Relationships between the results of hypo-osmotic swelling tests, sperm motility, and fertility in Estonian Holstein dairy bulls

As an attempt to find an inexpensive and simple laboratory method for artificial insemination (AI) bull semen quality assessment, the osmotic resistance of spermatozoa was measured using the hypo-osmotic swelling (HOS) test, developed by Jeyendran et al. (1984) (labelled HOS-1), and its modifications (HOS-2, HOS-3), with decreased osmotic pressure aimed at challenging sperm survival ability. The test results were benchmarked against sperm viability measurements performed using the Computerized Motility Analyzer (CMA), and field fertility was calculated as non-return rate (NRR). Two age groups of Estonian Holstein bull sires were included in this study to test possible age effect on semen quality parameters. The HOS-1 test in fresh bull semen correlated well with sperm general motility (GMot) (r = 0.63, P < 0.001 at batch level and r = 0.77, P < 0.001 at bull level) as well as with progressive motility (PMot) in frozen-thawed (FT) semen (r = 66, P < 0.001 at batch level and r = 0.81, P < 0.001 at bull level), which makes the test suitable for the prediction of post-thaw semen quality. However, the HOS-2 and HOS-3 values in FT semen had high correlations with NRR (r = 0.65, r = 0.66, P < 0.001 at batch level and r = 0.63, r = 0.71, P < 0.01 at bull level), which was comparable to those between GMot and NRR or PMot and NRR. A combination of motility parameters and the results of the HOS-1 and HOS-3 tests provided a good model for predicting the potential fertility of bull semen. Values of sperm membrane post-thaw intactness, assessed using HOS-2, as well as of sperm motility measurements were higher in mature bulls compared to those in young bulls. Short conclusion: different modifications of the hypo-osmotic swelling test are useful for routine bovine semen quality assessment at AI stations.