Understanding Goat Digestion
No, they don't eat tin cans! The old myth apparently grew from someone watching them peel off the paper on the can to get at the old-fashioned, sweet glue. And they do nibble at just about everything.
Goats are ruminants, more specifically, browsing ruminants. The goat can digest fibrous and woody materials like grasses, leaves, even small twigs, such as she would find in her native habitat. This makes her a very efficient feed converter with a far larger forage capacity than cows or sheep (25-40% of body weight, as compared to 12.5-15% for cows, 12.5-20% for sheep), which are strictly grazers.
During the initial food intake, she chews rapidly, breaking down the forage only enough to swallow. In the rumen, the action of microorganisms begins the "digestion" of the forage and releases valuable nutrients. The goat later regurgitates small amounts of rumen content – the "cud" – consisting mainly of bits of roughage and fluids. She then chews her cud thoroughly in a leisurely fashion. Further action of rumen bacteria then helps to release more nutrients as the ingesta passes on to the abomasum and small intestines where nutrients are absorbed.
The normal rumen environment is slightly acidic with a pH of around 6.5. This is maintained by the buffering action of saliva. However, large amounts of concentrate fed to high producers lowers the pH to sometimes dangerous levels and also changes the proportions of the different fatty acids in the rumen: acetic acid, which is important for butterfat production, decreases, while butyric and proprionic acid levels rise. The result is low butterfat and off-flavors in the milk. Low pH will also kill of rumen bacteria as well as providing a favorable environment for harmful bacteria, such as Clostridium perfringens. As those bacteria die off with a further drop in rumen pH, they release the deadly toxins which cause enterotoxemia. Severe enterotoxemia can develop and kill quickly. Acute acidosis (rumen pH 4.0 to 4.5) can also kill the animal.
We often feed concentrates because toady's high-producing milker, in most situtations, cannot easily meet her energy requirements from forage alone, especially during peak production. Energy is derived from the breakdown of fibers and the starches and sugars thus released. The lower the quality of the roughage, the more she will need to be supplemented with grain to maintain that high output. Part of the available energy in feed (gross energy, GE) is lost through fecal (digestible energy, DE), urinary and methane (gas) output (metabolizable energy, ME), as well as for maintaining body heat (heat increment). What is left (net energy, NE) is first used for body maintenance (NEm), and then for things like growth (NEg) and lactation (NEl), which, in the overall picture, is very little.
The carbohydrates in grain are not as complex as those in hay, therefore are more readily available. Since they are also highly digestible, they stay in the rumen a much shorter time (the bacteria devour them much faster), i.e., the rate of passage is faster, leading to decreased rumen action which in turn leads to decrease in muscle tone (of the rumen) and shrinkage (loss of rumen capacity). This shrinkage causes further decrease in hay intake, which means she'll need even more grain to maintain her production. Thus a vicious cycle begins, leading to the above-mentioned change in rumen environment.
The second limiting factor in feeding is usually protein. Some grasses are very low in protein, so knowing the grass species you are feeding can help you evaluate your (her) needs in that respect. Generally, though, if energy is met in a ration, protein will also be met. (Exceptions do exist)
One big advantage of the goat's browsing habits lets her sustain herself and even flourish in overgrown pastures full of weeds. Such forage can also easily be brought to her. The only caveat would be the presence of poisonous plants which would need to be identified and eliminated before turning her out.