Nutrition Principles for Sheep

Sheep

KEY MESSAGES

• Energy and protein are the key nutrients that limit sheep production.

• Deficiencies of vitamins and minerals can be of particular importance in lambs.

• To feed sheep successfully, we must aim to meet the nutrient needs of the microbial population in the rumen of the sheep.

Definition of animal production

Animal production is concerned with the conversion of chemical components in forages and grains into meat, wool and milk. The nitrogen, carbon, and minerals in pasture and other feeds are converted to muscle, milk and wool through the processes of digestion, absorption and assimilation within the body of an animal. How efficiently this occurs depends on the quality and quantity of the feed available as well as the type of animal eating the feed.

Uniqueness of ruminant animals

Sheep are ruminants and as such are characterised by having a specialised stomach with a number of compartments, one of which is known as the rumen or paunch. The rumen is basically a number of compartments, one of which is known as the rumen or paunch. The rumen is basically a 4 to 10-litre vat in which millions of microbes ferment incoming feed into products that the sheep then uses to grow.

Without these microbes sheep could not exist because the microbes possess the special ability to break down the otherwise indigestible cellulose component of plant material. Cellulose forms a large part of the fibrous feed of ruminants.

No animal has the enzymes capable of breaking apart the cellulose bonds to release the energy they contain. Thus, without the microbes, the sheep could not access the energy contained within fibrous plant feeds.

Another special feature of ruminants is their ability to ‘chew the cud’ or ruminate. As a normal part of eating, food is regurgitated from the rumen into the mouth and re-chewed. This makes the feed particles smaller and increases their surface area so that when they re-enter the rumen it is easier for the microbes to access the feed and continue their fermentation. Without this process of rumination, the feed particles would remain too large for the microbes to access the carbohydrates and protein contained within them.

The microbes in the rumen ferment feed carbohydrates to produce volatile fatty acids. These volatile fatty acids are the major source of energy for the sheep. Carbohydrates such as soluble sugars and starch are easily broken down by the microbes. Others, such as the celluloses contained within the cell walls of plant material are more complicated in structure and take longer to break down.

Another complex material known as lignin can not be broken down at all by the rumen microbes and its presence in mature plant material considerably limits the efficiency of fibre digestion by the rumen microbes.

Plant protein is broken down into its component amino acids by the rumen microbes. These are either incorporated into microbial protein or degraded further to produce ammonia that is then used to synthesise microbial protein or is absorbed into the blood stream to be recycled or excreted as urea in the sheep’s urine. When the microbes themselves pass from the rumen, through the acid stomach and into the small intestine, they are then digested into amino acids and these are absorbed into the sheep’s blood stream. Microbial protein can be of high quality in that it generally contains the right balance of the amino acids needed by the sheep to synthesise muscle, wool and milk protein.

Nutritional requirements of the ruminant

When we consider the nutritional requirements of the ruminant we must also consider the nutritional needs of the microbial population in the rumen. In essence we aim to ‘feed the rumen microbes to feed the sheep.’ If we are able to maintain a healthy, productive microbial population then we can be confident that the sheep will receive adequate energy (in the form of volatile fatty acids) and protein (in the form of microbial protein).

Pasture

Energy

Energy provides the power needed to drive all the metabolic processes of an animal. Without it, chemical reactions would not occur and muscle, milk and wool could not be synthesised. Provided there is adequate protein in the diet, the amount of energy available to an animal determines how productive the animal can be in terms of meat, wool and milk production.

The key factor in supplying energy for sheep production is how easily the energy can be extracted from a feed. In other words how quickly the sheep’s digestive processes can access and use this energy to produce meat, wool or milk.

The speed with which the energy becomes available depends on how digestible the feed is. Feeds such as green pasture, good hay or grains are easily digested because they contain large amounts of soluble sugars and starches and only a small proportion of the less digestible structural carbohydrates such as cellulose and lignin found in cell walls.

As plants age, more and more of these structural carbohydrates are laid down in the stems and the soluble, more simple sugars stored in the stems are moved to form the seed heads or grain. This means that feeds such as dry pasture or straw take longer to digest because they contain less soluble sugars and the chemical bonds that bind the cell walls together are very strong and require more time and energy to break than the bonds that hold simple sugars together.

When pastures begin to ‘hay off’ or sheep are put onto stubbles, their food intake generally drops off. This is because it takes longer for the microbes to break down the structural carbohydrates contained in these feeds and, in consequence, the feed remains in the rumen for longer. The sheep can not eat more feed until the rumen empties.

The total energy content of a feed is known as its ‘gross energy’ and is a measure of the amount of heat generated when the feed is completely combusted to ash in the presence of oxygen. However not all the energy in a feed is available to an animal since some remains trapped in the plant cells and is unable to be extracted by the animal’s digestive processes. Additional energy is lost in the gaseous products of digestion such as CO2and methane and a small amount of energy is also lost in the urine of the animal. Once these losses of energy are taken into account, the remaining feed energy is known as the ‘metabolisable energy’ which is the energy available to the animal for the metabolic processes of maintenance and growth.

It is the metabolisable energy content of a feed that determines how much production can be achieved by an animal, that is, whether an animal’s weight will be maintained, increased or decreased. It is essential to know the metabolisable energy content of feeds when formulating diets and developing feed budgets for ruminants.

Since metabolisable energy is difficult to measure, it is usually calculated from the digestible dry matter of the feed using an appropriate equation. The greatest loss of energy when forage is digested is that lost in the animal’s faeces. Put simply, digestible dry matter is the proportion of the dry matter consumed that is not excreted in the faeces. Digestible dry matter is expressed as a percentage, that is, the proportion of dry matter in the feed that can be digested by the animal.

A young ryegrass/clover pasture can have a digestible dry matter value as high as 80 per cent and a calculated metabolisable energy of 11.8 megajoules per kilogram of dry matter.This is similar to a reasonable quality barley grain. This means that the animal can digest 80 per cent of the dry matter in the diet and that for every kilogram of feed eaten, 11.8 megajoules of metabolisable energy will be released to the animal.

A mature grass-dominant tropical pasture that has recently finished flowering and is starting to become lignified, may have a digestible dry matter value of about 60 per cent with an metabolisable energy of 8.6 megajoules per kilogram of dry matter. On the other hand, a wheat stubble that has little leaf material present, is heavily lignified and contains a significant amount of indigestible silica, may have a digestible dry matter value of only 40 per cent with a metabolisable energy of 5.6 megajoules per kilogram of dry matter.

Short seasons such as that experienced in 2000 produce pastures and stubbles of a relatively high digestible dry matter. When the season stops abruptly, the plants do not have time to lay down structural carbohydrates and there is less movement of soluble sugars out of the stem and into the seed heads. The nutritive value of this dry feed will remain high throughout the summer as long as there is no summer rain to leach out the soluble sugars in the stems. Long seasons give stubbles of poorer quality because of the high proportion of structural carbohydrates and low amounts of soluble sugars remaining in the dry forage.

Animals require energy to maintain their basic metabolic processes such as kidney, liver, brain and heart function as well as to lay down muscle, fat, protein and wool. The energy required to maintain processes such as heart function is termed the ‘maintenance energy requirement’ of the animal. The amount of energy needed to maintain an animal depends on what the animal is doing and what environment it is living in. For instance, sheep produced in hilly country will need extra energy for maintenance to walk up hills in search of food. Similarly, sheep grazing a sparse pasture will have to walk further and therefore require more maintenance energy than a sheep grazing a relatively dense, green pasture or one that is housed and lot fed.

A 50 kilogram sheep walking three kilometres to water and back again each day will increase its maintenance requirement by 0.78 megajoules or 14 per cent. If it climbed a total of 600 metres on the way its maintenance requirement will increase by an additional 0.42 megajoules making a total increase of 1.2 megajoules or 21 per cent. In other words, to maintain its liveweight, the sheep will require 20 per cent more energy each day than a sheep with water close by and grazing flat land.

The need for extra maintenance energy under certain production conditions such as those described above become particularly important when we want to get sheep to specific target weights for milk and meat production. Part of the reason why weaners grown for prime lamb production grow faster in a feedlot is because they expend less energy searching for food and more energy growing than lambs produced on pasture alone. Another reason is that lot-fed lambs also tend to be given a feed ration capable of supporting high rates of growth.

Once the energy requirements for maintenance have been met, the sheep can use the additional metabolisable energy from the diet for productive processes such as meat, milk and wool production. Thus, the metabolisable energy intake required for production is principally determined by the desired growth rate or level of milk production. For example, lactating ewes require at least double their normal energy intake to produce enough milk to sustain the growth of their lamb/s.

Protein

Non-ruminant animals rely solely on the protein in their diet to produce proteins such as muscle and enzymes within their bodies. Consequently, if protein is deficient in the diet, the animal will have insufficient protein for these processes.

Ruminants have an advantage over non-ruminants because the microbes in the rumen are able to produce protein from non-protein nitrogen as well as from the protein in their diet.This means that the microbes can use nitrogen in the form of urea or sulphate of ammonia to make microbial protein. When these microbes are eventually digested in the small intestine of the sheep, this non-protein nitrogen ultimately provides the sheep with protein. This is particularly useful when sheep are grazing diets low in protein such as straw stubbles.

However, when the crude protein content falls below seven per cent, the microbes in the rumen are not able to reproduce themselves and the population begins to decline. As the microbial population falls, fewer microbes are available to break down the carbohydrates entering the rumen and in consequence the feed intake and growth rate of the sheep starts to drop off.

Growing sheep need between 12 and 15 per cent crude protein in their diet while sheep fed for maintenance only need about 8 per cent. Lupins can contain 30 to 40 per cent crude protein and it would therefore be wasteful to feed lupins alone since up to 65 per cent of this lupin protein could be wasted in the urine of the sheep. The aim with protein supplementation is to feed enough protein to raise the protein content of the base feed and thereby stimulate the feed intake (and therefore energy intake) of the sheep.

One of the disadvantages associated with the rumen microbes is that they fully degrade much of the protein entering the rumen into ammonia. This ammonia can be resynthesised back into microbial protein but any excess ammonia is absorbed into the sheep’s bloodstream with much eventually being wasted when it is excreted in the urine as urea.

When the feed is low in protein the amount of nitrogen excreted in the urine is relatively low. However, diets high in protein cause more ammonia to build up in the rumen and, in consequence, much of the incoming protein is wasted in the urine of the sheep. Microbial protein produced in the rumen travels to the sheep’s small intestine where it is broken down yet again into amino acids that are absorbed into the blood stream of the sheep for subsequent protein synthesis. All these processes are energetically inefficient and some of the protein entering the rumen is consequently wasted because of this inefficiency.

Some heat and chemical treatments will protect proteins from attack by the rumen microbes. These protected proteins ‘by pass’the rumen and are fully available for digestion in the small intestine. Canola meal is a good example of a protein source that is partially protected during the canola oil extraction process. Trials have shown that including some of the protein source as canola meal leads to small increases in the efficiency with which feed is converted to liveweight. Some commercial pellet formulations include a proportion of the protein as canola meal. It is only likely to be economic to use a source of protected protein if the difference in price between the protectedand unprotected protein source is small.

Minerals

While protein and energy are the major nutrients that limit animal growth, certain minerals can also reduce production if they become deficient, particularly in fastgrowing production animals. Macro-minerals are those needed in relatively large amounts (grams per kilogram) and include calcium, chlorine, magnesium, phosphorus, potassium, sulphur and sodium.Trace minerals such as cobalt, copper, iodine, iron, manganese, molybdenum, selenium and zinc are needed in much smaller amounts (milligrams per kilogram).

The intake of minerals by sheep varies throughout the year. Adult sheep tend to have enough reserves of minerals to provide for their own needs and those of the foetus during temporary seasonal deficiencies. However, animal reserves of minerals will be depleted when they are fed only a grain-based diet for more than one month. Periodic feeding of a multi-mineral and vitamin mix will be needed for any long-term feeding in feedlots.

All grains are low in calcium and have relatively high levels of phosphorus leading to a calcium to phosphorus ratio well below the ideal of 2:1. An imbalance of calcium and phosphorus while feeding grain can lead to reduced appetite and growth, soft bones and fractures,and the formation of urinary stones that may obstruct the urinary tract, especially in wethers and young rams. If urinary stones do block the urinary tract this can lead to rupture of the urinary bladder, leakage of urine into the abdomen, and the fatal condition called “water belly”.

To avoid all these problems it is recommended that 1.5 per cent of finely ground limestone be added when feeding cereal grains to sheep. Trace element deficiencies are more severe in foetuses and young animals. Newborn lambs, pre-ruminant lambs, and weaned lambs may not get enough trace elements for optimum health and growth during seasonal deficiencies.

The trace elements most likely to be deficient in Western Australia are selenium, cobalt and copper. Selenium deficiency is most often encountered in winter and spring (in lambs and calves) and summer and autumn (in weaner sheep). Cobalt and copper deficiencies usually occur in spring, particularly in years with rapid pasture growth after good winter rains.

Oral drenches and injectable products are available for the prevention and treatment of deficiencies of each of these elements, and intra-ruminal pellets are available for the prevention and treatment of selenium and cobalt deficiencies. Adding selenium and copper to fertilisers used to top-dress pastures will prevent deficiencies of these elements in grazing stock. Selenium, cobalt and copper are all included in most mineral supplements. Selenium deficiency may cause nutritional myopathy (“white muscle disease”) in young sheep, but the same condition can also result from a vitamin E deficiency.

The actual cause needs to be determined to ensure the correct treatment is provided. Copper deficiency in livestock may also result when copper in the diet appears to be adequate, but there are high levels of molybdenum and/or sulphur. The molybdenum and sulphur interact with the copper and make it less available for absorption from the digestive tract.

Vitamins

Vitamins of importance in ruminant nutrition are either fat or water-soluble. The fat-soluble vitamins include vitamins A, D, E and K and the water-soluble vitamins are those in the B group and vitamin C. Green pastures generally contain adequate levels of vitamins A, E and K for sheep and vitamin D is synthesised in the skin provided animals are exposed to enough sunlight.

Sheep usually synthesise enough vitamin C to meet their requirements and the microbes in the digestive tract are usually able to synthesise adequate amounts of the B-group vitamins. Cobalt is required for the synthesis of vitamin B12 by the rumen microbes and cobalt deficiency shows as a deficiency of vitamin B12 with the main effect being a disturbance in energy metabolism leading to reduced growth in young animals. Vitamin A deficiency can occur after prolonged dry periods. Sheep need to eat green feed,such as weeds, for at least a day to get enough vitamin A to last for three months. Some feeds, such as subclover and lucerne, have higher vitamin A levels.

Thiamine (vitamin B1) deficiency is usually caused by the presence of the enzyme “thiaminase”, which is ingested in certain plants or is produced by microbes in the rumen. Thiaminase destroys the thiamine entering and produced in the rumen, and deficiency of this vitamin leads to the development of a neurological disease. A change to a high-grain diet may precipitate thiamine deficiency, and the condition has also been reported in sheep fed diets that have insufficient fibre to encourage good rumen motility.

Vitamin E, like selenium, has a major role as an anti-oxidant and the metabolic roles of vitamin E and selenium are interchangeable to some extent. Sheep that have experienced reasonable lengthy periods on green feed and are only fed concentrated diets for short periods are unlikely to experience mineral and vitamin deficiencies. However, if young animals have only had a short period of access to green feed, or have been off green feed for more than three months, their body reserves are likely to be low.

Periods of rapid growth, pregnancy and lactation, generate high demands for minerals and vitamins and animals with low body stores may become deficient if they are not able to obtain all the vitamins and minerals they need from their feed source. Feed produced in high rainfall environments tend to have lower levels of vitamins and minerals because their concentrations are diluted in the fast growing plants.