Even at this time of year she shouldn't have a rough-looking coat. Check the skin underneath - is it flakey, dry? A good shampoo (on a warm day and rugged up straight afterwards) will take off all that winter dirt/dandruff and also help to get rid of her winter coat. A good horse brush is helpful and they love a good brush. Using gardening gloves (with some grip on them so they are rough) and just rub all over... you will have her going into a trance!
I guess she does NOT have lice or some kind of mange (check her heels for scabbiness - if yes treat for heel mange with Eprinex).However, I would give her a mineral/vitamin booster - either Caprivite (fi she takes it, most of mine don't anymore) or a sheep drench containing copper. Sprinkling garlic granules and seaweed on their (damp) feed - start with really small quantities and work up.
Also branches are welcome at any time of the year - mine love to peel the willow branches right now, but anything with green bark on will be liked, and there are weeds already out - docks, dandelion and chickweed. Just vary her diet a bit as well - apples, carrots, any kind of brassicas (in moderation if she is milking) etc etc