The Impact of Sub-Fertility in Bulls
5 February 2025This factsheet highlights the impact poor bull fertility can have on suckler herds, some of which can be less obvious. Indicative costs are provided, but the cost on your own farm may be different.
Fertility is a key driver of profitability in the suckler herd. A range of factors can influence fertility including nutrition, health, and genetics in both cows and bulls. However, farmers should aim to have 94-95% of their cows in-calf within a 9-week bulling period (6 weeks for heifers). A failure to achieve these key performance indicators (KPIs) will result in herd management issues and be detrimental to enterprise finances.
What is Sub-fertility in Bulls?
While the bull may only be 50% of the fertility story, the cow to bull ratio means that one bull can have a much greater impact on herd fertility. It is reported that up to 5% of bulls can be identified as being completely unable to get cows in-calf (i.e. they are sterile), but around 20-25% of bulls are sub-fertile (i.e. they will get cows in calf, but the bulling period will be extended). Bulls of all ages and breeds can be found to be sub-fertile due to problems associated with sperm defects, sperm quantity, libido, or physical problems with reproductive organs or locomotion. Injuries and infections can also negatively impact bull fertility.
Fertile bulls should achieve a 94% pregnancy rate in 9-weeks where the conception rate (CR, the number of females in the herd that conceive on the first try of artificial insemination or natural service) averages 60% to each service. By contrast, a sub-fertile bull may only achieve a 40% conception rate to each service. The impact of this is demonstrated in Figure 1 which shows that a sub-fertile bull will only achieve a pregnancy rate of 77% compared to the 94% achieved by a fertile bull over a 9-week mating period.
Figure 1. Comparison of number of cows conceiving in each 3-week cycle of a 9-week mating period for two different conception rates in a breeding group of 35 cows.
Sub-fertility can Lead to an Increase in Barren Cows
Furthermore, 23% of cows are barren after running with the sub-fertile bull compared to only 6% in the group running with a fertile bull in Figure 1. Currently (January 2025) cull cow values are at record levels, with many in good condition exceeding £2,000 per head, driven by a global shortage of beef. The cost of keeping a suckler cow per day varies by system but is generally around £2 - £3 per day. The sale value greatly exceeds the cost of maintaining an empty cow for another year, so culling barren cows makes financial sense. The question is whether to replace these cows or not.
Short term, a smaller herd will reduce variable costs e.g. feed, bedding, keep taken and sundry livestock expenses etc. Although, you need to consider that fixed costs e.g. labour, utilities, insurance, machinery purchase or repair, may increase as they are spread across a smaller herd size.
If the herd size is to remain stable and viable, homebred or purchased heifers or cows with calves at foot will be required. Purchasing replacements offers the advantage of introducing new genetics to the herd but can come with the risk of introducing disease, drug resistance and unknown traits. Contraction of the national herd by 12% in the last decade, means there are less replacements available, which has pushed up their value.
To determine the cost of culling, you should calculate the difference between cull cow returns and purchasing replacements.
The management of replacement heifers in the suckler herd is covered in more detail in Technical Note (TN744).
By the Numbers - The Impact on the Calf Crop
An increased barren rate results in less calves available for sale be that as weaned calves, yearlings or finishing depending on your system. As calves are the only output from the suckler herd, this can have a significant impact on gross margins in this enterprise.
The potential lost calf crop value from the group with a 23% barren rate could be £24,150 compared to £6,300 in the herd with a 6% barren rate by the time calves reach weaning at 200 days. This calculation is based on the assumption that no cows had twins, a calf birth weight of 40 kg, daily liveweight gain of 1.2 kg/day and market value of £3.75/kg (January 2025). The cost of keeping the cows and bull in each group would be the same.
It can be tempting to simply run sub-fertile bulls with cows longer as they may eventually conceive but this is false economy. Extending the mating period leads to an extended calving period, increasing labour requirements during this time and making management of cow nutrition challenging. Moreover, calves conceived in later cycles will be younger and lighter in weight at weaning. Not only does this pose a greater risk of health issues e.g. scour and pneumonia from mixing different ages of calves, but it will have a financial impact. Assuming a growth rate of 1.2 kg per day, and market value of £3.75/kg (January 2025), each cycle missed results in calves being 25.2 kg lighter at weaning at a cost per cow of £94.50. This may be an underestimation if health issues present in the group reduce daily liveweight gains. In the example shown in Figure 1, 7 less calves are born in the first cycle to the sub-fertile bull than the fertile bull which would total £661.50.
The impact on herds breeding their own replacements may be more acute. Ideally replacement heifers should be born in the first three weeks of calving. As shown in Figure 1, less calves are born in the first three weeks of calving to select from in the group running with a sub-fertile bull. Such herds may have to consider buying-in replacements.
How to Identify Sub-fertility in Bulls
All bulls should undergo a Bull Breeding Soundness Examination (BBSE) at least 6 – 8 weeks before each breeding season to highlight potential fertility issues in sufficient time to plan an alternative strategy. This should include newly purchased bulls too, unless they were tested before sale, and this was less than 8 weeks prior to using the bull. Your vet will conduct a physical and semen examination before classifying as either unsatisfactory or satisfactory and suitable for breeding. The cost of a bull breeding soundness examination including semen analysis, is typically £100 - £150 per bull.
It is vital that farmers observe bulls showing libido, mounting and serving cows throughout the mating season. Sub-fertility is more likely to go under the radar when bulls are rotated. Monitoring cows for return to service is also beneficial. Should there be any concerns, pregnancy scanning can be done 28 - 35 days from last service to confirm pregnancy.
What to do if a Bull is Sub-fertile
The key to success is early identification of sub-fertility. If a bull is found to be sub-fertile during the breeding season, there are two main options to limit the impact on the barren rate and calf crop:
- Source another bull
- Use artificial insemination (AI).
Sourcing another bull is the simplest option to replace a sub-fertile bull, but this bull should also have undergone a BBSE. If you need to buy or borrow a bull from another farm, consider the risk of introducing disease and the need for quarantine.
Artificial insemination (AI) is uncommon in commercial beef herd but offers the opportunity to access greater genetics and sexed semen is also available which can be useful for herds seeking to breed their own heifer replacements. Successful conception to AI is dependent on good heat detection by visual observation of behaviour or using aids such as tail paint, heat mount detectors, teaser bulls or activity monitors and recording. But the need for heat detection can be reduced or eliminated with synchronisation – administering drugs to ensure all cows and heifers are cycling at the same time. Synchronisation can therefore help tighten calving periods. Age, breed, body condition and nutrition all influence conception rates following AI, but they are not dissimilar to natural service. AI costs can vary depending on synchronisation protocol, cost of semen straws, and AI technician fee, but may not be dissimilar to the cost of retaining a bull.
Top Tips to Minimise the Impact of Infertility in the Suckler Herd
- Keep records of when bulls go out, which bulls run with which cows, and service dates. This will be useful to refer to if something goes wrong.
- Book your vet to BBSE all bulls 6 – 8 weeks before the breeding season each year.
- Prepare a plan B with your vet as part of your herd health plan.
- Don’t forget health and nutrition for both cows and bulls.
More information on suckler herd fertility can be found in the Sustainable Beef Systems Fertility Webinar.
Laura Henderson, SAC Consulting
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