I feed Purina Strategy all year long with their grass hay. When the mare is in her last trimester I switch to Purina Omolene 300 for lactating mares and foals. I'll feed that through the last trimester and then for 4 months or until I wean the foal. It's 16% protein and has vitamins and minerals that the pregnant mares need for optimal lactation. I also feed Vitamin E & Selenium because our area is known to be very Selenium deficient. I noticed very quickly that if I didn't feed that supplement through the 3rd trimester that the foals were born very down at heel and that they needed longer to tighten up than foals whose dams were supplemented. I also supplement the mares with Fast Trak Probiotics, I think it helps them utilize their food better than when they aren't on it. Not as much poo, better texture and not as much stink.
If you can't get a specifically designed feed for Mares and Foals, then I'd formulate a 16% protein ration and add a good multivitamin and probiotics. I wouldn't add the E-Se supplement without finding out if your area is deficient, too much Selenium can be poisonous.
An aside about foal heat: There's a less than 50% conception rate on breed backs on foal heat. I personally don't do it, I like them to have AT LEAST 30 days to heal and get back to normal before I'll even think of doing a re-breed. Since my stallion is NSBA nominated, I'll breed back in March for another Feb. Foal but even so, I'll wait as far into the month as I can to give her time to regroup. If I don't feel she's optimal for re-breeding, I'll wait til April. Due to heat of summers here, I prefer to be done by June 1. If I have a mare open after June 1, I tend to let them have the year off. I like to let them have a year off in between every couple of babies anyhow.
And then we have a foal feed - and i'll have a look into the selenium - I haven't a clue about it - thanks
At least I don't have to worry about the breeding concept - we've no stallions in site and i'm planning on this being her last foal x