We do not normally allow anything weaned to go back with the mature cows. They stay with their age group, separate pastures, fed a little better for growing needs. Then after they have calved, decisions are made as to who goes to what pasture/which bull. I particularly have to watch the calves I raise on nurse cows, as they will try to steal off another cow. Had one this year that went back on a cow that was supposed to be dry. We were shuffling around cattle and so this group of 10 weaned calves got put back with the DRY nurse cows. Been weaned nearly 90 days. She started sucking on a dried off nurse cow and they all got moved out again, to go to the barn facilities for bangs vacc and a few dehorns. The cow calved back in with very little udder and is only raising 1 calf this time. I am hoping that she will do okay and when bred back, then dried off, will come back next time and be able to raise several. She has a great disposition as a nurse cow, raised 7 her first lactation. That weaned heifer was not one of her original calves, but off another nurse cow. If she gives me any more trouble, she will go even though she is a nice heifer.
I feel the weaned calves need to be fed differently so they get their own field/lot in which to get a chance to eat and grow as they should. The older cows don't need the grain etc even though they think they are entitled to it!!!