Influence of sperm number per straw on the post-thaw sperm viability and fertility of Swedish red and white AI bulls

Semen from 5 Swedish Red and White bulls, approved in the fertility and progeny testing programme of a bull center, was split-frozen to produce straws with 15 or 10x10(6) spermatoza each (control and treatment dose, respectively). Post-thaw sperm viability was evaluated by visual assessment of sperm motility (MOT), measurement of ATP (Adeinosin Tri Phosphate) contents by luminometry, assessment of membrane integrity with combined fluorophore probes [Calcein AM (CAM)/Ethidium homodimer (EthD-1)] and by using a hyposmotic swelling lest (ORT). The straws were used for a total of 16651 artificial inseminations (A.I.). No statistically significant difference was recorded between the 2 treatments for any of the post-thaw sperm viability parameters. In addition, a significant bull effect was evident for most post-thaw sperm traits assayed. Significant variation in overall fertility (56-days NRR) was recorded among the bulls used. A.I. with a reduced number of spermatozoa (10x10(6)/straw) resulted in a 2%-units decrease (n.s.) compared with controls (67.8% +/- 4.8%, means +/- SD) in overall fertility. In the control split-sample (15x10(6) spermatozoa/straw), MOT did not show any statistically significant correlation with fertility (r = 0.41,p = 0.07). However, MOT was correlated with the percentage of spermatozoa depicting progressive motility (category Al, r = 0.45, p<0.05) as assessed with CAM/EthD-1. The latter was correlated with ATP contents (r = 0.57, p<0.01), expressed as the percentage of viable spermatozoa. Both CAM/EthD-1 and ATP contents showed a statistically significant correlation with ORT (r = 0.45, p<0.05 and r = 0.61, p<0.05, respectively). In the straws with the reduced sperm number (10x10(6) spermatozoa), post-thaw motility was significantly correlated with fertility (r = 0.50, p<0.05) and ATP-total contents (r = 0.48, p<0.05). CAM/EthD-1 and ATP contents (as million viable spermatozoa) were significantly correlated (r = 0.47, p<0.05). CAM/EthD-1 was significantly correlated with ORT (r = 0.48, p<0.05). In conclusion, the results indicated that the freezing-thawing of straws with a reduced sperm concentration (10x10(6) spermatozoa) did not alter the post-thaw viability or overall fertility of the bull semen used. However, in view of the significant bull effect found in the limited population studied, we recommend that such a reduction in sperm number/straw be based on the fertility of the bull in question.