AN OSMOTIC RESISTANCE TEST FOR BOVINE SEMEN

An osmotic resistance test (ORT) was developed for bull semen. Raw semen was incubated for 40-60 min at 35-degrees-C in a fructose/sodium citrate solution with an osmolarity of 150 mOsm kg-1. Semen processed in skim milk, egg yolk and glycerol diluent was mixed 1:4 with a similar solution at 100 mOsm kg-1. On examination with a phase contrast microscope (x 400), cells with intact membranes were swollen. Some of these showed residual activity, i.e. a non-progressive flickering of the mid-piece. The proportion (%) of cells showing this activity was taken as the test result. Repeatability was 0.85 for raw semen and 0.93 for post-thaw. Over 401 ejaculates from 80 bulls (three straws per ejaculate), 51.2% of variation was due to bull, 33.7% to ejaculate within bull and 15.1% to straw within ejaculate. Five or six ejaculates from yearling Holstein/Friesian bulls were used for a fertility trial. Each was ORT tested post-thaw and two groups of ejaculates formed for each bull according to ORT result. Friesian/Holstein cows were inseminated and 49-day non-return rates (NRR) obtained for each group. Raw ORT correlated with post-thaw ORT (r = 0.56), but not with NRR (r = 0.08). Over a series of 48 bulls, the post-thaw ORT:NRR correlation was 0.37, but after removing bull effects a correlation of 0.79 was obtained. As the method is simple and rapid, it shows promise as a routine assessment technique in commercial AI laboratories.