CATTLE need to consume more feed to put on fat than muscle and we, the consumer and often the butcher, trim off excess fat anyway. So why not breed leaner cattle?
One reason is that leaner heifers are less fertile than those carrying more fat. The second reason is that the leaner steer progeny, at least when grass finished, are more likely to incur sale penalties for failing to meet company specifications.
Researchers from a multidisciplinary team under the banner of the Beef CRC ran 394 Angus heifers in South Australia and Western Australia to look at this question. They purchased registered Angus heifers from the top and bottom 10 per cent of the Angus herd for rib fat estimated breeding values (EBVs).
These 2005 to 2007 drop heifers, when aged 15 months, were joined to Angus bulls for nine weeks. Prior to joining, the high fat EBV heifers were found to be 13 days older, 9cm shorter and, as expected, fatter than the low fat EBV heifers.
At pregnancy testing, the researchers found that conception rates averaged 91.2 per cent for the high fat heifers compared to 83.0 per cent for the low fat heifers.
They also found a strong relationship across the range for conception compared to pre-mating fat depth. The leanest heifers with a fat depth of 1-2mm achieved conception rates of less than 80 per cent while heifers at the top of the range with 10mm of fat were nearly all pregnant.
Under the management systems of most commercial beef producers on the tablelands, which includes annual pregnancy testing and sale of the not pregnant cows, these less fertile cattle would soon disappear from the herd.
When the researchers modelled their findings, they concluded that 58 per cent of the higher fat line would remain in the herd after 10 nine-week joinings while only 45 per cent of the low fat line would remain. Of interest they also found that under a six-week joining regime, only 20 per cent of both genotypes would remain in the herd after 10 joinings.
The steer progeny of these heifers were also studied by the group, led by long-term cattle researcher Mick Deland based at Struan in South Australia.
While both lines of steers had the same average live weights at 20 months, 69 per cent of the high fat line steers met the minimum requirement of 6mm of fat while only 41 per cent of the low fat line achieved that specification.
Under the grass finishing system of the trial, Mick Deland found that when the steers were sold at a constant age, more from the low fat line were penalised because they were too lean. When scanned and sold at a fat constant basis, more of the low fat line were discounted for age (based on dentition) and for excessively heavy carcases.
I asked Mick if he had any evidence for the suspicion that cattle bred to carry a bit more fat are more resilient during tough seasons. He believed this to be the case.
I think these research results show that tablelands cattle producers running self-replacing herds of British breed cattle, aiming to finish steers for the bread and butter supermarket trade, should be cautious about breeding leaner cattle.
Breeding for muscle, however, is a different story, for a future article.