Calves that have been weaned at about 500 pounds (6 to 7 months of age) will resume nursing their dams after having been separated for several weeks. Several years ago, we did some research in an attempt to reduce milk production in cows by "temporarily" separating weaning-age calves (7 months old) from their dams. Calves were separated at 4, 8, or 12 days from their dams, then rejoined with their dams. All calves resumed nursing their own dams after 4, 8, and 12 days of separation. You should be prepared to keep the calves and cows separated for about a month before running them in the same pasture. If you are working with a small number of calves, perhaps you might consider fitting each calf with a "calf weaner" device that prevents the calf from nursing. These devices cost about $2 each and can be purchased through several farm/ranch supply distributors.