Typically, you only find grain overload in animals that are penned and fed improperly (not enough hay but heavy on the concentrates - a very expensive error; hence, not common) or in animals that escape their pens and find their way to an unsecured feed room (this, unfortunately, occurs much more often). Also, grain load comes into play around the first weeks prior and after parturition. I agree with CJB that it is the sudden complete switch in diets that cause more issues...
