Monday, 15 October 2012

Farm animal nutrition: Proteins

Proteins are needed for balancing pH, liquids, nitrogen and acids in the body. They are also needed for cell to cell junctions, immunology, signal transmission, genetic regulation and entzymatic reactions. Proteins are built in the cells via DNA translation, when RNA sequences are used to build chains of amino acids.

Proteins are organic compounds, built from amino acids, which are linked together with peptide bonds. So again we have basic building blocks (~20 amino acids), which can be combined to create an immense variety of peptins (short amino acid chains) and finally proteins (usually they have 100-300 amino acids). Animals need proteins form their diet to form amino acids. Out of the 20 amino acids, nine are essential for most animals (the list varies by species):
  • threonine
  • methionine
  • isoleucine
  • leucine
  • valine
  • tryptophan
  • phenylalanine
  • histidine
  • lysine.
Picture from lecture materials, original source unknown
Animals can synthetize the other amino acids by deamination and transamination. Deamination means removing the amino group from the acid, and transamination means attaching it to a suitable carbon molecule. Often this doesn't create enough of the needed amino acids, so adding them to the feed may be required. The amino acids, which are presently not needed in the body, are deaminated, transferred to the liver and secreted in urine / uric acid. Thus feeding the animals too much amino acids increases the amount of nitrogen in their excrements.The carbon skeletons of the excessamino acids are stored as carbohydrates or fatty acids.

Farm animals fed with corn-based diet often have deficiency of lysine and isoleucine, but enough tryptophan and methionine. With legume-based diet there's little tryptophan and methionine, but enough lysine and isoleucine. All amino acids contain approximately 16 % of nitrogen. Thus the amino acid (or protein) amount in an animal feed can be estimated by determining the amount of nitrogen in the feed. Even after careful animal breeding, the ingested vegetable protein to animal protein ratio in farm animals is still only approximately 30 %. Most of the proteins are used in metabolism, tissue reformation and for producing offspring.

Ruminants and amino acids

Again, ruminants have their own ingenious system for creating and using amino acids. The microbes in the rumen use simple nitrogenous compounds from the feed to build amino acids for their own needs. These acids become a part of the microbe. When the microbe dies, it is transferred with the feed mass to the stomachs and to the small intestine, where the microbe is digested, and its amino acids are absorbed through the intestinal wall. This is called microbial protein or rumen degradable protein.

Animal feed may include some (mostly artificially coated) amino acids or proteins, which the rumen microbes cannot use. These particles move to the small intestine, where their coating can be broken down and the amino acids can be used. Because these proteins are not digested in the rumen, they are called bypass proteins.

Picture from lecture materials, original source unknown
Another term closely related to proteins is NPN, non-protein nitrogen. These are often created by decarboxylation of amino acids, and the NPN compounds may be toxic. Some NPN compounds form in plants, for example sinapin in rape and betaine in sugar beets. Urea and uric acid are also NPN compounds.

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