151 Effect of season on the in vitro fertilizing ability of frozen - thawed bovine spermatozoa

Abstract

Previous research indicated that the season during which oocytes are harvested affects quality of in vitro-produced embryos (Gupta et al. 2016 Anim. Reprod. Sci. 164, 162). In our own studies, sperm kinematics, membrane integrity, acrosome status, mitochondrial potential and reactive oxygen species were affected by season of semen collection (Sabés-Alsina et al. 2017 Vet. Rec. 180, 251). The aim of this study was to investigate effect of season of semen collection on in vitro fertilizing ability and embryo development of the same sperm samples from the sperm quality study, using lower than normal sperm doses to detect small differences between groups (Ward et al. 2003 Theriogenology 59, 1575). Frozen semen was available from 8 Holstein bulls kept outdoors in northern Spain, collected during winter, spring and summer. Bovine ovaries, Holstein and Swedish Red breeds, were obtained from an abattoir in spring. Oocytes were matured and fertilized in vitro with a low dose of frozen-thawed sperm (sperm:oocyte ratio 2500:1). After fertilization, presumptive embryos were evaluated at 44h for cleavage and on Day 8 for blastocyst development. Number of sperm binding to the zona pellucida and number of nuclei in developing blastocysts were assessed after staining with Hoechst 33342. There were 2 or 3 replicates per bull. For 555 oocytes inseminated, cleavage rates for winter, spring and summer semen collections were 42, 49 and 47%, respectively; blastocyst rates were 7, 12 and 8%; blastocyst cell numbers were 8.7, 10.2 and 7.8; and mean number of sperm bound was 0.70, 0.63 and 0.50. Although there were no differences (P>0.05) due to season of semen collection for any of these end points, individual bulls had considerable variation in cleavage rate. In conclusion, despite previously published differences in sperm quality with season from these bulls, ability of frozen-thawed sperm to fertilize bovine oocytes and initial embryo development in vitro were not affected by season of semen collection, at least for oocytes collected in spring. However, bull-to-bull variation in cleavage rate was high. The authors gratefully acknowledge the financial support of the Ministerio de Economía y Competitividad and FEDER (AGL2016-79802-P). M. Sabes-Alsina was supported by a PIF from the Universitat Autònoma de Barcelona and by the STS of the Epiconcept COST Action; J. M. Morrell was funded by the Swedish Research Council for the environment, agricultural sciences and spatial planning (FORMAS; 221-2010-1241) and the Swedish Farmers’ Association (SLF; 1330039).