I feed purina strategy healthy edge. My mare is chubby as hell.
make sure your not feeding it all in one go - she won't put the weight on and it could be detrimental on her digestive tract in such quantity, the best would be splitting it up into 4 feeds a day,however understandably this isn't always possible so 2 times would probably do too. I'm not familiar with the feed you mentioned but I personally find that broodmare mix - feed for pregnant or lactating mares is an incredible fat builder for the skinnies :) also flax seed oil is good or any type of oil - we used to buy the budget canola cooking oil and that did wonders to the condition and coats of our thoroughbreds who were a bit harder to keep. Also I find that 'fiber protect' has more nutritional value than the more plain types of alfalfa or chaff and helps with weight too. hope this helps