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Approximately 4% of stock bulls in the UK are infertile, with a further 16% of herd sires being sub-fertile, AI Services vet Brian Kennedy told farmers at a CAFRE online seminar last week.
“The bull that is fertile, but not sufficiently so, he’s the boy that does the damage by spreading your calving pattern. Watch a bull serving, and keep a look out for repeats,” advised Kennedy.
Ahead of the breeding season, he said it was important to check a bull and ensure he isn’t lame. Even a mild case of lameness can reduce both sperm production and quality within days.
At our site at Ballycraigy, we use a turnover crush – it is by far the easiest and least stressful for both the bull and the handler
Where a bull has his feet lifted and pared before going to cows, Kennedy said this should be done 60 days in advance, to give the animal time to recover, and for semen to mature.
“At our site at Ballycraigy, we use a turnover crush – it is by far the easiest and least stressful for both the bull and the handler,” said Kennedy.
Scrotal circumference
When buying young bulls, it is important to check the size of the testicles as this is directly related to bull fertility. As scrotal circumference increases, so too does the bull’s ability to produce viable sperm. A bull 12 to 15 months old should have a scrotal circumference of 30cm, rising to 34cm in a bull over two years.
There is a lot to recommend buying a bull on-farm
As bulls are selectively bred for more extreme carcase traits, scrotal size tends to reduce, thereby limiting the number of cows the bull can cover.
“There is a lot to recommend buying a bull on-farm, but be sure to look in between his legs. There is a possibility he wouldn’t pass at an official sale – do pay attention to that,” advised Kennedy.
For normal sperm production, the temperature in the scrotum should be 2°c to 6°c lower than the animal’s core body temperature, so a bull in good working order should have testicles that hang freely, with a defined narrowing where they attach to the body. Where testicles do not hang freely, scrotal temperature is higher, potentially reducing semen quality.
Condition
At the outset of breeding, Kennedy maintained that a stock bull should be in a body condition score of 3 to 4.
Where a young animal is bought from a society sale, he is likely to have been on a high-concentrate diet (up to 20kg per day), so the transition on to a farm can cause significant stress and leave the bull infertile.
I’m not sure how to square that circle
Heavy feeding can also lead to increased fat deposition in the scrotum and lower fertility, as well as laminitis, joint problems and an animal that has reduced interest in working. But if the bull isn’t well fed, he doesn’t sell.
“I’m not sure how to square that circle. Moderate forage based diets are the way to go. At AI Services we bring them down slowly (off a high concentrate diet) over a period of weeks,” said Kennedy.
Age
He was also asked whether sperm quality or production deteriorates with age. He maintained that it was not something he has seen, even in bulls up to 10 years old. When collecting semen, a mature bull (over six years old) will produce twice the straws of a bull that is two to three years old. Usually it is ill health or stiffness in joints that determines when an older animal is culled.
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