SELECTING replacement heifers by weight alone may be a thing of the past for Angus breeders with new on-farm research around genomic tools showing it is a poor indicator of genetic merit.
Environmental factors such as health and nutrition, date of birth and the age of the dam have a big influence on weight.
Six Angus breeders in Victoria's north-east are trialling the HeiferSELECT tool over five years and early insights are showing a weak correlation between genomic predictions for yearling weight and raw weight data.
The producers took DNA samples of their first heifer progeny in spring last year and are putting the genomic data to use now as those heifers become ready for joining.
They will test 100 per cent of their heifer crop as part of the Meat & Livestock Australia-funded producer demonstration site research.
The early insights around weight have meant some of the participating producers are giving light heifers with strong genomic predictions and moderate mature cow weight an opportunity.
Fourth generation beef producer and research leader Julian Carroll, Black Star Angus in Mudgegonga, said for as long as cattle scales had been commonplace, breeders had been selecting replacement heifers based on weight.
"Good yearling weight in replacement heifers is critical to optimise pregnancy rates and minimise dystocia," he said.
"However, it is clear that weights alone are a poor indicator of genetic merit.
"This is because the noise from environmental factors renders the raw weight data somewhat meaningless.
"Considering date of birth alone, in a six-week joining, we might get an eight week spread of calving dates.
"Fifty six days difference in age could easily account for a 56 kilogram difference in weight."
Mr Carroll said the producers had identified other negative effects from selecting heifers primarily by weight - avoiding heifers from heifer dams as they typically wean lighter being the key one.
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Critical mating weight for British-bred heifers is typically considered to be 60 or 65 per cent of mature cow weight.
"Our group uses 60pc which we think is enough in the high rainfall zone in spring calving herds where winters can be limiting but the springs are reliably productive," Mr Carroll said.
"We see strong weight gain during joining from October and December and on average, our pregnancy-tested-empty cows in condition score 2.5 and weigh 575kgs at weaning."
That sets an average critical mating weight of 345kgs.
However, because HeiferSELECT provides a genomic prediction for MCW, that average critical weight of 345kgs can be a little less rigid, Mr Carroll explained.
"In theory, a heifer with a lower-than-average MCW can be forgiven for having a joining weight lower than the average," he said.
Where light heifers with strong genomic predictions and moderate MCW have been joined, it will be very interesting to see how successfully they get pregnant, calve, get back in calf and wean a quality calf, he said.
Benchmarking
The Genomics for Commercial Angus Cattle PDS project is also giving breeders an effective platform for benchmarking because the HeiferSELECT scores are based on all commercial Angus heifers tested in Australia, currently around 12,000 head.
PDS projects aim to increase the rate of adoption of key management practices and technologies that improve business profitability, productivity and sustainability.
Mr Carroll said in this PDS work, clear-cut back stories about breeding objectives explained most of the variation of average scores for key traits.
"That has built a lot of confidence that the HeiferSELECT product accurately reflects the direction each herd had taken historically," he said.
"It also provides valuable insight for producers considering how to, or whether to, take corrective action on any traits with comparatively lower scores."
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