Subfertile rams? Multiple causes to consider!

The latest research carried out across multiple veterinary practices, including the results of 583 pre-breeding examinations, demonstrates that over 22 % of rams are sub- or infertile (Price, 2021).

As I used to say to my clients (when discussing the benefit of a pre-breeding examination (PBE)) ‘there’s no lamb, without the ram’. And, if more than one-fifth of rams being used in a flock are sub- or infertile, it isn’t unreasonable to expect that it will be significantly harder to achieve target lambing percentages.

Table 1 - Production targets for UK sheep enterprises (AHDB, 2016; Farm Advisory Service, 2020)

Sub- or infertility in rams can be the result of several factors working alone, or in combination. Throughout this article we will consider some of the major factors and, discuss why a pre-breeding examination (with semen evaluation) should be performed annually.

A reminder about sperm production; it takes an average of 61 days to produce new sperm cells and, the sperm spend ten or eleven days maturing and passing through the epididymis (Walters, 2020; Van de Hoek et al., 2022). When we consider the factors affecting ram fertility, we must look at the previous eight to ten weeks and not just the health of the animals on the day they are introduced to the group of breeding ewes.

Body Condition Score (BCS)

Body condition score is one of the five parts of the ‘RAM MOT’ advocated for by the AHDB and other industry bodies (AHDB, 2020). It is sensible to check the rams’ BCS as far out as ten weeks before the intended start of the mating period, as it can take as long as eight weeks to increase a BCS by a measure of 1.0. Too high a BCS will make rams over-conditioned and lazy, whilst too low a BCS results in rams not having enough energy reserves to last the entire mating period (Charles, 2023). Obese rams may develop an excess of sub-scrotal fat and this can affect temperature regulation and, sperm survival. A target BCS of 3.5 – 4.0 / 5 is advised as rams can be expected to lose as much as 16 % of bodyweight throughout two cycles (Farm Advisory Service, 2020).

Anatomical conformation

As part of a PBE you should expect to see your vet palpate the testicles, scrotum and, spermatic cord of each ram to feel for any growths or abnormalities in the testicular conformation. A healthy and sexually mature ram may hold 100 billion sperm in his testes and reproductive tract, 75 % of these may be found in the epididymis (Van de Hoek, Rickard and de Graaf, 2022). Growths within the spermatic cord can affect the release of sperm from the testes whilst inflammation of the epididymis (epididymitis) can kill live sperm, inhibit the passage of sperm, or cause significant pain that prevents the ram from breeding.

Anatomical abnormalities (which could be naturally occurring, genetic or trauma-related) to the penis and prepuce should be checked for and, if found, their impact on the ability of the ram to fully exteriorise its penis and achieve intromission should be assessed.

Measurement of the scrotal circumference is also performed. The required circumference to pass the Sheep Veterinary Society’s standards (2014) are age-dependent to reflect the growth and maturation that occurs in rams. Some breed societies have their own guidelines, and these should be followed when relevant.

Figure 2 - Minimum scrotal circumference measurements to pass the PBE (SVS, 2014)

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Infectious diseases including antibiotic usage

Systemic infectious diseases can affect the ability of the ram to produce good quality, healthy sperm. Similarly, ‘minor’ illnesses or infections which may just cause a reduction in appetite for a few days can cause a ‘stress spermiogram’ which is enough to still have a negative impact on fertility.

Recent studies have demonstrated that the use of tetracyclines in small ruminants can have a detrimental effect on sperm quality and production (Mercan Yücel et al., 2021). Some diseases which are commonly treated with tetracyclines, such as transient lameness, can be controlled with other first-line (or category ‘D’/prudence (EMA, 2022)) antibiotics. As a result, it is worth considering which antibiotic products are used close to the mating period, although this can be a very flock and disease specific decision and guidance from the farm’s vet should be sought.


In cattle, it is known that the proportion of bulls which pass a pre-breeding examination decreases with each year of life after a peak pass percentage at 3 – 4 years of age (Pardede et al., 2020; Walters, 2020). The same research has not been performed in rams. However, in my experience carrying out PBEs over the last five years, age is an important factor with older rams having a higher chance of failing.

Likewise, it is not always advisable to introduce only ram-lambs or shearlings as some of these will not have reached sexual maturity and may be sub-fertile. Age-related sub-fertility usually affects the motility and amount of sperm cells in the ejaculate (rather than the physical conformation); therefore, semen analysis is required.


Lameness, and other orthopaedic conditions, can be a significant factor in the fertility of rams.

When mounting a ewe, the ram’s forelimbs leave the ground entirely with the majority of the body weight (often > 100 kg) transferring to the hindlimbs and feet. Any lameness due to infectious or conformational causes in the feet will make rams reluctant to go onto their hindlimbs, rendering them unable to breed. In a similar fashion, acute or chronic injuries or changes to the legs and joints can have the same effect.


Extremes of temperature can affect the desire of rams to breed, with too hot a temperature reducing libido and energy levels. More importantly, extremes of temperature can also affect sperm survival rates and after extended periods of time (heatwaves etc.) sperm production and motility can be affected. If rams cannot keep the testes cool over the six – eight weeks before the mating period, this will be significant.

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Several factors influence rams’ suitability for reproduction, many of which can be detected during a pre-breeding examination. Importantly, not all these changes can be detected just by physical examination of the animal. Veterinary assessment of a sperm sample is required in many scenarios, and the Dynescan is a useful tool for this.

Some of these changes will be irreversible, and, in these cases, the most productive option may be to cull the ram in question and purchase a replacement. Not doing this will increase the likelihood of experiencing poor scanning and lambing percentages. If the effect is transient or correctable, then the benefit of a pre-breeding exam before the mating period is that there may be time to correct the problem (e.g. increase BCS or treat a transient lameness).

Lamb prices this season peaked at 742.9 p/kg in May (AHDB Online, 2023). With an average lamb deadweight of 19.7 kg (Shahbandeh, 2021), it is easy to see how even a small increase in the number of lambs reared to point of sale as a result of a pre-breeding examination with semen analysis can lead to a significant return on investment.

David is an experienced production-animal veterinary surgeon. He completed his CertAVP at the University of Liverpool.  In 2023, David received the RCVSKnowledge Award for Quality Improvement. He sits on the UK-Vet Livestock Journal editorial board and regularly delivers clinical and non-clinical CPD.

Dr David Charles



AHDB (2016) Lamb: cost of production benchmarks | AHDB, AHDB Online. Available at: (Accessed: 1 September 2023).

AHDB (2020) Ram MOT leaflet | AHDB, AHDB Online. Available at: (Accessed: 4 November 2022).

AHDB Online (2023) GB monthly deadweight sheep prices | AHDB. Available at: (Accessed: 3 September 2023).

Charles, D. (2023) ‘How to: ovine clinical examination’, Livestock, 28(1), pp. 41–49. Available at:

EMA (2022) Categorisation of antibiotics for use in animals. Available at: (Accessed: 28 June 2022).

Farm Advisory Service (2020) ‘An Introduction to Benchmarking for Sheep’. Available at: (Accessed: 27 August 2023).

Van de Hoek, M., Rickard, J.P. and de Graaf, S.P. (2022) ‘Motility Assessment of Ram Spermatozoa’, Biology, 11(12), p. 1715. Available at:

Mercan Yücel, U. et al. (2021) ‘Adverse effects of oxytetracycline and enrofloxacin on the fertility of Saanen bucks’, Tropical Animal Health and Production, 53, p. 466. Available at:

Pardede, B.P. et al. (2020) ‘Decreased bull fertility: age-related changes in sperm motility and DNA fragmentation’, E3S Web of Conferences, 151, p. 01010. Available at:

Price, R. (2021) More than one-fifth of rams are sub-fertile, research shows, Farmer’s Weekly Online. Available at: (Accessed: 2 September 2023).

Shahbandeh, M. (2021) • Average sheep carcass weight 2003-2020 | Statista. Available at: (Accessed: 2 September 2023).


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