They have evolved, over hundreds of thousands of years, to have a digestive tract that is capable of digesting roughage and only roughage. A four-chambered stomach (or rather, several forestomachs) is this key to a ruminant's ability to break down fibre, cellulose, hemi-cellulose and lignin that is commonly found in plant material to get to the "good stuff," i.e., the energy, protein, minerals and vitamins that are in the leaves of the grass they eat.
Actually it's only part of the key. The real "heroes" in this situation is the enormous army of bacteria, protozoa and fungi that live in the anearobic environment of the cow's rumen. They are the ones that break down the roughage material so that the cow can get all her necessary nutrients in order to live and function normally. They are the reason that fermentation takes place in the rumen, because their ability to soften coarse plant material enough that volatile fatty acids, non-protein nitrogen, carbohydrate, vitamins and minerals can be released, used by them to keep reproducing and replicating, then used by the cow. VFA's and carbohydrates provide energy, NPN's provide protein, and vitamins and minerals provide, well, vitamins and minerals. These protozoa only have a 15 minute lifespan, so the turn-over rate in the rumen is huge, large enough that the spent bacteria, protozoa and fungi can be moved through to the abomasum and small intestine where they are used as sources of energy, protein, and other nutrients. Thus it should be remembered that a producer is actually feeding the microflora of the cow when providing feed, pasture, water and mineral sources; the microflora then feed the cow.
Since grass is usually swallowed whole, the cow needs to regurgitate the feed/grass that she ate and rechew it again, further aiding in the break-down process. The teeth grind the cud (of partly digested plant material), and the saliva adds more moisture to the cud. Once it's broken down enough she then reswallows it and it returns to the reticulorumen to complete the fermentation/break-down/digestion process by the microflora. Once the digesta in the rumen is digested as much as possible, it is moved into the omasum, which acts as a filter that absorbs all the "water" or liquid out of the digesta. That dried up digesta moves into the abomasum, the true stomach, to undergo further digestion in terms of digesting proteins and amino acids. The reticulum's main responsibility is to acts as a stomach where foreign materials are stored and broken down by the acid in the stomach of the cow. However, it too is an aid in the fermentation/digestion process of the GI tract of the cow.
One thing that you should know is that unlike one user stated previously (answer was deleted by the way), grass isn't all cellulose. Grass, especially that which is between the 4-leaf vegetative stage and when it is about to reach maturity, is comprised of water, carbohydrates, proteins, vitamins and minerals. It's only the cell walls of plants that have the cellulose, lignin, hemi-cellulose and fibrous material that makes it harder to break down for other animals like humans (which are monogastrics, not ruminants, by the way). Grass in the vegetative stage is highly nutritious for cows--especially if the pasture is getting proper nutrients from other nitrogen-capturing plants (legumes) like sainfoin, alfalfa, clover, etc., and being fertilized by not only the producer, but also from the feces and urine that are excreted by the cows themselves. Cows that have come out of winter on just hay actually gain weight on grass that is in the vegetative stage; it's that nutritious. Calves that are weaned from their mothers, if weaning time is timed right, also gain remarkably on only grass, with no supplementation from feeds like grain.
As the grass reaches maturity, however, nutrient level decreases. When cows are stockpiled grazed on pastures that have 99% of the grasses in a dormant state are getting some nutrition, but not enough to make them gain weight on it. Stockpiled pastures are best grazed by dry pregnant cows, not for lactating cows, growing animals (heifers, steers and bulls), nor for bulls that need to gain weight until the next breeding season.
Another thing to note is that cows cannot live on ONLY grass. They need a fresh source of water and loose minerals too, as often the soil that the grasses grow in is often depleted of different microminerals like Iron, Copper, Iodine, Manganese, Molybdenum, etc. Most fields/pastures have a good source of macrominerals (phosphorus, calcium, magnesium, sulfur, potassium etc.) but often need to be supplemented if there is a mineral lacking or missing.
One thing that you should know is that unlike one user stated previously (answer was deleted by the way), grass isn't all cellulose. Grass, especially that which is between the 4-leaf vegetative stage and when it is about to reach maturity, is comprised of water, carbohydrates, proteins, vitamins and minerals. It's only the cell walls of plants that have the cellulose, lignin, hemi-cellulose and fibrous material that makes it harder to break down for other animals like humans (which are monogastrics, not ruminants, by the way). Grass in the vegetative stage is highly nutritious for cows--especially if the pasture is getting proper nutrients from other nitrogen-capturing plants (legumes) like sainfoin, alfalfa, clover, etc., and being fertilized by not only the producer, but also from the feces and urine that are excreted by the cows themselves. Cows that have come out of winter on just hay actually gain weight on grass that is in the vegetative stage; it's that nutritious. Calves that are weaned from their mothers, if weaning time is timed right, also gain remarkably on only grass, with no supplementation from feeds like grain.
As the grass reaches maturity, however, nutrient level decreases. When cows are stockpiled grazed on pastures that have 99% of the grasses in a dormant state are getting some nutrition, but not enough to make them gain weight on it. Stockpiled pastures are best grazed by dry pregnant cows, not for lactating cows, growing animals (heifers, steers and bulls), nor for bulls that need to gain weight until the next breeding season.
Another thing to note is that cows cannot live on ONLY grass. They need a fresh source of water and loose minerals too, as often the soil that the grasses grow in is often depleted of different microminerals like Iron, Copper, Iodine, Manganese, Molybdenum, etc. Most fields/pastures have a good source of macrominerals (phosphorus, calcium, magnesium, sulfur, potassium etc.) but often need to be supplemented if there is a mineral lacking or missing.
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