Weekly genetics review: Shorter gestation length can fine-tune a herd

Genetics editor Geoff Phillips, 15/08/2017

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THERE’s an Estimated Breeding Value (EBV) for just about everything these days.

Gestation Length (GL) is one that doesn’t seem to rate as highly as others, but is associated with lighter birth weights, improved calving ease and faster re-breeding. Gestation Length EBVs are estimates of genetic differences between animals in the length of time from the date of conception to the birth of the calf.

Calves born from a shorter gestation period may be heavier at weaning due to more days of growth.

The generally accepted gestation (pregnancy) period in beef cattle in Australia and most overseas countries is 283 days, but a New Zealand farmer-owned artificial breeding cooperative is marketing short gestation length (SGL) Hereford and Angus semen that can shorten the gestation length by a massive 10 days.

The NZ cooperative, LIC, claims the gestation length trait is a highly heritable 48pc in progeny, and that’s a big number.

Early autumn calvers six days earlier

In addition, American research indicates that cows calving in early autumn can have a gestation length of up to six days shorter than cows that calve two months later in late autumn. There doesn’t seem to be much work done on this aspect in Australia.

Dr Alex Ball

Hereford Australia’s Alex Ball said selection for lower birth weight and shorter gestation length were complementary, and that gestation length in Australian Hereford cattle had “probably decreased by three or four days over the past 10 years.”

Age of cow and sex of calf can also affect gestation length.

While it is generally accepted that Angus have a lower gestation length than Herefords and some other breeds, Beef Central found some 74-year-old irrelevant but interesting statistics that indicate industry gestation length benchmark was then 283 days, and that hasn’t changed to this day.

In 1943, the American study found that Herefords had a gestation period of 289 days, Aberdeen Angus, as they were known then, 272.8 days and unsurprisingly, a Hereford X Aberdeen Angus 281 days. The phenotypes and the genotypes of these two most popular traditional breeds have changed dramatically over the past 74 years. Maybe some ‘belt buckle’ types skewed that historical data.

Hot weather generates earlier calvings

The Oklahoma State University’s Glenn Selk suggests that cows that gestate in hot weather to calve early-autumn often calve a few days earlier than expected.

The OSU used data from two years from a herd of 60 Angus x Hereford cows artificially inseminated to provide early and late autumn calving groups. Semen from the same bull was used for all cows which were exposed to a single clean-up bull for 35 days, from four days after the AI.

The average maximum temperature in the week before calving for the early group was 32C while for the late group, 18C. The calf survival rate for all calves was 100pc with re-breeding 90pc and 92pc respectively.

Mr Selk reported the average gestation period for the ‘early’ cows was six days shorter (279 days) compared to the ‘late’ cows (285 days) in year-one. In year two, the ‘early’ cows averaged four days shorter (278 days) than the ‘late’ cows (292 days).

The researchers pointed out that these figures are ‘averages’ meaning that around half the cows could have calved earlier than the gestation days suggested.

Stats on millions of cows

Records from millions of Holstein dairy cows throughout America endorse these findings. The ‘early autumn’ Holstein calvers’ average gestation period was two days shorter than the ‘late’ calvers.

The Australian EBVs for Gestation Length are calculated using the joining date and the calving date. Only AI and hand joinings are used. Natural joinings are not used even though they may be observed. It cannot be guaranteed that the observed mating resulted in the calf.

To improve accuracy of gestation length findings, ET calves and twins are excluded. The data is also rejected if only one animal is represented in a single contemporary group or if two thirds of the animals in a contemporary group have the same gestation length.

New Zealand system can shorten gestation by 10 days

The New Zealand artificial breeding cooperative company, LIC, explains that its SGL (Short Gestation Length) system works to tighten calving periods and allows late-calvers to ‘catch-up’ to their herd mates.

LIC also warns that its SGL Angus and Hereford bulls are specially-bred for their gestation length. They protect their investment in SGL genetics by banning the retention of progeny as herd replacements or for breeding. The collection of genetic material from cattle carrying their SGL genetics is also banned. That’s real terminal breeding.

LIC explains the dairy plan simply: A typical dairy cow has a gestation length of 282 days which equates to a Gestation Length EBV of zero. LIC’s team of SGL sires have an average Gestation Length EBV of minus 20. This means that when mated to an average dairy cow they will pass on half of their GL EBV to the offspring, delivering on average a 10 day shorter gestation length.





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  1. Alice Greenup, 17/08/2017

    Caution!! Select for fertility – great, but be wary of selecting for trait without considering the whole systems and its implications. Increased post partum mortality rates have been linked to lower birth weight and lower vigour calves being born as a result of reduced gestation length.

  2. Phil Barrett-Lennard, 16/08/2017

    I am a bit surprised the article didn’t mention the long gestation of most tropical cattle. For example, Brahmans have an average GL of 292 days. Reducing that GL down closer to 283 days would surely have a positive impact on total herd fertility? Just a thought.

  3. Margo Hayes, 16/08/2017

    Ausline Cattle, derived from traditional Aberdeen Angus genetics, have gestation length as short as 267 days. My average is 274 days after 22 years of recording. Their low birth weights, shorter gestation and easing calving make them perfect heifer bulls for the commercial producer.

  4. Jo-Ann Strong, 15/08/2017

    An Australian Phd thesis by Dr Sara McClintock in 2004 notes the effect and benefits of short gestation. The thesis also notes the inheritability of short gestation length through the maternal line. Many Angus breeders have been using short gestation length as an effective breeding tool for some years. Particularly for first calving heifers, it ensures the cows are ready to rebreed in time and ensures high pregnancy rates of these cows.

  5. matt arkinstall, 15/08/2017

    if they are banning keeping breeders, you may as well needle the cows early and breed the cows to whatever bulls you want.

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