Improving reproductive efficiency in the ram: the role and applications of the neurohormone melatonin

Reproductive seasonality is perceived as a limiting factor for Australia’s sheep industry, with the non-breeding period characterised by reduced productivity. Melatonin, the natural regulator of ovine reproduction, has been proclaimed as a possible strategy to surmount this issue. Despite successful application in the ewe, the extent of effects of melatonin treatment in the ram is yet to be elucidated. This thesis explores how melatonin can be applied in the ram to mitigate the decrease in reproductive efficiency occurring in the non-breeding season, with the overarching aim of improving ram productivity and reproductive competence. The findings of this thesis further clarify how melatonin influences ram reproductive seasonality, and its direct effects upon spermatozoa. Moreover, these collective studies provide evidence that reproductive seasonality is not as detrimental to ram fertility as previously postulated. Herein, it is demonstrated that melatonin improves the functionality of cryopreserved ram spermatozoa, increasing sperm motility and DNA integrity, likely through suppression of mitochondrial superoxide production. Additionally, as a slow-release implant, melatonin alters ram testicular function, upregulating testicular size, sperm production and ram libido, and confers subtle improvements to the functionality of cryopreserved spermatozoa from implanted rams. Collectively, these studies reveal compelling evidence for a lack of pronounced reproductive seasonality in the ram, finding little difference in ram fertility, semen quality, or epigenetic modifications to spermatozoa between seasons. Through validation of the role and applications of melatonin in ram reproduction, this thesis has directly contributed to tangible outcomes in the Australian sheep industry