We wean in the fall, and don't let the replacement heifers back with their dams till pasture turn out time the following spring. The cows have new calves at that time, and usually don't allow their year old daughters to nurse. This doesn't always work---I have one from last year that will suck any cow in the pasture. She sneaks in from the rear while the calf is sucking and steals. She is destined for the freezer after a couple months of fattening. Nice yearling too. Wish I could have kept her for a replacement.