After weaning, I use a true creep feed, either A&M, Okie, Tindle, or Bonanza. I also put out a little a time or two a week when the cows are on the lease to keep them used to coming up for easy handling. I don't free choice creep my pre-weaned calves because I want my cows to perform in conditions that they might be expected to face in commercial herds.
I would allsways keep these out. I creep from one month on. Would say if you are going to creep and use a product with an ionofore(sp) that you keep them full of it. I read an article in The Cattleman mag. about it causing illness when they run out for a long period and then go back in forth between having sone and not having any. Meybe someone else can help elaborate.