It all depends on your deer numbers and the amount of other natural foods. If your property is short on food but has decent numbers then an acre can get gobbled up pretty quick. William is correct--when I used to plant cowpeas I always added some sorghum into the mix to allow the peas to climb. This really increases their ability to produce forage. The problem occurs when the deer nip 'em right out of the ground in the seedling stage.