The Beef CRC’s Maternal Productivity project, a big study that tracked breeders from their first calf through five subsequent weanings, examined the effect of fat levels on heifer fertility and performance.
The project wanted to see what might be happening to cow performance as breeders pushed for more lean meat yield, a trait known to have an adverse effect on fertility.
When the project’s numbers were crunched, what stood out most starkly was the strong linkage between the Rib Fat Estimated Breeding Value (EBV) and heifer fertility.
As a rule, the higher the Rib Fat EBV, the better the fertility.
Dr Pitchford noted that Rib Fat EBV isn’t just a number: studies show that the value accurately reflects an animal’s propensity to lay down actual rib fat.
As the study poved, the EBV is also a handy indicator of how likely a heifer is to get into calf at her first joining.
In the study, after a nine week joining, the conception rate in heifers with the genes for high fat levels was 91 per cent, versus 83pc conception for those with genetically low fat levels.
When the joining was compressed to six weeks, the difference in genetics became more pronounced: a 78pc conception rate for the high-fat heifers against 65pc for the low-fat heifers.
Even when genetically lean heifers were put on a high-nutrition diet and carried plenty of fat, they didn’t conceive at the same levels as the genetically high-fat heifers.
But these differences didn’t hold up as the heifers matured into cows.
Across four of the project’s joinings, geneticall high-fat cows produced on average 3.6 cows, and low-fat calves produced 3.5 calves.
The researchers concluded that early culling of dry heifers – of which there were more low-fat animals than high-fat – had erased any differences, effectively selecting for fertility regardless of genetic fat levels.
If the culling hadn’t been done, the difference in fertility between high and low-fat cows may have persisted into maturity.
The message, Dr Pitchford said, “is that if you cull your dry heifers, you set up your herd for high conception rates later on”.
Which isn’t news to most producers, he admitted, but now it has some scientific rationale that may help breeders improve the balance between lean meat yield and fertility.