Perhaps it is because humans and dogs have a faster GIT, I'm not sure on that one.
I am NOT picking on you Ron - you just get the synapses firin'.
OK, here's the scoop (warning, long and not exactly dinner talk):
Ruminants (cow, sheep, deer) are equipped to digest cellulose via a system of stomachs, specialized gi flora, and highly specialized dentition (cutting apparatus up front, heavy duty grinders in back). They chomp grass, swallow (they do not chew at this stage), then the rumen mixes the chopped plants with a soup full of probiotics but NO gastric juices (the rumen is not a true stomach). The animal fills its rumen, then stops grazing for a bit. During that time, the rumen will send back up the glob of plant material and bacteria (plus some gasses produced by the probiotics). The animal chews, chews, chews, chews and chews, swallows, then the probiotics go to work on the newly exposed cellular matter. The glob may be sent back up four or five more times (along with more gas), then the last time it goes into the stomach, having been converted to material the stomach can work with - sugars, protein chains, complex carbs, and a little fat.
In a ruminant, grain, for the most part, bypasses the rumen and goes straight to the stomach. The rumen, sitting idle, begins to shift the gi's ph down towards the acidic side, which in turn makes it difficult to digest the protein in the grain, which activates the pancreas to produce more acid, and so on until the animal dies of massive indigestion. This situation only arises when grain is fed exclusively, long term, but it is inevitable. To prevent this, feed lots lace their feed with two different antibiotics - one to kill the rumen bacteria and help fend off salmonella and e.coli, and one to prevent a disease which results from standing around in highly acidic manure.
We can use a small amount of grain-based feed (no more than 10% of the diet) to manipulate breeding cycles and maintain condition during stressful situations, but good grass or hay is always essential to have on hand. A sick sheep (or one in labor) will not eat grain, but will nibble hay or grass until literally unconcious.
http://faculty.fortlewis.edu/LASHELL_B/Nutr2-Rumdigestion.pdf#search=%22ruminant%20digestion%22
Non-ruminant herbivores (monogastric) have a specialized area in the large intestine (cecum), where cellulose-digesting biotics work to break down plant material though a slow fermentation process. The digestive system up to this point is very short and very rapid. The stomach is comparatively small and nearly useless as a storage area - anything eaten goes right through. It is VERY important that soluble carbs such as those found in young tender grass, and grains of course, reach the cecum fermentation area in VERY small amounts.
http://www.merricks.com/digestion.html
The fermentation process is indiscriminate. A large amount of soluble carbs reaching the lower intestine all at once will kill a horse extremely quickly when gas (see above) is produced and has nowhere to go. Unlike with ruminants, modern science has not figured out a way to force horses to thrive on grain alone. As we know, however, horses can thrive with judicious use of grain to enhance performance, with hay or grass making up the bulk of their diets.
People have a similiar ability to absorb some nutrients in the large intestine, but we have a much slower upper gi which is very difficult to overwhelm. We have an extremely large stomach capacity for our size which can reserve a huge amount of food and only complain a little - or if we REALLY overdo, we can throw up (horses cannot). You can eat your cake and have it too. [
] Humans have a tremendous capacity to abuse their systems, which is probably a highly adaptive trait if you think about it. We can survive in the widest range of climates of any large mammal. We have a longer lifespan to average out our nutritive choices.
Dogs can also vomit (think of how important this is for a niche feeder) and have a comparatively large true stomach. Food material sits in this chamber for much longer than it does for people, allowing for a longer period to break down proteins and fat (see ruminant, true stomach, above). However, their gi is much faster (similiar to the horse), but they lack, for the most part, the large fermentation area in the cecum which causes problems for the horse in absorbing soluble carbs quickly.
I don't know what implications you can draw from this comparision. I know that nutrtionists are moving away from emphazing grain in any herbivores other than high-performance/high-production animals.
http://ohioline.osu.edu/gsg/gsg_1.html
Even among some of those camps there are movements to go to completely grass-based feeding programs. More and more of your milk will likely come from managed grazing operations, where dairy cows (there is no higher-need animal on earth) are fed nothing but grass and hay all year.
[linkhttp://tinyurl.com/ejb6p]Here's an article on grass-based production.[/link]