Advice on running stock bulls with breeding heifers

According to AHDB data, natural mating is now used to produce fewer than 5% of dairy heifer replacements in the UK.

However, with routine euthanasia of bull calves discouraged and a soaring beef price, milk producers are giving greater attention to the genetics of dairy-beef progeny and the bulls used to “sweep up” heifers not in-calf.

See also: Benefits of weighing dairy heifers to improve efficiency

Previously, farmers might have run a home-bred dairy stock bull with heifers after cycles of artificial insemination (AI), with little thought given to the type of calf produced.

But many are now buying a high genetic merit beef bull to capture those higher returns for beef calves.

The bull should never be an afterthought, insists AHDB Dairy knowledge exchange manager Laura Awdry. She says selecting the animal is as important as selecting sire semen for AI.

“It should never be a case of ‘I have inseminated my heifers, I have done the important stuff’,” she adds.

Bull criteria

Running a bull with heifers comes with a different set of criteria to breeding from mature animals. No sweeper bull should be older than four years of age or younger than 15 months, Laura recommends.

For use on heifers, selecting an animal at the lower range of that age scale is advised because weight is an important consideration.

However, to be sexually mature and able to serve, a bull must be well grown too. At 15 months, he should have achieved 50% of his mature weight, increasing to 85% by two years of age.

Bull-to-heifer ratio

As fewer mature bulls are used on heifers, greater numbers should be used than the bull-to-cow ratio to compensate for the poorer performance of younger bulls.

Chris Burke, a senior scientist at DairyNZ, recommends one bull to every 15-20 heifers not in-calf.

The number of bulls required can be calculated by estimating how many heifers are expected not to achieve conception from AI, says Laura.

For example, if a pregnancy rate of 70% is predicted in a group of 100 heifers, 30 of these will be on heat when they run with the bull, so two bulls will be required.

However, Chris advises ensuring there are always at least two sexually active bulls running with each mob throughout the mating period.

“Bulls are typically run with yearling heifers on an all-in basis,” he says, adding that it may be prudent to factor in replacements for any bulls failing to perform.

Breed and trait selection

The shorter gestation period and calving ease associated with Aberdeen Angus and Hereford often make these breeds popular choices for mating dairy heifers.

Even so, selecting bulls based on estimated breeding values (EBVs) is always advisable and can open up choices among other breeds.

Laura, who is researching sustainable and profitable dairy-beef production for a Nuffield Scholarship, says any bull used on heifers should have a positive EBV for short gestation and direct calving ease.

“The ambition is that any calving should be as easy as possible – nothing should compromise a heifer’s first lactation and fertility,” she says.

Resources to help inform choices include the AHDB National Beef Evaluations service and the pedigree index.

Breed aside, using a pedigree or purebred bull is key to producing a marketable beef calf, Laura adds.

“We sometimes see dairy farms keeping one of their own crossbred bulls, a Holstein cross Angus perhaps, to run with their heifers.

“That is fine for easy calving, but it means the beef calves won’t necessarily be right for the market,” she cautions.

They should instead aim to produce a beef calf that can capture a higher tier of value.

Some meat processors will have their own integrated beef schemes – for example, initiatives that cross the best Aberdeen Angus genetics with dairy animals.

Laura advises that if a farmer is using a breed list of high genetic merit bulls, they should check if the pedigree breeder they are considering sourcing a bull from uses the listed genetics.

This will make that animal’s progeny eligible to enter the integrated scheme.

Geography plays a part too, she adds. “If you are in a part of the country where there is a processor with preference for calves from a particular breed, you might want to consider that.

“For example, in Cornwall, we see lot of Aberdeen Angus used because the St Merryn plant at Bodmin gives farmers an outlet for those calves.”

Young bulls that are sexually mature, but not overly large, should be used on heifers © Debbie James

Compatibility of management system

For a dairy farm that is predominately forage based, it is advisable to select bulls compatible with that system, and likewise for more intensive feeding systems.

“It is probably sensible for a grazing farm to not source from a breeder feeding a lot of barley, for example – although the bulls might look lovely, the systems are very different,” says Laura.

“A bull that has been used to a lot of concentrates won’t maintain condition if the intention is to turn him out with heifers at grass.”

For bulls reared on forage and introduced to a higher yielding system, mobility and fertility can be challenged, especially if they will be housed on concrete. A feed transition of three or four weeks is advised.

“It takes 21 days for the rumen to make a functional change,” says Laura.

“That period of adjustment to the diet is important, because any upset or stress to the bull can have a negative impact on his semen production and fertility.”

If buying multiple bulls, these should ideally be sourced from the same mob to reduce the risk of fighting when they are introduced to the heifers.

If that is not possible, Chris advises taking delivery of bulls earlier to establish their social order well before mating starts.

Bull vaccination

Bulls should receive the same vaccination programme as the dairy farm’s heifers and cows.

Chris says buyers should insist on bulls that are certified as fully vaccinated for leptospirosis and bovine viral diarrhoea (BVD), allowing time for two initial vaccinations four weeks apart.

They should then receive an annual booster.

“They must have been vaccinated twice initially, four weeks apart, and then boosted with a single shot annually for each of these diseases,” he says.

He advises also asking what disease exposure bulls may have on the source farm, and selecting those certified free of BVD, infectious bovine rhinotracheitis and other economically important diseases.

If a buyer is unsure about Johne’s status, Chris says they should discuss this, and the steps needed to develop a bull drenching programme for parasites, with their vet.

He advises veterinary examination of bulls at least one month before mating, to include a physical assessment as well as fully testing semen quality.

Body condition score

Bulls should be in good condition before, and during, the mating period, ensuring diet and body condition score (BCS) are appropriate. Laura recommends aiming for BCS 2.5–3 at mating.

“If he is underconditioned, that, in combination with the stress factor of introducing him to the heifers, can have a negative impact in terms of semen production,” she says.

“Equally, if he is overly fat, that can potentially also create fertility challenges.”

Condition scoring should be done well before mating (at least three months) to allow sufficient time to make changes to the diet.