You'll be splitting the concentrates into two feedings, yes? You might even consider thee feedings a day if you have time. I don't think you can go wrong with beet pulp. However, she really needs all the good grass hay she can eat, IMHO.
My mare (who's really a pig in disguise) gets fed the following:
Morning and evening:
1-1/2 scoop alfalfa pellets
1-1/2 scoop Paddock (It's a locally milled feed for broodmares and performance horses - it's probably very similar to yout Ultimum Growth)
1 scoop soaked beet pulp
1 - 1/2 scoop alfalfa pellets
1/2 scoop Paddock
I've never weighed it, but the scoop is a standard 4 quart scoop. Dancer foaled 06/28/10 and is in a 2-3 acre pen, but the grass is getting pretty sparse because she's shares it with daughter's mare, Mystie. (currently nicknamed Auntie Mystie because she's appointed herself chief babysitter to Dancer's filly, Rain.) We keep a round bale of good prairie hay in there at all times.
It may sound like a lot of feed, but - we had an "incident" while we were on vacation - the person who was paid to feed our horses for the two weeks we were gone...well, didn't. The horses lost a LOT of weight - especially Dancer. Guess the unborn foal pulled her down faster.
At any rate, that's why she's fed that much, but because we are feeding her so much, it's split into smaller feedings. She's gaining weight slowly but steadily.
We also found a source of pelleted beet pulp and are trying it out on our other horses. The jury is still out on that because the pelleted version contains molasses. (The horses love it of course, but we don't know if it will make them hot or not.) The shredded beet pulp we've been feeding has molasses, too, but not very much and we drain the liquid off of after soaking, which removes a lot of the molasses.
Plain Old Dee, horses Dancer and Rain I believe in dragons, unicorns, good men and other mythical creatures!