15 QUANTIFICATION OF BULL SPERM TRAITS AS ASSESSED BY COMPUTER-ASSISTED SEMEN ANALYSIS AND THE RELATIONSHIP TO PREGNANCY RATE FOLLOWING CONTROLLED BREEDING

The bull’s contribution through artificial insemination to reproductive efficiency is of great biological importance. The objectives were (1) to compare the oestrous synchronization response of Bonsmara and Nguni cows; and (2) to find the relationship between cow’s conception rate (in vivo and in vitro fertilization) and bull sperm motility rate assessed by computer-assisted semen analysis (CASA) following AI. For the in vivo sperm fertility test, 100 Bonsmara and 482 Nguni cows were randomly selected and subjected to oestrous synchronization protocol and AI with frozen–thawed assessed semen by CASA before AI. Briefly at Day 0, cows were inserted with an intravaginal CIDR® (1.9 g), which was removed on Day 7. Prostaglandin was then administered (2 mL) on Day 8 and a heatmount detector was placed on the hindquarter of each cow. For the in vitro sperm fertility test, collected oocytes from slaughterhouse were in vitro matured (n = 360) and in vitro fertilized (sperm/mL) in 100-µL droplets (final volume) of BO-IVF medium per treatment bulls (Bonsmara or Nguni bull). The frozen/thawed semen straws of Bonsmara and Nguni bulls were randomly selected and used under the same IVF conditions. The thawed bull’s sperm characteristics were examined by CASA before in vitro fertilization. Data were analysed using ANOVA. Treatment means were compared using the Fisher’s protected least significant difference t-test. There was no significant difference in oestrous response for the Bonsmara (83.0%) and Nguni (90.8%) cows, respectively. The Bonsmara cows recorded a significantly higher pregnancy rate (59.0%) compared with the Nguni (37.1%) cows (P < 0.05). Sperm traits such as total motility (TM), progressive motility and rapid were found to be positively correlated with conception rate (r = 0.06, 0.03, and 0.08, respectively; P < 0.01), although correlations were low. There was no difference in the average frozen–thawed sperm TM rate of Nguni (92.2%) and Bonsmara (81.0%). There was a lower fertilization rate following IVF with Bonsmara and Nguni bull sperm. In conclusion, Nguni cows had similar oestrous response as Bonsmara cows. The sperm traits from Bonsmara and Nguni bulls were found to be related to in vivo conception and in vitro fertilization rate when sperm cells were assessed by CASA technology. However, the pregnancy rate was lower in Nguni cows.