How long does it take for a beef cow to dry up after separating the calf, and will her milk production resume if she is put on a winter hay ration and/or the calf is returned to her?

Whether or not she begins to lactate again has little to do with the diet she is eating. We have weaned calves for 4 days, 8 days, and 12 days and then turned the calves and dams back together. Calves weaned at 4 days, 8 days, and 12 days all mothered-up and began suckling their dams. We measured milk production, and all cows that had their calves at 4 days, 8 days, and 12 days produced milk after the calves were returned. Milk composition didn’t change much depending on whether cows had their calves weaned for 4 or 8 days, but when calves were weaned for 12 days, milk composition was slightly different, indicating that the cows whose calves had been weaned for 12 days were beginning to dry up. So calves would probably have to be weaned for at least 30 to 35 days to “dry” the cow up, and then cows and their calves could be commingled.