There is a big difference in gestation lengths between many Shorthorn bulls. There is a difference in gestation length between bulls of many breeds.
I first noticed this, in two bulls we imported from Ireland, that being Highfield Irish Mist and IDS Duke of Dublin. Irish Mist was on average 11 days shorter gestation than Duke was. Obviously, birth weights for Duke sired calves were higher than the Irish Mist calves, yet we did not have many calving problems from either bull. I have been using an ET full Irish bull sired by Irish Mist on my heifers for the past three years. This bull has the shortest gestation length of any Shorthorn bull I have ever used, and he is very easy calving. Even though these calves are about a week shorter gestation length than our other bulls, they are extremely vigorous and healthy at birth. They are up and sucking immediately, and the heifers breed back faster simply because the birth was so easy.
I have found that in my herd, Salute has a gestation length 7 -10 days longer than some of my other sires. His calves have had more birth weight, but we have not assisted a Salute calf yet.... we probably have had a little luck, in that regard. I do not think that the sire is the only thing that affects the gestation length. I have seen wide variations in birth dates in my ET calves from the same matings. Three years ago, I had 6 ET calves born from the same mating. There was 18 days between the first born and the last born, and all were heifers. All 6 were DNA parentage tested to the ET matings. I have seen some variations between other ET matings as well. I usually find that male calves will go longer in gestation than female calves, but there are exceptions to this as well.
Another general trend I have noticed is that my fastest gaining calves usually come from the longer gestation calves. I often wonder if there is a correlation and if any research has ever been done on this. As I mentioned on here before, last year we did not have one bull index over 100 for ADG or WPDA that was in the lowest 1/2 for birth weight. This year we have 62 bulls on test, and half way through the test, the same thing appears to be happening.
I also noticed that there was a big difference in gestation length in some of the original Maine bulls when we used them. Cunia had a shorter gestation that Covino III and Dollar 2 for example, and Capone was between these bulls. I also saw the same thing in our Charolais cattle when we had them. The calving ease Charolais bulls were almost always in gestation length than other sires
I have felt for a long time that more calving ease is closely related to gestation length. It is not the only factor affecting calving ease, but it is one of them. Much more needs to be done in this area.
