Ruminants have a minimum protein requirement for maintenance, growth and reproduction. The figure of 7-9% crude protein (CP) is often used as the minimum requirement for an adult animal that is not reproducing or growing. For growing ruminants, dietary requirements are 14-16%.
Protein requirements are often presented as a range for different classes of ruminant livestock, due to the unique relationship between the ruminant, the microbes within the rumen and the diet which makes defining more specific requirements impossible.
A proportion of all protein consumed is degraded by the microbes in the rumen (rumen degradable protein). Depending on the amount of energy supplied by the diet, some of this degraded protein is converted back to microbial protein by the microbes and then passes down the digestive tract for absorption as amino acids.
Within most diets, a component of the protein is resistant to microbial breakdown (undegraded dietary protein). This passes through the rumen and is either absorbed lower down the gastrointestinal tract or excreted in the faeces.
Therefore the amount of protein available for absorption depends on the type of protein consumed, the energy available for microbial protein synthesis and the protein content of the diet. Protein deficiency has been associated with grazing sub-tropical perennial grasses during the dry season and with grazing dry annual pastures in summer.
Table 1 demonstrates the low protein content of perennial grasses (in autumn and spring) and annual ryegrass (in autumn). Introduction of annual or perennial legumes or improved nitrogen nutrition would alleviate the protein deficiency in a perennial grass system.Alternatively protein could be provided through grain legume supplements (such as lupins) or a protein lick.
Crude protein is estimated by analysing a pasture sample for nitrogen content and multiplying the resulting figure by 6.25 (as protein generally contains 16% nitrogen). However, this crude calculation may over-estimate the true protein in feeds due to the presence of some non-protein nitrogen compounds (e.g. nitrates, betaines).
These non-protein compounds may be converted into microbial protein in the rumen, but the extent to which this occurs depends on the availability of energy from the diet. In the absence of sufficient energy, the compounds are converted to ammonia in the rumen, which is absorbed by the animal, converted to urea and excreted in the urine.
Sulphur is primarily used along with nitrogen for the production of microbial crude protein in the rumen. The recommended dietary N:S ratio for sheep is 12.5:1. Perennial pastures such as saltbush provide high levels of sulphur.
Vitamins and minerals:
Livestock also have minimum requirements for a range of minerals and vitamins. In summary, naturally occurring deficiencies of copper, selenium and cobalt (vitamin B12) will occur in some regions if supplements are not provided or pastures have not been fertilised with the appropriate trace elements.
For selenium and cobalt, deficiencies are most likely in the medium to high rainfall areas in spring. Deficiencies will result in reduced growth and wool production and in extreme conditions incapacitation and death. Deficiencies of calcium can also result if livestock are fed high levels of grain supplements. Responses to zinc supplements have been reported in grazing sheep.
Of the essential vitamins, only vitamin E deficiency is consistently of commercial importance. Vitamin E (-tocopherol) is a powerful antioxidant present in green plants. Deficiency can cause nutritional myopathy in weaner sheep, especially when they have grazed dry feed for an extended period.
Sub-clinical nutritional myopathy is not detrimental to liveweight gain or wool production and can often go unnoticed, but clinical deficiency may cause the death of the animal. Perennial pastures offer significant opportunities to supply young animals with vitamin E when they remain green over the summer and autumn. Weaner sheep grazing saltbush have been found to have significantly higher vitamin E levels in their muscle and liver tissues than sheep grazing stubbles.
Additional secondary compounds that impact on intake and nutritive value:
Increasing sodium chloride (salt) in the diet decreases feed intake, digestibility, liveweight gain and wool growth significantly. This is particularly relevant to plants growing in saline soils where the salt content of the leaves of halophytes can be up to 35% of DM.
Condensed tannins (CTs) are a large and diverse group of compounds, present in a number of species including tagasaste, sulla, Lotus and Acacia spp. They may have a positive contribution to animal production at low concentrations (0.2% or less of dry weight) through resistance to plant disease and pests and bloat control. They are also thought to assist in the control of internal parasites in lambs, but this is yet to be demonstrated under Australian conditions.
However, at concentrations of more than 1% CTs bind protein – protecting it from ruminant digestion. At concentrations more than 5% CTs reduce the voluntary feed intake and digestibility of the herbage. Tannin concentrations in herbage vary with plant genotype, season, soil fertility, soil acidity, water stress and temperature.
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