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Evidence supporting a direct relationship between the scrotal circumference (SC) of bulls and the age at which their female offspring reach puberty is scarce and outdated.
While the results of some studies indicate that increased SC is correlated with a reduction in the age at which female offspring reach puberty, none of these correlations were found to be statistically significant.
However, bull SC measurement is a relatively simple, reliable and repeatable procedure that may still have value as a selection criterion for reducing the age of puberty in female offspring.
When reviewing the results from a recent bull breeding soundness examination visit to a beef cattle stud, you notice substantial differences in the scrotal circumference (SC) measurements between animals, despite them being the same age and of the same breed. You have often heard it stated that the age at which female offspring reach puberty decreases with an increase in sire SC. However, you would like to confirm this statement with evidence from the literature before discussing breeding recommendations with your client.
In the [beef bull], does increased [scrotal circumference] measurement directly reduce [puberty age] in [daughters]?
The keywords (bull OR cattle OR male OR beef OR “Bos Taurus”) AND (“scrotal circumference” OR “scrotal size”) AND (“puberty” OR “first service”) AND (progeny OR offspring OR heifer OR daughter) were used to search CAB abstracts and Scopus.
Nineteen papers were found in the Scopus search.
Fourteen were excluded because they did not answer the question or were duplicates.
Three were excluded because they were half-sibling studies.
In total, two relevant papers were obtained.
Twenty-eight papers were found in the CAB search.
Twenty were excluded because they did not answer the question or were duplicates.
Six were excluded because they were half-sibling studies.
One was excluded as it was not written in English.
In total, one relevant paper was obtained.
Overall, three relevant papers were identified.
Search last performed: October 2019
Four additional papers were sourced from the reference lists of the papers returned by the search. However, all four of these were excluded because they were half-sibling studies.
Summary of evidence
Paper 1: Relationships of sire scrotal circumference to offspring reproduction and growth1
Patient group: A total of 60 Hereford, Angus and red Angus bulls from which SC measurements were obtained. These measurements were age-adjusted to give the SC at 365 days of age.
Study type: Cohort study.
Outcomes: The age at which 779 female offspring of these bulls reached puberty was recorded.
Key results: The results reveal a distinct, but not statistically significant, correlation indicating that an increase in sire SC decreases the age at which female offspring reach puberty. For every 1 cm increase in SC, the age at which female offspring reached puberty was found to decrease by 0.796 days. A definite seasonal effect on age of puberty was also observed, with heifers born later in the season reaching puberty at an earlier age.
Study weaknesses: While a correlation between sire SC and the age at which female offspring reach puberty was determined, it was not found to be statistically significant. The sample size was also relatively small, and the method by which the age of puberty was determined was not stated. In addition, the study was performed in a well-managed, purebred research herd, and it is unclear whether the results could be generalised to the wider cattle population.
Paper 2: Estimates of genetic correlations between testicular measurements and female reproductive traits in cattle2
Patient group: This study included a total of 36 Hereford bulls from two herds that were involved in a long-term selection programme. The testicular circumference, diameter, length and volume of these bulls were measured at the ages of 205 days and 365 days.
Study type: Cohort study.
Outcomes: Age at first breeding and age at first calving were determined for 645 female offspring of these bulls, and genetic correlations were estimated from sire-daughter analyses.
Key results: In the sire-daughter analyses, the SC of bulls at 205 days old favourably correlated with their daughters’ age at first breeding (–1) and age at first calving (–0.56). However, no such favourable correlation was observed between the SC of bulls at 365 days old and their daughters’ age at first breeding (0.16) or age at first calving (0.58).
Study weaknesses: The sample size used in this study was small, and only the heifers’ age at first breeding and calving were examined, not age at first oestrus or puberty. Additionally, the study assumes that no maternal effects on these measures were present.
Furthermore, although the jack-knife procedure used in this study is a useful method for estimating genetic parameters and their standard errors, the large number of estimates obtained in this study may have affected the results of the correlation analyses.
Paper 3: Effects of selection for scrotal circumference in Limousin bulls on reproductive and growth traits of progeny3
Patient group: Eighteen Limousin bulls were arranged in nine pairs based on average adjusted yearling SC – one bull in each pair had a large SC (mean 36.3 cm) and one had a small SC (mean 28.5 cm). In addition to phenotypic grouping, non-parent SC estimated progeny differences (EPD) were used to group bulls into high, average and low lines. Each bull was then bred to a randomly assigned group of between 15 and 20 brangus/Hereford cross cows each season for one to three years.
Study type: Randomised controlled trial.
Outcomes: The age at which 210 female offspring of these bulls reached puberty – defined as progesterone levels being above 1 ng/ml for between two and three consecutive weeks – was recorded.
Key results: The female offspring of bulls with a large SC reached puberty an average of 6.9 days before the female offspring of bulls with a small SC, but this difference was not statistically significant. Additionally, a significantly greater percentage (P<0.05) of heifers bred by sires from the high EPD line had reached puberty by 13 months of age, compared with those bred from sires from the average and low EPD lines.
Study weaknesses: The number of bulls assessed in this study was very small, so it is unclear how generalisable these findings would be.
Evidence supporting the direct relationship between sire SC and age of puberty (AOP) in daughter heifers is scarce and outdated. The study by Smith and colleagues is the only study to report a direct effect of sire SC on daughter AOP, concluding that with every 1 cm increase in sire SC the AOP was reduced by 0.796 days.1 However, the sample size was relatively small (60 sires from a well-managed, purebred research herd), and, although a correlation was observed, the result was not statistically significant.
Moser and colleagues also examined the direct effect of sire SC on AOP in heifers.3 They found an average decrease in AOP of 6.9 days between large and small SC groups, but this result was again not statistically significant. This study also showed that the proportion of daughter heifers reaching puberty by 13 months was significantly increased for sires with higher SC EPD, implying that a genetic relationship may be present between the two traits.
Recent advances in genetic analysis methodology have seen studies on the relationship between SC and AOP switch to gene analysis – a more practical, repeatable technique for which large sample sizes are more easily obtained. For example, a genome-wide association study assessing the genetic relationship between Brahman bulls (n=1105) reaching an SC of 26 cm and the presence of the first corpus luteum in Brahman heifers (n=843) has been performed.4 Of the 280 single nucleotide polymorphisms associated with these traits individually, 32 (approximately 22 per cent) were associated with both traits and all were located in the same region of the same chromosome. This suggests that the genes located in this region are related to age at puberty in both sexes in Brahman cattle, potentially assisting in genomic selection for earlier puberty in both males and females.
Another study examined these genetic correlations of bull and heifer puberty traits in two tropical beef cattle genotypes.5 While scrotal circumference was correlated with heifer AOP, only low to moderate correlations, ranging from –0.06 to –0.41, were seen. These results contradict those of the genome-wide association study.4 The correlations were higher in both genotypes when using SC measurements taken at six and 12 months of age. However, by 24 months of age, no significant association between SC and heifer AOP was detected in either genotype.5
It should be clarified that the majority of studies discussing the relationship between SC and AOP have examined the correlation between AOP in heifers and yearling SC in half-sibling bulls.2,6–13 This approach is based on the premise that very strong genetic relationships indicate that AP and SC are essentially the same trait,14 as would be suggested by the results of the genome-wide association study.4 Further reasoning for this approach is provided by Land and colleagues’ study, demonstrating a correlation between testicle size in male mice and ovulation rate in female mice of 0.97, and a genetic correlation in both mice and sheep between the quantitative expression of reproductive characteristics in male and female animals.15
The results of half-sibling analysis in cattle have indicated variable genetic correlations between AOP in heifers and SC in half-sibling bulls. While many studies have demonstrated similar negative (ie, favourable) genetic correlations (–0.32,10 –0.39,2 –0.57,7 –0.7113 and –1.078), the relationship is uncertain and appears to be reduced in studies examining larger numbers of heifers.16
Despite the lack of published evidence of a direct relationship between sire SC and offspring AOP, it is understandable that SC is still purported as an effective selection criterion for reducing AOP in female offspring. SC measurement is easily performed, cost-effective and can be performed in bulls of any age. Additionally, it is widely accepted that greater selection intensity can be applied to this male trait than direct female reproductive traits.7
If further conclusive phenotypic studies are not performed, genetic markers associated with improved fertility must be further evaluated to enable vets to appraise the true relationship between sire SC and offspring AOP.
Critically Appraised Topics (CATs) are a standardised, succinct summary of research evidence organised around a clinical question, and a form of evidence synthesis used in the practice of evidence-based medicine (EBM) and evidence-based veterinary medicine (EBVM). Access to CATs enables clinicians to incorporate evidence from the scientific literature into clinical practice. CATs will be published regularly in the Clinical Decision Making section of Vet Record.
We thank Kate Allman from the Charles Sturt University library for her valuable assistance with database searches and referencing.
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