It is always best to keep the cows out of the acorns. They love them and they will almost starve to death eating them. It is very important if you have them in the acorns that they have alot of good green grass, so they will still eat some grass. The biggest problem with acorns is that they pack into the stomach if there isnt enough grass to cushion and the sharp points scratch the stomach lining and then it becomes infected. They get cut up and when that happens the stomach doesnt digest and keeps packing tighter and tighter with the acorns and the infection and lack of nutrition kills them. Usually, by January the acorns and frozen and swollen and weakened enough that they wont be as likely to do the damage as they can when they are green. I have several pastures that have acorns and cant keep the cows out of all of them, but it is a worry