When the grass starts growing consistently, for the haylage and I would judge the necessity for hard feed on the mare's weight and condition score, which should be monitored weekly.
We buy in haylage as a group on the yard but the YO has said, as of this week, we are not buying anymore. There are 7 horses and 2 ponies out 24/7 over 35 acres. I have my own supply of haylage I use for the hour or so I bring her in, to groom etc.
I have just downloaded, World Horse welfare's, guide to weight scoring, so i will check her tomorrow, I have a feeling she may be a bit overweight and if so, will gradually cut back on her hard feed.