Bull fertility tests show a third of all beef bulls and half of those more than six years old will fail to deliver a compact calving pattern, report vets working in south-east Scotland Between March 2003 and July 2005, the vets completed 368 tests on 319 bulls in 72 commercial beef herds.
Lysan Eppink of the University of Edinburgh, Easter Bush Vet Centre reckons this level of sub-fertility easily goes unnoticed on farms because of the practice of rotating bulls.
Sub-fertile bulls will get some cows in calf, but herds will suffer an increased calving interval.
“Few bulls are totally infertile.
But a fertile bull should get 60% of cows pregnant in six weeks and 90% in nine weeks,” she told the British Cattle Vet Association congress in Devon.
There is little information on UK bull fertility available because few bulls have been tested. But results of UK bull evaluations were similar to USA studies, which also showed 30-40% are sub-fertile, she said.
The evaluation considers sperm quality and scrotal circumference for age, as this influences daily sperm output, which is important when a bull is running with more than 40 cows.
Sperm samples were taken using electro-ejaculation.
Under a microscope progressive motility is scored and the number of normal sperm assessed.
More than 70% normal sperm is required.
Bulls are then classified as fertile or sub-fertile, added Miss Eppink.
In the 368 tests, 33% of all bulls failed, increasing to 52% of those more than six years old.
Of those failing, 39% failed on semen quality alone, 34% on semen quality and physical examination and 27% on physical examination.
But a sub-fertile bull on one test should not be seen as useless.
“The sperm quality can be affected by bull health, a vet treatment or a rapid change in condition score.”
A test could only show his fertility on that day, so a later test may show regained fertility, added her colleague Colin Penny.
Bulls failing on physical examination alone could be put with fewer cows to compensate for their lower daily semen production, advised Miss Eppink.
And those with 50-70% normal sperm could also be run with fewer cows or managed differently to avoid affecting calving interval, although this would only reduce bulls failing to 30%.
Mr Penny added that it was often not necessary to cull young bulls that failed.
Managing them in a different way could often be advised.
But he suggested older bulls were less likely to regain fertility.
He also stressed it was vital to watch a bull serve cows when he first went out with cows, as the breeding soundness evaluation didn’t assess libido or ability to serve.
Miss Eppink added this observation should be close enough to see his penis locating properly in the cow.