Nearly half of the dairy cows in New Zealand calve to natural service sires. It's surprising, given the economic consequences of poor fertility, that only a tiny proportion of the bulls used in dairy herds are put through any form of breeding soundness examination before they're run with the herd.
Bulls are potential carriers of disease. Venereal diseases such as campylobacter, trichomoniasis and ureaplasma are occasionally identified but are of limited threat. Bovine Virus Diarrhoea (BVD) virus infection is a greater threat. Given the size of the fertility losses that can occur from BVD, testing and vaccinating bulls for BVD before arrival on the farm is highly recommended.
By far the most important health problem of bulls in dairy herds is lameness. This occurs as a result of the strain on limbs and ligaments from the high workload of mating as well as from walking along races and yarding. Once lame, a bull is unable to mate. If the bull recovers, it is highly likely that the stress and pain of lameness will have adversely affected his semen quality for the rest of the breeding season.
Any sick, injured or lame bulls should be removed from the herd. Such bulls may no longer be able to serve cows on heat and, if they are dominant bulls, may keep other fertile bulls away from these bulling cows. Bulls that become lame are generally ‘out' for the season. Depending on the condition or lesion, lame bulls can be infertile or less fertile for several weeks.
Prevention of lameness is better than cure. Ensure that a bull's feet are sound before mating starts. Keep the bulls in the paddock or ensure there are enough bulls to maintain a quick rotation within the herd. The three most common causes of bull lameness are bikes, dogs and backing gates. All of these can be avoided if bulls are left in the paddock and not walked to the shed with the cows. This training of bulls to stay behind may sound like a difficult task but with the cost of bulls and empty cows, it may be worth the effort!
The lesson needs to start on the very first day the bulls are turned out. Make the bulls highly visible both day and night with paint and reflective tape. Make sure everyone is committed to leaving the bulls in the paddock. Train the staff before the bulls.
On the first night three to four people may be required. One person gets the cows out, and everyone else stands in a little way from the gate and turns back any bull that comes near the gate. Do the same thing the next morning and thereafter, with good fortune, getting the cows in needs only two people.
The number of bulls you will need varies hugely. If you're achieving a greater than 90% 21-day submission rate, a first service conception rate of 55%+, a low anoestrus rate, an Artificial Insemination (AI) period of six weeks and excellent heat detection efficiency, that should leave the bulls only 25% of the herd to mate. A poorer submission rate, only moderate conception, high anoestrus levels and shorter AI period may leave the bulls with 60% or more of the herd to mate.
The situation is very different if most of the returns are ‘synchronised', as is the case when CIDRs and/or prostaglandins have been used. Synchronisation programmes will concentrate the number of cows requiring insemination/mating over a few days rather than having them spread over three weeks. This also means twice as many bulls to mop up the herd, or five to six bulls in a 200-cow herd.
Usually, it is advisable to rotate herd bulls on a weekly basis - more frequently is better for the bull's feet! This issue is less critical if plenty of bulls are being run with the herd. In this situation, an infertile bull will be covered by the other bulls in the team. As a general rule it is better to ‘over-bull'. The old rule of thumb of 3% +1 for the number of cows that are expected to be empty at the end of the AI period is as useful a guide as any. The +1 means that there are never less than two bulls.
Summary
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Check bull:cow ratios...use plenty of bulls
- Yearling bulls must be well grown, and have good scrotal circumference (>34cm)
- Beware of lameness...bulls are best left in the paddock, not coming into the races and yard
- Monitor the bull mating period, so that you know if cows are holding or returning to natural sire services
- Aim to use easy calving sires
What can TVL do to help?
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Offer advice on any of the issues raised above
- Organise BVD testing of bulls and BVD vaccination programs
- Carry out breeding soundness examinations on bulls. These can range from a quick check of the general soundness of a bull for mating to semen quality evaluation and service capacity testing
Please give us a call at Totally Vets if you wish to discuss any of these procedures.
