe), manganese (Mn), zinc (Zn), molybdenum (Mo), cobalt (Co), selenium (Se), and fluoride (Fl).
Salt (sodium [Na] and chloride [Cl]) has an important regulatory function in the body. Inadequate salt intake can decrease feed and water intake, milk production, and growth of lambs. Animals desiring salt may chew on wood and/ or lick dirt. They are also more likely to consume poisonous plants. When adding salt to mixed rations, it is customary to add 0,5 percent to the complete diet or 1% percent to the concentrate portion. Salt is sometimes used to limit the intake of free choice mineral mixes. It can be used to regulate the intake of feed.
Calcium and phosphorus
Calcium (Ca) and phosphorus (P) are interrelated in the development and maintenance of the skeleton. Deficiencies may result in rickets. An imbalance of Ca and P in the diet can cause urinary calculi in male sheep, especially wethers. The calcium in most forages is usually adequate to meet the needs of sheep. Deficiencies of calcium most often result when high-grain diets are fed, as cereal grains and oilseeds are high in phosphorus and low in calcium. The ratio of calcium to phosphorus in the sheep’s diet should be at least 2:1.
Sheep require vitamins A, D, and E. Vitamin A is absent in most plant material, but is synthesised from beta-carotene. Vitamin D is required to prevent rickets in young animals and osteomalacia in older animals. B-vitamins are not required in the diets of ruminants because they are synthesized in the rumen. Vitamin K is essential for blood clotting. Dietary supplementation is usually not necessary.
Fibre adds bulk to the diet and keeps the sheep’s rumen functioning properly by increasing rumination and salivation. Most ruminant nutritionists agree that sheep should always have roughage in their diets, at least 453,6 grams per head per day. Sheep that do not consume adequate forage may chew on wood or wool.
Water participates in nearly all body functions and is the most important “nutrient”, though often the most neglected aspect of feeding sheep. A sheep will consume anywhere from 2,27 to 18 litres of water per day, depending upon its physiological state and the environmental conditions. Voluntary water intake is usually two or three times more than dry matter intake and increases with high-protein and high-salt diets. Decreased water intake may reduce milk production in ewes and growth rates of lambs. Animals that consume adequate water have fewer digestive upsets and a lower incidence of urinary calculi.
We thank the ARC Institute for Agricultural Engineering in South Africa who made their manual on sheep production and facilities available to the readers of ProagriMedia.