Why shouldn't cows eat corn?
Grain, including corn, barley, wheat, soybean, and rye, is an unnatural diet for cattle. Cattle have evolved to be able to efficiently digest a forage-based diet of grass and hay. When fed a grain diet, besides the benefits of increased weight gain before slaughter and boosting milk production in dairy cows, the bad often outweigh the good, as I mentioned above. When the bad outweigh the good, this means that a grain diet in cattle must be mixed with antibiotics and other additives to decrease the incidence of bloat and acidosis. When slaughtered, many animals that have undergone acidosis (acute and subacute) are found to have lesions or abcesses in their livers. Livers then have to be thrown away, as they are not suitable for food production. Trace amounts of antibiotics can also be found in meat from an animal that has been on a grain-fed diet.
Corn, which is quite high in starch, protein, and carbohydrates like many grains are, is considered a "hot ration" for cattle, and is digested very quickly in the rumen. This quick digestion is often too fast, resulting in maladies like acidosis and bloat. When any of these things happen, most often the animal goes down fairly quickly and dies just as fast. Acute acidosis is the main cause of death in many feedlot and corn-fed dairy cattle, and occurs when the body is overwhelmed with the spike in acid production in the rumen. Those that don't succumb to acute acidosis wander aimlessly around or cannot get up; after they are treated (with thiamine), they can also get what is called chronic acidosis, among other maladies like founder, bloat, and polio. Founder (inflamation of the laminae in the hooves) is often a precursor to acute acidosis, resulting in permanent lameness in the animal. Feed-tested bulls commonly have this problem more than any other animal on a grain-based diet. Bloat is another main cause of death and concern for cattle that have eaten a grain ration; bloat happens when protein digestion in the rumen goes hay-wire, resulting in froth or bubbles of gas that constantly build up in the rumen and are unable to be released through belching. It is often caused by feeding too much grain too soon, and if not treated quickly, the distended rumen, expanding all the time with the digestion of starch, protein and CHO, puts pressure on the lungs to the point where the animal suffocates and dies. Bloat is also found to be a precursor to acidosis. Polio (polioencephalomaladia) is a neurological disease that is often a result of a deficiency of thiamine. Thiaminase, an enzyme that breaks down and "uses" thiamine, is often put into high gear when the animal is digesting a high-grain ration which decreases the pH level in the rumen. Hence polio is also linked to acidosis in grain-fed cattle.