Evenly spread over an entire field they wouldn't have much of a choice would they? I see lots of dairies that cover their entire farm and the cows seem to graze away. Apply it to short grazed or mowed grass (4 or 5 inches). Then allow the grass to grow out. Don't apply it to 12 inch tall grass and expect them to eat it. That wont work, I don't care how much it rains.