5 min read
Successful heifer mating is essential for their productive future. Targets include reaching puberty at 12-13 months, conceiving by 13-15 months, and being of adequate weight at calving. Liveweight, not age, determines puberty onset. Mating should start 1-2 weeks before the milking herd. Conception is best when heifers reach 60% of their mature liveweight at mating. Options for heifer mating are artificial breeding and natural mating. When selecting bulls, consider fertility, health, temperament, and disease testing. Observe bulls regularly during mating to ensure their well-being and identify any issues. Mating strategies for underweight heifers require special attention. Consult with a vet for guidance.
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.
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.
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.
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.
Stock owners commonly use one of the following approaches when mating heifers:
If you are considering using AB for yearling heifers, consider the following factors:
A mating management plan should be in place before you start, if you are in a grazing contract relationship. It should cover:
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.
When using bulls there are four critical areas to manage to achieve a high pregnancy rate:
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.
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.
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.
Table 1. Mature liveweights for bulls of different breeds.
|Breed||Mature liveweight (kg)|
|Jersey||550 - 700|
|H-F x J crossbred||675 - 800|
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.
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.
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.
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.