This is a question that I would love to--and will--go into a lot more depth and detail in the Keeping & Raising Cows Blog in the future.
That said, basically cattle need decent quality roughage in the form of hay and/or pasture plants to graze. These are primarily grasses, legumes such as alfalfa or clover. A diverse diet that also includes forbs is also beneficial as a "salad bar" for a cow.
How much a cow should get largely depends on her body size. The larger the cow, the more she'll eat, and vice versa. Cows consume an average of 2.5% of their body weight per day in forage on a daily basis, on a dry-matter basis. (This is purely average. A cow will consume anywhere from 1% of her body weight to 3% of her body weight on a daily basis, depending on her nutritive needs and the quality of the feed.) For example, a 1000 lb cow may consume 25 pounds of dry-matter forage per day.
Dry matter is where all the water is removed from a forage sample. This is usually done when various feeds are sampled at a laboratory for their nutritive quality. Thus, when water is included in a feed sample, we call it "on an as-fed basis," or the moisture content is what it is when it is being fed to a cow; all the water is not completely, 100% removed as it would be in the feed-testing laboratory.
For instance, if the small cow in our example is being fed good quality hay, and the hay is 10% moisture (90% dry matter), she will be consuming approximately 27.8 pounds of hay on an as-fed basis. If the cow is eating pasture at 60% moisture (40% dry matter), she will be eating 62.5 pounds per day on a dry-matter basis.
The Bovine Practicum Q&A "blog" is an informative, just-for-fun section where I find a variety of questions that are often (and not-so-often) asked by inquisitive people like you, and answer them to the best of my ability. Much of the questions have come from a wide variety of reaches from the Internet.
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