Why do cows get fat if they only eat grass?

Answer 1

There are a lot of factors that come into play: nutrient quality of the grasses, forage conversion ability of the cow, climate, topography, soil, type of grasses being grazed, time of year, stage of grass growth, stage of life cycle of cow (pregnant and dry, pregnant and lactating, lactating), age of cow, etc. Putting all those factors together, cattle can get fat on grass because that grass contains most of the necessary nutrients (except for water and some minerals that needs to be supplied by a separate mineral supplement) that the microbes in the cows' rumens need to feed the cow and maintain her health and well-being. Grass that is growing often contains enough protein, energy, calcium, phosphorus and other nutrients to feed the microbes which feed the cow and which puts weight on a cow.

However, cattle who are on grass all the time aren'tnecessarilyfat. Since they tend to live to eat instead of eating to live, the energy, protein and other nutrients in the grass that the cattle don't need right away are simply stored as fat. The other thing that makes cows look "fat" on grass is how big their bellies get when on grass. This is simply the case of the rumen being large enough to digest all that roughage being eaten, which is a fairly good indicator of forage conversion ability.

Grass quality is the determinant of a cow's ability to gain or maintain her weight. Higher quality in grasses is needed for lactating cows with calves, as well as first-calf heifers and growing steers and bulls. The more marginal quality grasses are suitable for dry pregnant cows, since these grasses don't put more weight on the cows, nor does it make them loose it.

It should be interesting to note that not all cows get fat on grass. Dairy cows such as Holsteins or Jerseys that are raised predominantly in a confined feeding operation and selected to gain weight or milk readily on pre-determined rations that are fed to them often will lose condition or starve to death on grass that is considered by beef producers to be "good quality." Genetics plays a big role in a cow's ability to "do well" on grass. Thus if dairy cows are taken straight off a "factory farm" and put on grass and expected to do well on it--or better on it than what they were raised on in the conditions they were so "cruelly" kept in--it would come to a huge and very saddening surprise to find that those dairy cows will need to be heavily supplemented with grain or other high-quality feed otherwise they will literally starve to death.

Answer 2

No. The feed that they are fed may also contain forbs or legumes like alfalfa or clover which are not grasses. Some oilseed grain that they're fed, such as beet pulp, soybean meal or canola meal are not from grass either. However, 90% of the feed or pasture that they eat are of or from grass.

Answer 3

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.

Answer 4

Because grass contains a lot of nutrients, and the nutrients like energy and protein that the cow doesn't need gets stored into their fat reserves for the winter ahead. Cows should be fat on grass because that means that they are healthy and thriving, and the grass is going good for them.

Answer 5

A cow's four-chambered stomach allows her to digest the grass thoroughly enough that she can get plenty of nutrients from the grass without any other supplementation, provided the grass is of fair to excellent quality, and not poor quality.

Grass is a natural diet for cattle that contains a wide variety of nutrients and minerals for cattle to live off of. Most farms, though, are located in an area where a lot of minerals from the soil are lacking or almost non-existent, and mineral needs to be fed to the cattle to keep them healthy. Otherwise, grass is the cheapest and more nutritious source of forage that cattle can have. Nutrients from the grass is digested in the rumen through the process of fermentation and digestion from the micro flora that live in the rumen. This digestion process allows some of the protein and energy to be absorbed into the bloodstream through the stomach wall. Other chambers (omasum and abomasum) further digest the matter and move it into the small intestine where most of the nutrient absorption occurs.

Answer 6

Because their stomachs are built to digest roughage like grass and squeeze as much nutrient out of those grasses as possible. That's the beauty of being a ruminant: fore-gut fermentation is the only way that the tough plant fibres and cellulose can be broken down to get the nutrients out of them.

Answer 7

The primary reason cattle are fed grain is to gain weight and put on fat, whether it's for slaughter or because a cow (mature female bovine used in a breeding herd) is too thin and needs the extra energy to sustain herself.

Answer 8

because it tastes very good to them.