When starting any new feed you should always work your way up to the certain amount and change feeds slowly. If you do it all at once, it can cause colic and founder problems.
To add some weight onto my mustang mare who us 18 y/o and looked at a healthy weight everywhere but her ribs and by her withers stuck out and up. I was constantly told this was typical for older mustangs to look this way but that she was at a good weight, I refused to leave it be at that and all I had to do was add beet pulp to her current diet and it is covering up her ribs and withers in very little time.
She is eating
2lbs of tribute essential k ration balancer
20lbs of local grass hay
4lbs of beet pulp, soaked
Strongid c2x(once daily)
This is broke into 2 daily meals and when I first started trying to put some extra weight on her I fed a daily probiotic for a month. I used the probios powder, which some people do not like but it seemed to actually help out quite a bit with how much she was actually getting out of the feed she was eating. You can tell now that she is off of it that she is not gaining as quickly as she previously was.