By this time of the year, grass - even if present - has lost a lot of its nutritional content. Pregnant ewes need more than that! So yes, feed good hay or haylage, and in the last 6-8 weeks, unless you
know the forage is giving them enough, also feed pellets / cake of some sort - or feed blocks. In my experience, winter grass plus hay plus licks is not enough for twin-bearers (due March / April) in the northern uplands
.
Note that if the ewe is depleting her own resources to feed the growing foetuses, you may not see all of the effects in her condition score, and she may suffer metabolic problems - twin lamb disease and the like. So feed plenty of glucose (molassed licks are a good source), and if in doubt, a little cake, in the last 6-8 weeks, or risk problems such as TLD, difficult lambings, insufficient milk, etc, etc, etc. Of course,
too much cake can produce over-sized lambs and difficult lambings, so you do have to find the right balance for your sheep, land and conditions!!