I can not comment on the horse question but I would caution you against too rich a diet, in corn or ddg's, to pregnant heifers or cows ahead of calving.

Are you sure you need more protein ahead of calving?

You may end up with calves so large that cows have a terrible time giving birth. Cows don't need that much protein ahead of calving.  10% protein hay with a good mineral and lots of good water seems to me to be all they need.  

I had two heifers I was going to process with the steers graze standing corn in their second trimester, changed my mind and decided to calve them out, took them off of corn about the start of the third trimester.  Their calves were very large and both had a lot of trouble calving. The rest of the herd had no corn after 1st trimester and calved easily on their own.

I would suggest feeding significant amounts of ddg's and corn are for animals being fed out for harvest only. If you do feed females ahead of calving I would just use a very small and measured amount. But then you also need to make sure the larger more aggressive cows don't eat more than they should also.  jmho and beginner's experience.

Jim at Dawn