Age at puberty scanning provides new insight into heifer reproduction

The collaboration between the Animal Genetic Breeding Unit and New Zealand scientists, jointly funded by Meat and Livestock Australia along with Beef and Lamb New Zealand, investigated the heritability of age at puberty and related traits as potential ways to improve and monitor female reproduction.

A total of 981 spring born 2017 and 2018 weaned Hereford heifers from a southeast NSW seedstock herd and two in the New England region were serially ultrasound scanned three times to detect their first corpus luteum indicating puberty, before being synchronised for artificial insemination.

The study found only 52 per cent of heifers were pubertal at synchronisation prior to artificial insemination, a trait that genetic analysis showed was moderately heritability at 0.26.

Heifers that were not pubertal were given a penalised record, based on the maximum age at puberty for their contemporary group plus an additional 21 days or one cycle. This trait had a higher heritability of 0.38, just as high as some growth or carcase traits.

An additional 3200 Angus heifers were scanned at the same time and produced similar results, ruling out any breed influence.

The findings were presented for the first time at the Association for the Advancement of Animal Breeding and Genetics conference in Armidale recently, attended by researchers and breeders from Australia and across the world.

While the study wasn't the first of its kind, following on from work by Beef CRC and the Repronomics project work, project leader and AGBU scientist Dr Matt Wolcott said low number of females that were pubertal at their first mating was unexpected.

"Given that reproduction rates are generally good in temperate beef breeds, it means that in natural mating situations there has to be something inducing first oestrus pretty quickly once bulls are introduced to the heifer herd," he said.

"In the stud sector these results suggest that synchronisation for AI (artificial insemination) must be inducing puberty in a significant proportion of heifers, which we didn't expect to see."

Further data and results related to reproductive outcomes and the impacts on pregnancy rates won't be available until the current calving is complete and weaning data is submitted to Breedplan.

But, with an increasing number of artificial breeding programs, Dr Wolcott said the early findings warranted attention from beef producers.

"When we started this project we were comfortable running it in heifers we knew were going to be synchronised for AI, because the expectation was the vast majority would be pubertal going into mating," he said.

"The fact that only half the heifers were pubertal, and assuming reproductive outcomes are good, means that (synchronisation) treatment has to be inducing ovarian activity in animals that weren't pubertal up to that point.

"I think it is a result that people should be conscious of. The fact we don't have a problem at the moment is good, but I think this may be a bit of a warning.

"Of all the traits we don't want to see problems developing for, reproduction is at the top of the list. Once you start seeing reproductive rates going down it is a long term proposition to improve it."

Days to calving is currently the key selection trait for female reproductive performance in the Breedplan evaluation and describes the days from the start of mating until calving, in naturally mated females. A female synchronised for AI cannot currently get days to calving records.

With the rise of AI breeding and some recent confusion around the requirements for days to calving data submission to Breedplan, recording levels for the trait are low or declining.

"If we assume that we don't have a reproduction issue in these breeds at the moment, which I think is generally true, we still need good description of female reproduction in their genetic evaluation," Dr Wolcott said.

"As AI rates continue to increase the amount of information coming into the Breedplan evaluation on female reproduction is going down.

"Age at puberty is a heritable alternative which may present opportunities for breeders to monitor female reproduction as they apply selection pressure for other aspects of productivity like marbling, other carcass traits and mature cow size for temperate beef breeds."

He believed a new fertility trait for genetic evaluation was achievable and could be a combination of age at puberty, lactation anoestrus interval (how quickly females cycle after their first calf), and days to calving records.

"The strategy adopted for tropical breeds has been to retain days to calving as the descriptor of female reproduction, but to allow information from age at puberty and lactation anoestrus interval to contribute to the evaluation as correlated traits," he said.

"This may be a reasonable strategy for temperate animals, but more research will be required if it's to be adopted for these breeds."

The Southern Multi-breed Project, involving NSW Department of Primary Industries and University of New England researchers, has recently received Meat and Livestock Australia funding approval.

This research will focus on the issue of accurately describing female reproduction in the Breedplan evaluation, and better understanding the genetics of these traits for temperate beef breeds.