My horse has been on canola oil for a couple of years. In fact, he usually won't eat his feed without the oil on it. But lately he just stopped eating. I finally fed him with no oil and he's back to gobbling the feed down. So I guess he's just decided he doesn't like oil anymore?
One of mine stopped eating it as well as he did before. he didn't stop eating the oil, but wouldn't always finish. I stopped using oil and started adding amplify to his feed and giving him an extra flake of alfalfa. This has taken care of the weight and the barn owner prefers it to dealing with the oil.
My old mare did the same thing. In the last few years I've made so many changes to her diet...she loved her oil, quit that. She absolutely went nuts over her beet pulp - she quit that a year ago. She hated mashes, now she loves them. She never used to care about sweet feed, but about 2 years ago it was the only thing she would eat - that lasted about 6 months. That girl has thrown me for a loop so many times
I have one that would do it- I would back him down to a very small amount and then gradually build it back up over the course of several days. I finally switched to a fat supplement- Progressive's Envision- and that seemed to help.