22 Jul 2025
Phil Elkins BVM&S, CertAVP(Cattle), MRCVS delves into the latest research around the farm-specific factors influencing successful conception rates
Phil Elkins
Job Title
Figure 1. Pregnancy rate in 500-herd National Milk Records cohort over time.
Enhancing and maintaining cattle fertility has been a mainstay of the workload of cattle vets for decades. While this often takes place crush-side at an individual animal basis, it is important to regularly consider the farm-specific factors influencing fertility success.
National Milk Records (NMR) undertake a study each year looking at 500 sentinel herds to report on national level dairy statistics1.
The latest report shows continued significant progress in fertility, with the median herd achieving 39% of cows conceived by 100 days in milk, compared with 27% in 2010, and a pregnancy rate that has nearly doubled from 9% to 17%. This has occurred against a backdrop over the same time period of an increase in milk per cow per year of around 1,000 litres.
Two very important points are pertinent for the author to bring up now. Firstly, the definition of pregnancy rate; that is the proportion of eligible cows that conceive within a three-week period. To be comparable between farms, the definition of eligible must be standardised, which the NMR study will have achieved.
When using farm management software to derive the pregnancy rate, a number of farm-specific factors influence the figure that will not be standardised, such as voluntary waiting period (VWP), recording of cows not to be bred and defined mating periods.
The second point is that each of the farm data software providers calculates eligibility in a slightly different way. If we consider eligibility to be a cow that is past her voluntary waiting period, not designated as a “do not breed” (DNB) cow and not yet pregnant, this suggests a fairly robust definition. Once this is looked at across a three-week period, it is less robust; how many days of the 21 must a cow be past her VWP for her to be eligible? Does she still count if she is culled midway through a three-week period? The consequence of this is that entering identical data into two different software programmes will give differing pregnancy rates.
These two points combined make benchmarking fertility performance between farms, and in particular using farm-derived pregnancy rates, challenging and prone to over-interpretation.
What is more important is understanding the required level of fertility for success on each farm, and the benefits and pitfalls of going much beyond that. For example, if the pregnancy rate was a theoretical 100% with a 50-day VWP, this means that all cows not already designated DNB are pregnant by 71 days in milk. This leads to an increased proportion of the year as a dry cow, with the increased temporal risk of transition disease and the challenge of drying cows off giving very high yields.
Each cow will have an optimum time to conceive (for some it will be never). For many, it will sit in the range of 80 to 110 days, and the ultimate aim of fertility should be to get as many cows as possible pregnant within that window. One key to achieving this is having cows that are cycling normally as soon after calving as possible – this is primarily a factor of transition success.
A lower peak of energy deficit in early lactation is associated with a delay in resumption of normal luteal activity2, which will then delay service, and reduce the potential for conception. This negative energy balance has complex consequences for metabolic processes, one of which is an increase in non-esterified fatty acids (NEFAs) and circulating ketones, both of which have disruptive effects on oocyte quality and immunity3. Individual milk protein percentage is a barometer for the metabolic status of that animal. Lower milk protein percentages in early lactation are associated with delayed first service, reduced first service conception rate and reduced proportion of cows pregnant within six weeks of the start of eligibility for all lactation yields4.
So, it is clear to see that transition nutrition has a significant role to play in fertility, particularly in early lactation. In fact, in the author’s opinion, the improvement in pre-calving and fresh-calved nutrition during the past 10 years is responsible for a significant proportion of the overall increase in fertility experienced by the national herd.
Nutrition and its role in fertility is obviously an incredibly complex subject; considering the balance of rumen degradable protein (RDP) to rumen undegradable protein (RUP), here is indicative of the role of nutrition in fertility, rather than a specific area to target. An Australian two-by-two study compared two different proportions of rumen degradability (37% versus 15% RUP), measuring both production and fertility outcomes5. The diets were identical for energy provision and total protein provision. Cows fed the higher RUP diet (more bypass protein) significantly outperformed those fed the lower RUP diet:
In particular, where early lactation fertility is sub-optimal, or where fertility is improving with amount of time calved, particular attention should be placed on nutrition around the transition period.
It has been suggested that genetic factors that affect both protein yield and reproductive performance together are responsible for the improved performance in both – in other words, that the increase in one has been driven by association with breeding for the other. A single, retrospective cohort study of more than 50,000 lactations found, in fact, that both protein yield and genetic fertility breeding values were associated with improved fertility independent of each other6.
The heritability of fertility is low, at between 0.5% to 10% depending on which trait you are considering. This means that between 90% and 99.5% of the variability between cows is influenced by factors other than genetics (such as nutrition and management).
The near-ubiquitous use of genomics allows the “noise” around fertility to be removed, with the current fertility index giving producers the best opportunity to maximise the benefits of genetics.
The relatively low heritability of fertility means that short-term gains will be more feasible from improving nutrition and management. However, the other side of the coin is that genetic fertility improvements will be sustained over the long term, and will be accumulate over generations. For the sustainable future of dairy herds (breeding their own replacements), farmers must engage with genetic selection for fertility.
The author would suggest, as a bare minimum, only selecting bulls with a fertility index at least as good as their current herd, accepting that genetic selection must balance all the requirements of the herd.
It would be remiss when discussing fertility to not consider the role of the bull. Even in fully artificially inseminated herds, bull fertility plays a significant role in overall fertility.
Semen analysis has historically consisted of a single time-point assessment of motility and morphology leading to a binary result – fertile or not. Considering the distance sperm must travel prior to fertilisation, it is important that the spermatozoa are not just motile, but motile at a suitable velocity for an adequate duration of time. Anything challenging that motility will lead to reduced conception rates.
A case study by the Scottish technology company, Dyenval, looked at a suckler herd using three bulls, where all bulls had undertaken breeding soundness exams according to the BCVA protocol and passed7. Each bull had been run with a group of 30 cows. Two of the bulls had only succeeded in getting a single cow pregnant over the summer. The third bull got 28 out of 30 cows pregnant. Semen from the three bulls was collected and subjected to assessment of progressive motility every five minutes for 15 minutes. The charts from the three bulls are shown in Figure 2 (bull 2 had a second sample tested) – it is clear to see that bull 3 maintained progressive motility for the full 15 minutes. Both of the other bulls had moderate initial progressive motility, but this rapidly drops off over time with negligible motility by 15 minutes.
The use of technology to allow continued assessment of progressive motility has many applications for cattle vets, including improving the breeding soundness exam and investigating sire-specific reduced fertility on dairy units.
The improving cattle fertility within the national dairy herd is obviously excellent for farmers. However, this will continue to be an area of interest for cattle veterinarians, and a significant topic for advice for our clients. Awareness of the role of nutrition, genetics and bull fertility is key to ensuring the trend continues.
Phil Elkins qualified in 2005 from The University of Edinburgh and, following stints in Cheshire and New Zealand, spent the majority of his career in clinical practice in Cornwall, during which time he gained a certificate in advanced veterinary practice in cattle. Following 15 years in clinical practice and a stint working for an agri-tech company, Phil now works as an independent consultant to both farms and industry bodies. He is a former council member of the BVA.
