My mare is in heat and got into the stallions paddock. No one was home while she was in there so I'm not sure if he got her or if he did, how many times. Is there anyway I can tell before the next time she comes into heat? If she is pregnant any vitamins or supplements I should put her on?
Talking from experience, that's all it takes, just get the 2 together, they'll take care of the rest. If you know the exact date she was in with him, or dates, go approximately 17 - 21 days out from that date and have her palpated and ultrasounded to find out if she's pregnant. If you reallllllly don't want her pregnant, she probably is. The earlier you get her checked, (you can even have her checked at 14 or 15 days but that's not as fool proof) the easier it is to terminate the pregnancy if she's pregnant. Call your vet tomorrow, he might be willing to give a shot or just flush her at some point without knowing she's pregnant, kind of a "just in case" kind of thing. You'll still need to check her to be sure though.
Feed, vitamins, and all that, don't change much until the 2nd trimester when she starts to lactate and the foal puts on the last growth spurt. I keep an eye on my mares to make sure no one loses a bunch of weight once they're pregnant, and I just keep them in good flesh and I give a daily vitamin anyhow. A good, daily multivitamin would be all she'd need.
If you decide to go through with the pregnancy (if she's in foal) then ask your vet about how selenium deficient, or not, your area is. Here, I give a Magnesium, E and Selenium supplement because we're known to be pretty much ZERO for Selenium.