Pretty much an old wives tale here in western U.S. Many old timers won't integrate the pastures because "that's the way it's always been". However studies show that pastures actually improve due to the different grazing habits and physiology. There are some parasites and diseases that could possibly be passed from sheep to cows, but those are pretty well taken care of with good herd health. It is important that the pasture be large and well managed, however...