I usually buy a calf or two ahead of time. Availability is key. A person don't realize how hard it can be to locate a baby calf, or several for that matter, in a timely fashion. And when ya need 2 or 3 baby calves, ya need em NOW (I try to buy em from 50 to 200 bucks, you can sure go to a sale and buy em for 400 or less but that's silly)
One thing I've learned, is that it is easier to get calves on the nurse cows as soon as possible after calving while the hormones are still forcing that cow to be a mother! They just seem to accept em better.
If I already have baby calves in reserve (on a bottle) when they calve, I wait 24 to 48 hrs for the born calf to get colostrum and put em together. I also withhold a bottle 12 hrs before grafting usually. I want that calf AGGRESSIVE and HUNGRY
Bessie runs with the herd. Her new set keep with her just fine. No issues at all. I just keep her penned a week or two with the new calves and the calves just know that's where to go. Thats their mama.
I HAVE had some that were on prior nurse cows that would steal from any cow when all turned out and running with the herd. I've never seen a born calf nursing anything but it's own mother tho