Butler Stephen T, Crowe Alan D, Moore Stephen G, Lonergan Pat
Teagasc, Animal and Grassland Research and Innovation Centre, Moorepark, Fermoy, Co. Cork, Ireland.
Teagasc, Animal and Grassland Research and Innovation Centre, Moorepark, Fermoy, Co. Cork, Ireland; School of Agriculture and Food Science, University College Dublin, Ireland.
Animal. 2023 May;17 Suppl 1:100775. doi: 10.1016/j.animal.2023.100775.
A unique aspect of seasonal-calving pasture-based systems of dairy production is the intense focus placed on achieving a concentrated herd-calving period in late winter and early spring. Hence, excellent reproductive performance is required during a short breeding period. A concentrated calving period also produces a problem in the form of a large number of male dairy calves being born at the same time; as these calves have little economic value due to poor beef merit, they present a potential welfare concern. A solution exists in the form of sex-sorted semen, but this is typically associated with poorer pregnancy per artificial insemination, and hence, the use of sex-sorted semen must be carefully considered. The logical strategy to use sex-sorted semen is to target the best genetic merit dams in the herd to generate replacement heifers, thereby accelerating herd genetic gain. On the other hand, if all dairy farmers adopt such a strategy, there will be a corresponding reduction in elite genetic merit male dairy calves being born, potentially reducing availability of the next generation of future bulls to be used for artificial insemination. Use of in vitro embryo production on elite dairy donors could avoid this problem by acting as a multiplier, potentially in tandem with Y-sorted semen to skew the offspring sex ratio towards more male calves. Use of sex-sorted semen on the best genetic merit dams can also facilitate a marked increase in the usage of beef semen on any dams that are deemed unsuitable for sex-sorted semen. The use of "beef on dairy" requires selection of beef bulls that generate offspring with traits that meet the key requirements of both the dairy farmer (e.g., gestation length and calving ease) and the beef farmer that must be motivated to purchase the calves (e.g., growth rate, age at slaughter, carcass value). Beef breed dams that have elite genetic merit for these traits could also be considered for in vitro embryo production, potentially in tandem with Y-sorted semen, to facilitate genetic gain for the growing "beef-on-dairy" market. It is possible to transfer a beef embryo (75-100% beef breed genetics) into dairy dams that are not required to generate replacements, but this is likely to remain a niche practice as there are many barriers to widespread adoption. Such combinations of assisted reproduction have the potential to improve the efficiency and sustainability metrics of seasonal-calving pasture-based dairy herds.
季节性产犊的牧场型奶牛生产系统的一个独特之处在于，其高度专注于在冬末和早春实现集中产犊期。因此，在短繁殖期内需要优异的繁殖性能。集中产犊期也会产生一个问题，即大量雄性奶牛犊同时出生；由于肉用性能不佳，这些犊牛经济价值不大，因此存在潜在的福利问题。性别分选精液是一种解决方案，但通常与每次人工授精后的妊娠率较低相关，因此，必须谨慎考虑使用性别分选精液。使用性别分选精液的合理策略是针对牛群中遗传价值最高的母牛来培育后备小母牛，从而加速牛群的遗传进展。另一方面，如果所有奶农都采用这种策略，出生的具有优秀遗传价值的雄性奶牛犊数量将相应减少，可能会降低未来用于人工授精的下一代公牛的可获得性。对优秀奶牛供体使用体外胚胎生产技术可以作为一种繁殖手段来避免这个问题，可能与Y染色体分选精液配合使用，使后代性别比例偏向雄性犊牛。在遗传价值最高的母牛上使用性别分选精液，还可以显著增加在任何被认为不适合使用性别分选精液的母牛上使用肉用精液的比例。“奶牛配肉牛”的做法需要选择能产生具有符合奶农（如妊娠期长度和产犊难易度）和有购买犊牛意愿的肉牛养殖户（如生长速度、屠宰年龄、胴体价值）关键要求性状后代的肉用公牛。对于这些性状具有优秀遗传价值的肉牛品种母牛也可考虑用于体外胚胎生产，可能与Y染色体分选精液配合使用，以促进不断发展的“奶牛配肉牛”市场的遗传进展。将肉牛胚胎（75 - 100%肉牛品种基因）移植到不需要留作后备的奶牛母牛体内是可行的，但这可能仍将是一种小众做法，因为广泛应用存在许多障碍。这种辅助繁殖技术的组合有潜力提高季节性产犊的牧场型奶牛群的效率和可持续性指标。
