Do sheep always feed their lambs?

No. Sometimes when a sheep does not recognise one of it's lambs it will not feed it. Indeed it may attempt to harm the lamb by butting it with it's horns. To rectify this situation the farmer uses a procedure called 'mothering on'. The mother sheep is tied to a post by it's horns so that it cannot turn on the lamb which can then feed from the mother without being harmed. Here are pictures of the 'mothering on' procedure.