Relationship between phenotypic subcutaneous backfat thickness and spermiogram outcomes in young beef bulls

This study investigated the relationship between subcutaneous backfat thickness (SCBF) and semen quality in young beef bulls. We hypothesized that bulls with increased SCBF would have decreased semen quality compared with bulls of adequate SCBF, despite being fed the same diet. Data collected from yearling beef bulls (n = 710) enrolled in two bull development programs were utilized in an observational retrospective cohort study. Bulls were developed according to industry standard practices and growth performance was evaluated over 84 or 112 d following a 14- or 21-d adaptation period. At the end of the growth performance evaluation period, carcass ultrasonography was performed to evaluate ribeye area (REA), SCBF, and intramuscular fat. Furthermore, breeding soundness examinations (BSE) were performed within 40 d after carcass ultrasonography. Bulls that failed the BSE for reasons unrelated to semen quality were excluded from this study. To evaluate the relationship between SCBF and fertility variables, bulls were categorized into three cohorts based on SCBF estimates using two distinct thresholds. Within each location and year, bulls were retrospectively ranked according to their SCBF and classified into the top 10% (TOP10; n = 71), middle 80% (MID80; n = 569), and bottom 10% (BTM10; n = 70). To further investigate the relationship between SCBF and fertility, bulls were also categorized into the top 20% (TOP20; n = 153), middle 60% (MID60; n = 419), and bottom 20% (BTM20; n = 138) based on SCBF. TOP10 and TOP20 bulls had greater initial and final body weight, SCBF, REA, and IMF (P ≤ 0.01) compared with their respective cohorts. In contrast, SCBF classifications did not impact average daily gain, scrotal circumference, and sperm motility (P ≥ 0.29). The percentage of morphologically normal sperm was decreased in TOP10 and TOP20 bulls compared with their respective cohorts (P < 0.01), which are similar among them (P ≥ 0.31). These differences resulted in a greater proportion of TOP10 (P < 0.01) and TOP20 (P < 0.01) bulls classified as deferred compared with their respective cohorts. In summary, elevated SCBF in bulls exposed to the same diet was associated with an increase in sperm morphological abnormalities and resulted in a larger proportion of bulls classified as deferred during their first BSE.