16 Sperm morphology in frozen-thawed bull semen: dairy versus beef and the effect of sire’s genotype

The success in cattle breeding using frozen semen in artificial insemination programs, in both dairy and beef production systems, depends on factors such as the environment, health, nutrition, management, and semen quality—the latter being of enormous impact. Frozen-thawed semen quality depends on factors related to the cryopreservation process and initial sperm quality, including sperm morphology. The presence of a high proportion of sperm with uncompensable sperm defects (USD) (>15%) in frozen semen is related to low pregnancy rates. This abstract reports the outcome of semen straws submitted for testing at the andrology section, veterinary school, Universidad Nacional de Costa Rica, and aims to analyse the results of sperm morphology, specifically, the percentage of USD based on breed purpose, dairy (DB) and beef (BB), as well as subspecies, Bos taurus (BT), Bos indicus (BI), and composite breed (CB). A total of 1,160 frozen bull semen straws were evaluated. The same operator performed the evaluation after thawing the straws in a water bath at 38°C for 30 s. Sperm head abnormalities (size and shape) were determined in smears stained with carbol fuchsin (200 cells/slide-1000×). USD such as diadem, simple nuclear pouches, proximal cytoplasmic droplets, knobbed acrosome, nuclear crater, and abnormal middle pieces were assessed in wet smears fixed with Hancock solution (phase contrast, 200 cells/slide-1000×). Statistical effects of dairy or beef bull, subspecies, and their interaction on USD percentage were analysed by a generalised linear model using arithmetic means (GLIMMIX, SAS). Due to non normal distribution, the Tukey-Kramer test was used under a log-Normal distribution assumption to compare least-squares medians for significant effects. DB and BB semen accounted for 38.4% (n = 445) and 61.6% (n = 715) of samples analysed, respectively. BT, BI and CB accounted for 61.9% (n = 718), 30.4% (n = 353), and 7.7% (n = 89) of samples, respectively. Significant effects of dairy or beef bull (P < 0.0001) and sire’s subspecies (P < 0.01) on USD were detected, while the interaction was not significant (P = 0.33). Adjusted mean USD for BB (21.76 ± 1.01, IC 95%: 19.8–23.7) was significantly higher (P < 0.0001) than DB (12.17 ± 2.05, IC 95%: 8.1–16.2). USD mean for CB (23.36 ± 2.74, IC 95%: 18.0–28.7) was significantly higher (P < 0.01) than BI (12.72 ± 1.91, IC 95%: 9.0–16.5), but not significantly different (P = 0.20) than BT (14.82 ± 0.79, IC 95%: 13.3–16.4). The difference between BT and BI was also significant (P < 0.05). Although, the interaction between dairy or beef bull and sire’s genotype was not statistically significant, frozen semen from beef, BT and CB bulls showed higher percentage of USD compared to dairy and Bos indicus sires. In addition, it is important to highlight that both BB and CB average USD percentage was >15%, which may negatively impact pregnancy rate. These results emphasise the need to maintain strict semen quality controls over the frequency of sperm abnormalities before freezing, particularly considering the increase in beef and cross breed sire use in fixed-time artificial insemination programs.