As the food is just to get the sheep to follow you to another field, you can use anything that they like. I use chopped up carrots at this time of year which shake nicely in a bucket. However, if you frequently go into the field, call the sheep and give them a treat eg carrots, apple pieces, a few stock nuts in your hand, then they will come to call anyway and readily follow you and you only need to dole out a few treats at a time.