It is normal that a ewe will lose weight during lactation. In fact, fat ewes at weaning time should be eyed very suspiciously, their milk production may be low. The key is not to let the ewes lose so much weight that they cannot regain it during the maintenance period. If the hay is low in protein then the diet may need supplementing with a protein supplement, soybean meal, cottonseed meal, etc. to get the protein content increased. Again, water and mineral are crucial.
Many producers lamb in the spring and then run the ewes and lambs on good spring grass.
This system works well, but remember the high nutrient requirements of the ewe during lactation and the often inconsistency of forage quality and quantity between years. Certainly, the weather in the spring will determine the amount of spring forage available.