I bought a registered Hereford bull at 6 months of age for a herd bull. He was about 16 months when I needed him to breed my cows. I gave him two or three months to breed my cows. I saw my cows keep coming back in heat every 18 to 21 days. On June 20th I put a new Bull in with my cows and pulled the old bull out at the same time. I have had two calves I know from my old bull. I had a calf on Sunday, one on Monday and one on Tuesday and one today (Thursday). When you count the days the calves this week should be my old bull. I think they are from my new bull. What do you guys think? It really doesn't matter, I guess I'm more curious than anything.