Mating

A heifer’s first calving sets her up for a productive life. The decisions made about her first mating play a key role in her future. Consider your options for mating heifers.

Targets for heifer mating

• reach puberty at 12-13 months of age
• conceive by 13-15 months of age
• are of adequate weight and size by the time they calve.

Setting heifers up for a successful mating

Age vs. weight

A heifer’s genetics and liveweight determines when she reaches puberty, not her age. This means that younger heifers can be successfully mated alongside older heifers as long as they have reached their liveweight target. In New Zealand, dairy heifers reach puberty when they are at 43-47% of their mature liveweight.

Mating Start Date (MSD)

Heifers take longer to cycle after calving than mature cows. Take this into account when setting the MSD; ideally heifer mating should begin 1-2 weeks before the milking herd’s MSD.

Impact of liveweight on conception

Conception can occur when a heifer reaches 47-49% of her mature liveweight, however the best results are reached when they are at or above 60% of their mature liveweight at MSD.

Heifer Breeding Options

Stock owners commonly use one of the following approaches when mating heifers:

Artificially breeding heifers

If you are considering using AB for yearling heifers, consider the following factors:

• a yard with a race is critical for the safety of both the animals and the AB technician
• when using heat detection, easy access to animals and a method to improve detection accuracy (e.g. tail paint or heat mount devices) are important to achieve adequate submission rates
• only use sires known for calving ease. Consult with your genetics supplier about which bulls will be suitable.

A mating management plan should be in place before you start, if you are in a grazing contract relationship.  It should cover:

• the AB mating period
• heat detection methods
• any synchrony programmes. This plan should be agreed to with those involved e.g. vets, graziers, genetics provider, and AB technicians.

At the end of the AB programme wait for 24 hours after the last insemination before introducing follow-up service bulls into the mob. If using a synchrony programme, ensure there are enough extra bulls in the mob to cope with returns – use the same bull ratios as you would for a natural mating programme. Double the bull ratio 18-24 days after the peak insemination date, or recommence a period of AB for 3-4 days following heat detection.

Natural mating

When using bulls there are four critical areas to manage to achieve a high pregnancy rate:

Adequate numbers

Bull ratios of 1 bull for every 15-20 heifers is appropriate if bulls are not being rested.  There should be a minimum of two bulls in a mob at all times to limit the risk of an infertile bull. Bulls can be rotated around different mobs to eliminate the risk of one mob being over-exposed to a faulty bull; however, this can increase the risk of spread of infection.

Healthy

Bulls need to be checked daily for ability to serve heifers, injuries, or lameness during mating regardless of the number being used. Lame bulls should be removed from the mating mob so they can rest. If any bulls are timid or failing to serve they will need to be replaced.

Well-grown

Bulls reach puberty when they are at 43-50% of their mature liveweight. For optimal fertility, bulls should be at least 60% of their mature liveweight going into their first mating season. Table 1 shows the different mature liveweights for breeds and can be used as a starting point for estimating the weights of well-grown bulls. When choosing bulls remember that heavier breeds pose a greater risk of injuring heifers during mating.

Fertile

Bulls must start the mating season with a body condition score (BCS) of 4.5–5.5 and be fed appropriately to maintain this BCS. Thinner or fatter animals commonly have lower fertility. 

Selecting bulls for heifer mating

All bulls sourced off farm should have gone through a verified vaccination programme and been tested for common diseases before they are brought onto a new farm - consult your vet about appropriate tests for the region where your heifers are being grazed.

Bulls should be calm and settled into their new environment prior to mating. InCalf recommends that purchased or leased bulls arrive on-farm three weeks prior to mating for quarantine and to let them settle in. Although some farmers have had success with lead-in times as short as overnight, this is not recommended because of the biosecurity risk and the lack of time to manage animal stress.

Considerations when selecting bulls for heifers

• Use bulls that will minimise the number of assisted calvings.
• Fertility test bulls before mating. A New Zealand survey of beef bulls found 18% of two-year old bulls were not suitable for mating due to poor mobility, serving ability, penis confirmation and sperm motility. 
• Select bulls from a bull supplier with a reputation for healthy animals of good temperament.
• Exclude bulls with deformed feet.
• Choose virgin bulls whenever possible.
• Bulls should test negative for tuberculosis (TB), enzootic bovine leucosis (EBL) and bovine viral diarrhoea (BVD).
• Bulls should be vaccinated against leptospirosis and BVD. Seek advice from your vet on vaccination protocols, and if testing is warranted for additional sexually transmitted infections (STI), e.g. Vibro, and Neospora.

Managing bulls during mating

• Regularly observe bulls to identify injury or dominant animals that may be reducing the service ability of other bulls.
• Only handle bulls in suitable yards to minimise the risk of injury to people and the animals.
• If running multiple mobs of heifers keep mating records for each mob (e.g bull numbers, date bulls put out, date bulls removed).

Mating strategies for underweight heifers

Yearling heifers should be at, or above, 60% of their expected mature liveweight by the MSD to optimise pregnancy rates. In practice, there may be times when some heifers are significantly behind this target.

If a group of heifers are at less than 50% of their expected mature liveweight at the planned start of mating, there will be a significant proportion that will not have reached puberty. Some heifers may be so small or of low BCS, that they are not suitable for mating. In this situation, a vet should assess if a progesterone-based synchrony treatment (e.g. CIDR) is an option for triggering puberty in some or all of the animals, as long as their welfare is not put at risk. Your vet will assist in identifying welfare risks.

Actions to take for underweight heifers

1. Review feeding levels - both quality and crude protein levels, as well as quantity.
2. Assess animal health with a vet and take corrective action as needed.
3. If feeding levels are sufficient and any animal health issues are resolved, oestrus synchronisation may be an option for underweight heifers.
4. If synchrony is used, AB should be used for the first mating. Afterwards, bull ratios should be 1:15-20 (the same as for a natural mating programme).
5. Select bulls of a size and temperament that are less likely to injure small heifers at mating, and for calving ease.
6. Excessively small heifers should be withheld from mating for their own health and wellbeing. They should either be grown until they meet minimum mating weights or culled.

To avoid a similar situation occurring again, review weight targets and the causes of low growth rates, and develop a suitable management plan to deliver better outcomes in the future.