Cows on pasture rarely get sick.
This is in direct contrast to animals in a feedlot. An article from the May/June 2000 Feedlot Magazine states "nearly every animal in the feedlot will experience sub acute acidosis at least once during the feeding period." This is a direct result of the feeding of grains to cattle.

Let them eat grass!

Although most feedlot diets supply enough nutrients to satisfy minimum vitamin requirements, mistakes do get made. In an incident reported in a veterinary journal, cattle being fattened in a feedlot were fed a diet deficient in vitamin A. (The vitamin had been added to the rations, but had been destroyed by heat and humidity.) Deprived of this key vitamin, the cattle suffered blindness and convulsions. Interestingly, heifers fed this same vitamin-A-deficient diet were free of symptoms, and, when tested, were found to have adequate levels of vitamin A in their blood. The researchers were puzzled until they discovered that the heifers had been able to forage on sparse grasses and weeds found along their fence row. Apparently, the grass was so rich in vitamin A that even these meager gleanings were enough to compensate for the vitamin-deficient feedlot diet.

("Divers TJ, et al, "Blindness and convulsions associated with vitamin A deficiency in feedlot steers." J Am Vet Med Assoc 1986 Dec 15;189(12):1579-82.")

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