Sire contribution to pregnancy establishment in cattle

A fundamental goal for optimising reproductive efficiency in seasonal pasture-based systems is to achieve a 12-month calving interval in order to coincide grass growth with herd dietary requirements. Pregnancy loss is one of the major causes of reproductive failure in cattle and leads to extended calving intervals which, in a seasonal system, can have a significant impact on profitability. Fertility of the bull is a major contributor to overall reproductive performance. Therefore, the first objective of this thesis was to characterise the bull’s contribution to pregnancy establishment in cattle (Chapter 1 & 2). The second aim of this thesis was to determine causative factors of subfertility in bulls (Chapter 3 & 4). This was explored by determining the phenotype and genetic background in a case of extreme subfertility in an AI bull and to determine the effect of nutrition during maternal calfhood on the reproductive development of future male progeny. In Chapter 2, the sire’s contribution to pregnancy failure was assessed by sperm transport, fertilisation failure, early embryo development as well as conceptus growth and survival. Using AI bulls divergent in fertility by approximately 17%, differences in embryo quality were apparent as early as Day 7. These differences likely contributed to the higher proportion of conceptuses surviving to Day 15 derived from high fertility bulls. Using semen from the same cohort of bulls, in Chapter 3, Day 15 conceptuses were generated and co-incubated with Day 15 endometrial explants to assess the cross-talk. Interestingly, conceptuses derived from high fertility bulls induced an immune response from the endometrium. This was not observed in conceptuses generated from low fertility bulls where the main response elicited from the endometrium was associated with cell cycle features. Chapter 4 focused on a case study of extreme subfertility in a Holstein-Friesian AI bull. This particular bull passed all of the typically quality control assessments in the AI centre yet had an average pregnancy rate of 18% from approximately 2,976 inseminations. After performing an array of in vitro and in vivo assays, it was ultimately determined by whole-genome sequencing that this case of subfertility due to a mutated variant of adenylate kinase 9 (AK9) in the homozygous state. This combined with an impaired caffeine-induced hyperactivation explains the sperm’s inability to reach the site of fertilisation, bind and penetrate the zona pellucida of the oocyte. Finally, Chapter 5 explored the effect of maternal nutrition during the prepubertal stage in heifers on the development of the reproductive tract of their future male fetuses. Heifers offered a high (HI) or moderate (MOD) plane of nutrition from 4.5 to 8.5 months, were later artificially inseminated by approximately 500 days of age and slaughtered on Day 100 to recover their male fetuses. Heifers offered HI diet in calfhood, reached puberty at a faster rate and resulted in male fetuses with altered reproductive development compared to heifers offered a MOD diet. The HI diet appears to favour male reproductive development with a strong tendency towards greater seminiferous tubule density, less interstitial tissue area and an upregulation of key genes involved in the development of the reproductive tract in 100-day fetuses.