How do cows eat grass with out being sick?

Answer 1

Their digestive systems are built to handle forages and roughage and use them adequately for their bodily functions. Certain microbes in the digestive system help break down the fibrous material, and the four different chambers in the stomach help with processing the forages in four different stages within that stomach. Cows don't get sick eating grass, they never have and never will.

Antiquality factors such as bloat, fescue toxicity, grass tetany and nitrate toxicity come up but that's mostly associated with the quality or lack there of in the grasses that make them sick. Bloat is caused by high levels of protein in the young grasses or in legumes, and mostly caused by turning animals out on a fresh pasture when they are hungry. Fescue toxicity is caused by a fungus (ergot) that is commonly grown in Tall Fescue, and makes for foot, ear, and tail gangrene in cattle if eaten in excess. Grass tetany is a lack of magnesium in the grass, which, in the cow, leads to symptoms of magnesium deficiency which kills quickly if not treated quickly. Nitrate toxicity occurs after a bad case of frost or drought and nitrates build up in the grasses. Too much nitrate in the cow causes illness which also needs to be treated immediately.

But as for grass being just grass without the antiquality factors I mentioned, ruminants were always designed to eat grass. It's just the way they've evolved over thousands and thousands of years.