Non-ruminant animals like humans and pigs cannot properly digest grains
because they lack the necessary bacterial enzyme, phytase, to break down the
phytates in grains. Bacteria in rumens produce phytase, but even ruminants
fare poorly when their diets include too much grains and not enough
non-grain grasses, as reported by Michael Pollan of the New York Times:

 "Perhaps the most serious thing that can go wrong with a ruminant on corn
is feedlot bloat. The rumen is always producing copious amounts of gas,
which is normally expelled by belching during rumination. But when the diet
contains too much starch and too little roughage, rumination all but stops,
and a layer of foamy slime that can trap gas forms in the rumen. The rumen
inflates like a balloon, pressing against the animal's lungs. Unless action
is promptly taken to relieve the pressure (usually by forcing a hose down
the animal's esophagus), the cow suffocates. 

 A corn diet can also give a cow acidosis. Unlike that in our own highly
acidic stomachs, the normal pH of a rumen is neutral. Corn makes it
unnaturally acidic, however, causing a kind of bovine heartburn, which in
some cases can kill the animal but usually just makes it sick. Acidotic
animals go off their feed, pant and salivate excessively, paw at their
bellies and eat dirt. The condition can lead to diarrhea, ulcers, bloat,
liver disease and a general weakening of the immune system that leaves the
animal vulnerable to everything from pneumonia to feedlot polio." ["Power
Steer," March 31, 2002, http://www.michaelpollan.com/article.php?id=14]

Birds such as chickens have gravel-containing gizzards that help them break
down seed hulls, but even chickens can develop nutritional deficiencies from
eating too much cereal grains. The species best adapted to consuming grains
are apparently smaller organisms such as the grain weevil, aphids,
armyworms, cereal leaf beetles and Hessian fly maggots (rodents are also
often listed as natural grain eaters, but they have to wait for grain seeds
to fall to the ground, so my guess is that, in the wild, grain provides a
much smaller part of rodent diets than insects and other small organisms).
Does anyone know how these small organisms are able to digest grains? Do
they use phytase? Also, does anyone know the purpose of phytase in
ruminants, given that they can only handle small proportions of grains in
their diet--that is, did they adapt evolutionarily to digest SOME grain, or
do non-cereal grasses also contain phytates, just in smaller concentrations?
I was unable to find any information on this.

Thanks,
Paleo Phil