PSV-15 Evaluating the relationship between subcutaneous backfat thickness and sperm morphology in young beef bulls

Abstract Literature indicates that high-energy diets can lead to reductions in both sperm motility and morphology in bulls. This study aimed to investigate the relationship between individual variation in subcutaneous backfat thickness (SCBF) and sperm morphology in young beef bulls exposed to the same diet. We hypothesized that bulls with excessive subcutaneous fat thickness have increased sperm morphology defects compared with bulls with adequate subcutaneous backfat thickness. Historical data collected from yearling Bos taurus beef bulls (n = 557) enrolled in a bull development program were used in an observational retrospective cohort study to test this hypothesis. Over the course of 5 yr, bulls in two locations were developed according to industry standard practices, and performance was evaluated over 84 d with a 21-d adaptation period. At the end of the performance evaluation period, a carcass ultrasound was performed to evaluate ribeye area, SCBF, and intramuscular fat. Furthermore, breeding soundness examinations (BSE) were performed according to the Society of Theriogenology guidelines within 40 d after carcass ultrasonography. Bulls that failed the BSE for reasons unrelated to semen quality were excluded from this study. Within each year, bulls were ranked based on SCBF and classified as top 10% (TOP; n = 55), middle 80% (MID; n = 446), and bottom 10% (BTM;n = 56) for SCBF. Initial body weight (BW) was greater (P < 0.01) in TOP bulls compared with MID and BTM (Table 1). Off-test BW was also greater in TOP compared with MID and BTM bulls (P < 0.01). In contrast, average daily gain and scrotal circumference did not differ between experimental groups (P = 0.35 and P = 0.65, respectively). TOP bulls had greater (P < 0.01) SCBF compared with MID and BTM, and MID bulls had greater (P < 0.01) SCBF compared with BTM bulls. Similar results were observed for intramuscular fat (P < 0.01) and ribeye area (P < 0.01). Percentage of normal sperm cells was greater in both BTM and MID bulls compared with TOP bulls (P < 0.01). Additionally, percentage of primary abnormalities tended (P = 0.07) to be greater in TOP bulls compared with BTM and MID. There were no differences in the percentage of secondary abnormalities between experimental groups (P = 0.45). There was a tendency (P = 0.07) for a greater percentage of TOP bulls to fail the BSE compared with MID and BTM bulls. Moreover, there was a positive linear relationship (P = 0.01) between SCBF and the probability of failing the BSE. In summary, elevated SCBF in bulls exposed to the same diet was associated with an increase in sperm morphological abnormalities and tended to increase the percentage of bulls failing the BSE.