Short Gestation


Short gestation length (SGL) semen not only means more days in milk, but also condenses calving and gives cows a longer period to recover, increasing their chances of getting back in calf to AB.


SGL dairy (Days)    Beef breeds* (Days)
Holstein-Friesian -15.6   Hereford -6.3
Jersey -15.4   Angus -6.9
KiwiCross -14.5      




*Beef information is within beef breed only. Hereford Pak includes some horned bulls.

Q. Why should I use SGL semen?
Your normal AB mating programme should be used to generate the numbers of replacement stock you need/want. When you are sure you have achieved that, you can then use SGL on the stragglers to tighten your calving interval. This will give you more days in milk, help you avoid inductions, and reduce future empty rates.

Q. Can I keep the progeny of SGL as replacements?
We don’t recommend that you do. SGL semen is supplied by bulls chosen only for their short gestation length BVs. Their BW and TOP traits are not part of the selection criteria.

Q. What is the difference between proven and unproven SGL?
Proven SGL bulls are proven for gestation length only. It only takes one year to get an SGL proof – we have the figures as soon as the calves are born the next year! They are not proven in the normal sense of the word (ie for BW and TOP, which takes 3 years to get information once the daughters born start to milk). Likewise, unproven SGL bulls do not have any calvings yet, so the SGL BV is based purely on ancestry. These unproven bulls will graduate as proven SGL bulls next year.

Q. How do I interpret the SGL BV?
The SGL BV is expressed in days. A negative number of days indicates a shorter gestation length than average. As with all BVs, on average, the bull passes half his BV to his offspring. So if the dairy bull’s gestation length BV is -10, his calves will be born, on average, five days earlier than would the calves of a dairy bull with a BV of 0. Depending on the SGL BV of the cow he is mated to, it could be even earlier than five days. 

Q. How does SGL work for beef bulls?
It’s a slightly different story for beef. Progeny of Hereford bulls have a longer gestation length – on average, 4 days longer than the progeny of the average dairy bull. This means that the progeny of a Hereford bull with a SGL EBV of -8 days (within the beef breed), will be born after the same length of time as the progeny of an average dairy bull (assuming they’re both mated to the average dairy cow). The benefit of Hereford is the white face marker, which prevents you accidentally keeping a replacement. Angus are polled, so you can’t make mistakes there, either. (Angus gestation length is about the same as dairy).

Q. How are SGL bulls chosen?
Bulls with the shortest gestation are chosen: EITHER from their ancestry information (unproven) and sold through Alpha, OR as proven bulls, having received SGL proofs from their usage. SGL is a highly heritable trait, which is measurable much sooner than production traits are.


Short gestation is a great product for condensing calving, giving cows longer to recover and get cycling again, and of course more days in milk. But remember, that “proven” here means proven for gestation length, not proven for production or TOP traits. Therefore you should not be keeping these calves as replacements.
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