Different horses respond to different treatments. I take care of horses professionally (I have 7 barns I do chores at every day and also barn-sit, exercise horses and do veterinary follow-up care). In some cases I've dealt with, the horse does fine on senior feeds, in other cases they don't. One reason my clients stick with me is I'm willing to work with them to come up with rations customized to work for their individual horses. In the 30 years I've been in the horse business (17 years doing this particular work) often the weight gain problems are not resolved by feeding senior unless it's fed as a complete feed or in combination with other feeds that have sufficient calories. I'm not saying feeding Senior is a bad thing, it just does not work for every horse.
Chip (Earen's mom for 23 years; Cheers' mom for 12)