so I have been letting the cows graze my alfalfa/orchard grass hay field for a few hours a day then take them off where they have accsses to dry hay. we did get a frost , do I need to worry about that or do I need to keep them off the hay field. I ask cause I have heard not to graze for a few days after a killing freeze. I am confused, what is considered a killing frost and do I need too even worry about it????