I am afraid of flax, and I know a lot of people swear by it.
Maybe a coincidence but a few years ago I thought I would try flax for conditioning and a shiny coat,
I had three mares abort at around 3 to 4 months gestation. All within a few weeks after starting the flax. It was the only change we had made so I stopped the flax.
I asked on this forum about that issue and all responses felt it had nothing to do with the flax, however, I have never tried it again.
I do use a small amount of canola oil in the hot mash we feed twice daily.
(1 feed scoop beet pulp,1 feed scoop sweetfeed,2 feed scoops oat pellet filler and about a cup of oil, then hot water to make the mixture increase 2 times it's size) I feed all the horses with a 2 cup container, some getting a bit more and some a bit less depending on their accomodations, i.e. if outside more, if older, if younger, etc.)
I watch their manure and also find that on colder days I worry less about them not drinking enough as the mash is pretty sloppy and they lick it right up.