All my girls are out on acres of grass with their babies when it comes to weaning time. I simply remove the foals and put the girls back out with their friends. Udders get full and then soften in a couple of days. Never had a problem, but I think some folk have difficulties because they a) wean too early when the foal is still taking quite an amount of milk from the mare, therefore the mare is 'geared' to produce more, and b) because they keep milking 'just a little' from the mare.
While, as has been said, I would cut out any 'extras' from your mares diet, but certainly wouldn't cut down on any of her basic rations.
Anna