the bull and the cow DEFINITELY both play a role in the gestation length.. I do think that repeatedly stacking short gestation length animals could eventually have ill effects... My herd has an average gestation of 288 days... The new bull I have may be a short gestation length bull, I've had 3 calves from him and I'm really scratching my head on if some of these cows were bred again later, since it did seem like I saw the bull with them a second time
Here's some data:
Prada, Bred May 4th, calved Feb 10th (281 days), 80 lb bull, she was only seen bred once.
Cenci, Bred April 28th, and seen again with the bull May 19th, calved Feb 10th, 70 lb bull, 292 days if it's the earlier date, 271 if the second.. She's a long gestation cow and has gone as long as 302 days before
Dusty, Heifer, bred Apr 30 and probably seen again May 17th, calved Feb 12, 80 lb bull, 295 if the first time, 278 if the second...
I'm scratching my head on this as to which are the correct breeding dates!
Here's a graph of 222 births according to gestation time and sex of calf..