17 Comparison of two methods to evaluate sperm morphology in bulls and their effect on the breeding soundness classification

The conception rate achieved in natural mating systems by breeding bulls classified as sound is clearly higher than those classified as unsound. As sperm morphology is an important part of the final classification system, differences have been established between bulls with <15% versus =15% uncompensable sperm defects (USD). A major concern, however, is that the sensitivity and specificity for methods used to evaluate sperm morphology would affect their value in bull evaluation. This study examined two methods of bull sperm morphology evaluation by comparing their sensitivity and specificity over the final breeding classification, based solely on sperm morphology. Sperm morphology in 240 ejaculates from bulls managed extensively in tropical Costa Rica was studied with microscopy (200 cells/×1,000). Each sample was examined by two methods for detection of all sperm abnormalities: (1) fixed buffered formol saline (FS) wet smears under phase contrast microscopy, and (2) eosin nigrosin (EN) stained smears under brightfield microscopy. In addition, head morphology was assessed further with carbol-fuchsin staining (CF) under brightfield microscopy. Data were analysed by analysis of variance for repeated measures (SAS). Sensitivity and specificity for the FS and EN methods were obtained through contingency tables using data for sound (sires with <15% USD) or unsound (sires with = 15% USD) breeding classification using the combined classification of FS plus CF methods as the gold standard. The sensitivity of EN (58.6%) was significantly lower (P < 0.001) than FS (93.8%). Specificity for EN (97.4%) did not differ from FS (96.2%; P < 0.68). The lower sensitivity of EN to detect an unsound bull based on USD arose from its inability to detect abnormalities in the sperm nucleus and the presence of proximal cytoplasmic droplets (Table 1). Thus, the exclusive use of EN for evaluating sperm morphology underestimated the prevalence of abnormal acrosomes, head (nuclear), and proximal cytoplasmic droplets. In contrast, assessment of morphology by FS smears resulted in detection of most defects. However, the additional use of CF staining increased the capacity for detection of USD even more.

Table 1. Mean percentages (±s.d.) of sperm abnormalities evaluated through eosin nigrosin, formol saline, and carbol-fucshin