All body activities need energy, which must be met by consumption of feed containing energy in its chemical form. Carbohydrates are the most important energy source in grazing sheep. Carbohydrates in pasture are mainly cellulose and sugars (Energy content of pastures).

Sheep need protein to build and repair various body structures, such as muscle and wool. Protein is also needed for milk production in the ewe and for the immune response to various infections, including worm infection. Proteins are made up of building blocks called peptides, which in turn are composed of long chains of small compounds called amino acids.

The protein supply in grazing animals(Diagram of rumen) comes from two sources:

1. Rumen microorganisms convert plant proteins consumed by the sheep to microbial protein. This is the major source of protein to grazing sheep.
2. Some proteins cannot be broken down by rumen microorganisms. These proteins are called bypass or protected proteins and are usually given to sheep as supplementary feeds(Providing extra protein). These bypass proteins are usually insignificant in the pasture grazed by sheep.

The classes of sheep most susceptible to worms are young animals during in their first year and ewes during late pregnancy and early lactation. These animals have increased protein requirements, needed for growth of the young animal and the foetus, and for milk production.

There is some evidence, gained from animal house trials, that supplementation with particular minerals may enhance resistance to worms. Supplementation will only be useful in regions where these minerals are normally deficient.

Cobalt deficiency can impair the immune function of sheep and make them more susceptible to worm infection. Many agricultural areas in Australia have cobalt-deficient soils, including large areas of South Australia. Early work has shown cobalt supplementation in these areas can be beneficial.

Diets with low phosphorus levels can also reduce immunity to intestinal parasites. In addition, worms impair the absorption of phosphorous from the gastro-intestinal tract. Pastures that are not fertilised may have low phosphorus concentrations, especially during months when pasture growth is low.

Abomasal (4^th stomach) infections, such as barbers pole worm and brown stomach worm, result in reduced absorption of copper. Early trials have shown that supplementation with copper can substantially reduce the establishment of barbers pole worm and brown stomach worm. Supplementation with copper will only be an option in regions where copper poisoning does not occur.

Dietary supplementation of penned sheep with molybdenum has also been shown to enhance rejection of established barbers pole worm and black scour worm. While this work is promising, further research needs to be conducted to refine the use of minerals under field conditions.

In this section:

• Energy content of pastures
• Diagram of rumen
• Providing extra protein

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 Nutrition - Energy content of pastures

Energy in pasture is mainly provided to animals by carbohydrates, such as cellulose and sugars, and to a very small extent by fats.

Cellulose is a lower quality energy source and sugars are a higher quality energy source.

The proportion of cellulose increases with plant age, while the proportion of sugar decreases.

Both cellulose and sugars are fermented by rumen micro-organisms to provide metabolisable energy (ME) to the animal.

ME is the amount of energy in a kilogram of feed dry matter (i.e. in 1 kg of feed after drying) that is digested and used for the metabolism of the animal. (Metabolism is the overall term for processes involved in staying alive, growing, feeding a foetus or producing milk.)

The ME content of feed is expressed in units of megajoules per kilogram of dry matter (MJ/kg DM) and is determined on the basis of three factors, namely:

• The total energy of the feed. The amount of heat generated when a feed is burnt. Typical values for high quality pasture are 19 MJ/kg DM and for low quality pasture 17 MJ / kg DM.
• The digestibility of the feed. The percentage of feed intake that is digested (i.e. enters the animal) by an animal and does not contribute to faeces. For example, if the daily feed intake for a sheep is 1 kg DM and it defecates 0.2 kg DM/day, the digestibility is calculated to be (1 - 0.2)/1 x 10 = 80%.
• The amount of digested energy that is lost as urine or via gases such as methane. Typically these losses account for 19% of the digested energy.

For the examples stated above, the ME of a high quality pasture would be calculated as follows:

19 MJ/kg DM gross energy x 80% digestibility x 0.81 = 12.3 MJ ME/kg DM

Laboratories now generally use Near Infrared Spectrometry (NIRS) to determine the ME of a feed. High quality pastures are easier for animals to digest and on these pastures animals are able to eat more. Low quality pastures are harder for animals to digest and consequently animals have a lower feed intake. As long as there is adequate protein and minerals, the greater the ME content of the herbage the greater will be the supply of microbial protein to the animal.

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 Diagram of rumen

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 Nutrition - Providing extra protein

Extra protein can be given to sheep in one of several ways:

1. The supply of microbial protein can be increased by improving the quality of the pasture and by ensuring animals have sufficient pasture to graze. Pasture quality can be improved by inclusion of legumes, and by improving soil fertility and grazing management. Sheep are able to optimise their intake of pasture at about 1500 kg DM/ha (i.e. measuring only the green herbage), which equates to a pasture height of about five cm.
2. To augment the supply of microbial protein animals can be fed protein-rich supplements, such as cottonseed meal, canola meal, copra meal and protein meals treated with formaldehyde. These supplements contain high levels (i.e. 30-60% or more) of bypass proteins. Instead of being largely broken down by rumen micro-organisms, a proportion (30-60%) of the protein in these supplements is digested and absorbed by the animal.