best supplement ?

When it starts getting colder I plan to give the horses a little something extra to help them out.
I'm in TX so it usually doesn't get veryyyy cold here but since I obviously don't know how the hay is going to be through the winter and I have a couple that are kinda hard keepers + get ridden, I know they're going to need a boost.

I've had all kinds of suggestions, but don't know what is going to be best.
There also seems to be quite the negative side to most of these.

Right now they are on coastal hay almost 24/7. Eh they have a few hours in the morning they might run out.
And a 12% feed. (+ mineral blocks and salt always available.)
They are not fat right now. Most you can can't see the ribs but are easily felt.
My mustang being a hard keeper his ribs are slightly visible.

They have never gotten 'hot' on anything I've fed before. That's not really an issue.

So, I'm wondering.. What do you feed your hard keepers through the winter?

My mare lives outside in wi year round. In the winter the horses get free choice [crappy] hay. She gets fed 3lbs of strategy daily. She is a pretty easy keeper and does just fine on that. The rest of the year she gets 1lb of strategy.

Thanks!
My guys are kinda hard keepers. They're not really on grass, just the hay. It's green right now but like I said, it's going to suck if it's like last winter again. Upping the hay amount would be hard as they're on so much already.

Rice bran, corn oil. Beat pulp,
Not sure which would be the best ..
There's an 8 % fat feed available I might be able to switch to if needed.