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Background  Early maternal separation may have long-lasting physiological effects on different organ systems. Although long-lasting effects of early maternal separation are mostly studied in connection with the development and function of the central nervous system hypothalamic–pituitary–adrenal axis, animal health, behaviour and productivity, there is a lack of information about its impacts on the reproductive system. In the dairy industry, calves are often separated from mothers 1 or 2 days after birth and are not nursed. In the present retrospective study based on data from an artificial insemination (AI) centre, we compared semen parameters and fertility in bulls that were separated from their mothers one day after birth with the semen parameters of bulls that remained with their mothers and were nursed for approximately 2 months. Semen parameters were followed in 3 consecutive years in 52 maternally separated and 22 nursed bulls.    Results  Ejaculate volume and total sperm count in ejaculate were significantly higher in nursed bulls in comparison to maternally separated bulls at the age 25–36 and 37–48 months, but interestingly, not at the age 12–24 months, during the first year in the AI centre. Non-return rates did not differ between separated and nursed bulls.    Conclusion  The results suggest that early maternal separation causes long-lasting effects on the functioning of the male reproductive system, evident by reduced production of semen in adult bulls. The data suggest that with a standard of 20 million sperms per straw of frozen semen, 27–78 fewer straws can be obtained from one ejaculate of maternally separated bulls in comparison to the nursed bulls.

Calves’ management conditions affect sperm count in adult bulls