Comparison of bull sperm morphology evaluation methods under field conditions

Sperm morphological assessment is a critical component in bull breeding soundness evaluations. Although sperm morphology is an important parameter to identify subfertile from fertile bulls, the evaluation can be biased because sperm staining methods used under field conditions may not be exact due to various factors, including artifacts. Objective was to compare 2 sperm morphological evaluation methods. Prebreeding season ejaculates of 1,216 Angus cross bulls collected via electroejaculation were evaluated. For each bull, an unstained (UNS) and an eosin-nigrosin stained (ENS) semen smear were viewed under a phase contrast microscope and a brightfield microscope with an oil immersion lens, both at 1,000 × magnification. Normal and percentage of abnormal sperm were identified by counting 200 sperm. Inter-rater agreements between 2 clinicians for the percentage of abnormal sperm determination and its categories were very good (ENS method, r = 0.84 – 0.96; UNS method, r = 0.76 – 0.96; p < 0.01). No differences (p > 0.1) were observed for abnormal sperm percentage determination and its categories between 2 methods. Correlation was very good between 2 methods for total abnormal sperm percentage determination (r = 0.91; p < 0.01) and its categories (r = 0.84 – 0.96; p < 0.05). Additionally, 60 ejaculates were evaluated by triple stain (TS), ENS, and UNS methods. Agreements between TS (percentage of sperm with damaged membrane) and ENS (percentage of abnormal sperm) and between TS and UNS methods were moderate (r = 0.58; p < 0.05) and fair (r = 0.43; p < 0.05), respectively. Based on our findings, either technique can be used for bull sperm morphological evaluation under field conditions. Considering the ease of semen smear preparation, the UNS method can be a viable alternative to the ENS method.