Bull sperm morphology analysis varies greatly by reader

Sperm morphology assessments are important for selection of a herd sire, with acceptable fertility influencing bovine industry economics. Studies have evaluated effects of reader experience and evaluation method on sperm morphology in other species, but apparently no study evaluated bulls. Semen was collected from 35 yearling bulls at a university bull test station and from each bull, 1 eosin-nigrosin morphology slide (with a monolayer of sperm) was created by a board-certified Theriogenologist. Seven individuals (blinded to bull IDs) assessed slides: 4 board-certified theriogenologists (DACT) and 3 fourth-year veterinary students (VS) who completed an advanced reproductive elective course. One-hundred cells from each slide were evaluated (2018 Society for Theriogenology classification) in oil immersion (100 x) objective utilizing a light microscope in 1 sitting. A second evaluation was completed by each individual > 1 week later. Three DACT’s performed additional evaluations of 200 and 400 sperm from first 5 slides to determine if assessing a greater number of sperm would increase agreement of morphologic characteristics within and between reviewers. Data were analyzed using separate mixed model analysis of variance (Proc GLIMMIX, SAS v9.4) to test fixed effects of reviewer type, reviewer, slide number or number of sperm read, and interactions. There was an interaction (p < 0.0001) of reviewer type and slide (VS versus DACT), indicating some slide ratings were very similar between reviewer types and some differed significantly. Among slides that differed, VS identified larger number (p = 0.0001) of morphologically normal sperm compared to DACT. In addition, there were differences (p < 0.0001) in coefficient of variation between reviewers, but no differences (p = 0.78) between reviewer type. Within DACTs, there were no effects (p = 0.96) of numbers read on percent normal sperm, indicating that evaluating additional sperm did not affect outcomes of BSE. There was an interaction (p < 0.0001) of number of sperm assessed and reviewer on time to complete and for some reviewers, there were significant increases in time to complete increased sperm assessment. Standard cutoffs for adequate morphology assessment utilized for decades are questionable. Bulls not classified as satisfactory potential breeders are possibly due to fewer number of sperm evaluated and variation between assessors. Further investigation into slide preparation, microscope, and reader experience is important to validate this method of evaluating bovine sperm morphology.

Keywords: Sperm, morphology, bull, assessment

This manuscript was originally published in the journal Clinical Theriogenology Vol 12(3) Sept 2020.  Clinical Theriogenology is the official journal of the Society for Theriogenology (SFT) and the American College of Theriogenologists (ACT).  This content has been reproduced on the IVIS website with the explicit permission of the SFT/ACT.