Sperm morphology of beef bulls evaluated by two different methods

The objectives of this study were to compare two different methods of evaluating bull sperm morphology, bright-field (BF) microscopy of eosin–nigrosin (EN) stained dry-mount semen smears and differential interference phase contrast (DIC) microscopy of wet-mount semen ‘fixed’ in isotonic formal saline, both at 1000×. Ejaculates (n = 72) were evaluated, representing both pre- and post-breeding season ejaculates collected from 40 2-yr-old beef bulls via electro-ejaculation. For both methods, 200 sperm were counted in random fields with defects categorized as major (MAD) and minor (MID). Sperm abnormalities were also placed into two other categories: those considered to be most influenced by process (wet or dry, METHDEF) and those with depictions that could be influenced by optics (BF or DIC, OPTIDEF). Differences (P < 0.05) occurred between DIC and BF methods respectively: MAD 23.3/16.1, MID 7.6/13.4, acrosome 3.8/1.1, midpiece 9.2/11.7, tail 2.0/4.7, droplets 8.3/4.2, METHDEF 14.2/21.4 and OPTIDEF 13.0/5.5, but not (P > 0.05) in percent normal sperm 69.1/70.4 or sperm head defects 7.5/8.3. Acrosome, tail and droplet defects were observed in 98.2/80.5, 86.1/100 and 98.2/94.4 percent of bulls for DIC and BF, respectively (P < 0.05). As percent normal sperm did not differ between methods, bright-field microscopy assessment of EN preparations was considered to be a satisfactory method to categorize breeding soundness of bulls. However, DIC was more effective in visualizing major defects, while BF (which included stained smear preparation) was considered to cause more minor defects. Thus DIC was considered to be the preferred method of semen assessment for accurate assessment of sperm morphology in bulls.