Friday, 12 October 2012

Farm animal nutrition: carbohydrates

Carbohydrates may be frowned upon in some human diets, but for animals they are the main source of energy. Chemically carbohydrates consist of one or many saccharides (from Greek, sacchar: sugar), and mostly follow the formula (CH2O)n, where n >= 3. They can be classified according to the number of saccharide units in the carbohydrate:
  • Sugars
    • Monosaccharides
      • Trioses, tetroses, pentoses, hexoses, heptoses (3-7 carbons, respectively)
    • Oligosaccharides
      • Disaccharides, trisaccharides, tetrasaccharides
  • Non-sugars
    • Polysaccharides
      • Homo- and heteroglycans
    • Complex carbohydrates
      • Glycolipids, glycoprotein
Animals receive sugars from plants, of which 2/3 of dry weight is carbohydrates. Plants create carbohydrates via photosynthesis, and by adding minerals from the earth, they can create carbohydrates with sulphur, nitrogen or phosphorus. Carbohydrates important to farm animals consist of three saccharides, which can be thought of as building blocks: glucose, fructose and galactose. By joining these in chains, different carbohydrates of varied length can be created. In the digestive tract all carbohydrates are broken down into monosaccharides (the basic building blocks).

Cellulose and hemicellulose are both carbohydrates, which can be found from plants. Hemicellulose includes all the polysaccharides (except cellulose) found in the plant cell wall. It is a branched  heteropolysaccharide, while cellulose is a straight monosaccharide. Hemicellulose makes the plant cell wall flexible by creating bonds between cellulose and lignine. Cellulose is a supportive material, which gives plant cell walls strength. It's amount increases when the plant grows: young grass has very little cellulose, but straw has much. Ruminants have rumen microbes, which can break the structure of these molecules and use them as energy.

Starch is formed in the green parts of plants, and stored in roots, seeds and tubers. Starch consists of amylose or amylopectin, which is the most important carbohydrate in grains.  Starch and glycogen are reduced to maltose and glucose.

Glucose (also known as dextrose) occurs free in honey, fruits and plants. It is a component of many other carbohydrates, and very important for nervous tissue, energy and blood sugar. Glucose is the single energy source for the brain. Glucose is stored to the liver and muscles as glycogen (animal starch). Muscles use the glycogen when needed, but the liver can return it to blood circulation to maintain bloog sugar levels. All animals can generate glucose in the liver via gluconeogenesis, using propionate, lactate and glycerol as sources. For ruminants, this is the main source of glucose, since all ingested glucose is used by the rumen microbes. Glucose and fructose together form sucrose (also known as saccharose or table sugar).

Lactose is an oligosaccharide, which the mammary gland forms from glucose and galactose. Cow's milk has 43-48 g of lactose per kg; if the cow has glucose deficiency due to low-energy diet, it cannot produce milk well.

Fructans are reserve material in roots, stems, leaves and seeds of hays. They consist of fructose. In silage, lactic acid bacteria turns fructans quickly into acid, lowering the pH and thus ensuring the silage doesn't spoil. Ovedose of fructans may cause laminitis in horses.

Carbohydrates and ruminants
Ruminants, with their complex gastric system, are able to digest many complex carbohydrates.  Ruminating breaks the feed to fine mass. The surface area of the feed increases, which allows the rumen microbes to attach to the feed in great numbers, and to begin their enzymatic reactions. The microbes get ATP (energy) from reducing carbohydrates, and thus use the energy for their own actions. As a side product, the rumen microbes create volatile fatty acids (VFA). VFAs are absorbed into the ruminant through the rumen wall, and they in turn become the major source of energy for the animal. If pure glucose needs to be administered to a ruminant, it has to be given as an injection or infusion, not orally, since otherwise the rumen microbes would use up all the glucose, leaving none for the animal.

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