Dairy farmers use artificial insemination (AI) to generate replacement female stock.
For the small proportion of dairy herds with elite genetic merit cows, male dairy calves are also potentially valuable as future AI bulls.
The next generation of AI bulls are identified shortly after birth using genomic testing, allowing elite bulls to be identified and purchased by AI companies, and these bulls produce semen for sale from as young as 12 months of age.
Currently, dairy farmers use conventional (non-sorted) semen, resulting in an approximately 1:1 female to male sex ratio, and AI companies screen potentially elite male dairy calves of interest using genomic testing (Figure 1).
While this has provided long-term genetic gain, there are unintended consequences of this approach to animal breeding that present welfare, social and environmental concerns.
Male dairy calves and sexed semen
Because of their poor future beef value, the majority of male dairy calves have small economic value (with some having zero or potentially negative economic value). This is now leading to concerns regarding the welfare and survival of these calves. In addition, ruminant production is being placed under greater scrutiny for the environmental impact of milk and meat production.
Use of conventional semen is no longer the only option for generating female replacements. Sexed semen reliably produces a 9:1 female to male sex ratio, reducing the number of male dairy calves. Farmers will correctly target the best EBI dams for insemination with sexed semen, allowing them to accelerate genetic gain within their herd.
Widespread uptake and usage of sexed semen, especially when used on the best EBI dams, will also reduce the number of elite EBI dairy bull calves born and therefore could slow or stagnate genetic gain. An alternative strategy is needed.
Using modern reproductive technologies, genetic gain for dairy traits could be accelerated despite fewer male dairy calf births and the next generation of AI bulls could be generated by design.
Greater usage of sexed semen to generate replacement heifers opens the door to also using more beef semen to generate beef-cross calves. The Dairy-Beef Index (DBI) is a breeding goal for Irish dairy and beef farmers to increase the proportion of high-quality beef cattle bred from the dairy herd. High-DBI calves are more saleable as youngstock and more profitable at slaughter, but have minimal consequences on calving difficulty or gestation length for the dairy cow.
The offspring would provide the next generation of high-DBI AI bulls and high DBI bull dams.
2021 field trial
In a collaborative project between Teagasc, University College Dublin and Vytelle a large-scale field trial was conducted during spring 2021 to compare pregnancy rates in lactating dairy cows bred by AI or that received embryo transfer (ET). The objective of the first part of the study was to assess the potential role of IVP embryos to accelerate genetic gain for both EBI and DBI.
To achieve this, IVP embryos were generated from oocytes collected by OPU from genetically elite Holstein-Friesian (n=29), Jersey (n=11) and Angus (n=21) donors.
The objective of the second component of the study was to evaluate the potential role of IVP embryos to improve the quality of beef calves derived from dairy cows that are not suitable for generating replacements.
To achieve this, ovaries were collected after slaughter from commercial crossbred beef heifers (n=119) and fertilised with semen from high-DBI bulls. The resulting embryos contained at least 75% beef breed genetics.
Following IVF and culture for seven days, a single embryo was transferred either fresh or following freezing and on-farm thawing into lactating dairy cows (predominantly Holstein-Friesian), which had been synchronised with a 10-day PRID-Ovsynch protocol. On the day of synchronised ovulation, 20% of the cows (n=240) were assigned to receive AI, and the remaining 80% of the cows (n=952) were assigned to receive ET seven days later (20% dairy fresh, 20% dairy frozen, 20% beef fresh, 20% beef frozen).
On the day of ET, 10% of the synchronised cows were deemed unsuitable for ET, and did not receive an embryo.
The average pregnancy rate per service event at day 32 was not different between AI (47.7%) and ET (fresh/frozen combined, 47.9%) and did not differ between dairy and beef embryos (dairy: 50.1% vs beef: 46.1%).
There was, however, a marked difference in pregnancy rate on day 32 between embryos that were frozen and thawed before transfer compared with embryos that were transferred fresh (dairy: 40.1% vs 61.1%; beef: 41.2% vs 51.7%, respectively).
Pregnancy loss between day 32 and 62 was greater for ET (15.6%) compared with AI (4.7%). Among the different ET treatments, greater losses were observed for frozen beef (16.6%), fresh beef (17.7%) and frozen dairy (22.8%) compared with fresh dairy (6.7%).
In conclusion, pregnancy per service event on day 32-35 was similar for AI and ET, although 10% of cows assigned to ET were deemed unsuitable on the scheduled day of transfer and were rejected.
Fresh ET had greater pregnancy rates compared with Frozen ET, and embryo losses were greater for cows bred using ET compared with AI.
The key finding from the study is that it is possible to collect oocytes from donors and produce IVP embryos in the weeks immediately before the breeding season begins, and that fresh ET to lactating dairy cow recipients can achieve pregnancy rates comparable with AI at first service.
This technology is likely to become important for the dairy and beef breeding sectors in the coming years.
Funding for the project was provided by Dairy Levy and FBD Trust. Hormones were donated by CEVA.