The role of sexed semen and evaluation of fertility in Irish dairy herds.

Motivation for sexed semen usage :

Sex-biased semen is a revolutionary technology that provides many potential advantages for the Irish dairy industry:

1. Replacement heifer requirements could be met using sexed semen in the first three weeks of the breeding season. Use of sexed semen instead of conventional semen would reduce the number of male dairy calves, which have poor beef traits and low economic value

2. Genetic gain for EBI could be accelerated by selecting the best dams to breed replacements

3. All dams that are not selected for breeding replacements should be inseminated with beef semen. This will help to increased the value of beef output from the dairy herd. The superior quality attributes of beef-cross calves relative to male dairy calves is vital for a sustainable beef industry

4. Dairy heifer rearing will be more streamlined, as all the female replacement animals will be born together within a short window of time at the start of the calving period;

5. Biosecurity can be improved by maintaining a closed herd.

Background

Research trials were carried out to evaluate the performance of sexed semen in lactating dairy cows in 2018 and 2019. During these trials, there was no sorting lab in Ireland, the trials were undertaken in collaboration with Irish AI companies, Cogent in the UK, and ST Genetics in the Netherlands.

2018 dairy cow field trial 1: Semen from both resident bulls (i.e. located at or near the sorting lab) and shipped ejaculates (i.e. bulls collected in Ireland, and ejaculates shipped to sorting lab within 6 to 7 h) were included in the study. Ejaculates were split and processed as either conventional semen (15m sperm per straw) or sexed (4m sperm per straw). Cows were inseminated after detected heats, and neither the farmer nor the AI technician was aware which straws were conventional semen and which straws were sexed semen.

In total, 7,236 inseminations events were analysed. The main findings were as follows:

• Overall, conception rate was greater for cows inseminated with conventional semen than for those inseminated with sexed semen (59.9% vs. 45.5%; 76.0% relative to conventional semen).

• The difference between conception rate achieved by conventional semen and sexed semen was less for resident bulls (60.3% vs. 50.2%; 84% relative to conventional semen) than for shipped ejaculates (58.6% vs. 40.7%; 70% relative to conventional semen).

• In 33% of the herds, the conception rate achieved with sexed semen was ≥ 90% of the conception rate achieved with conventional semen.

The findings highlight large herd variation in conception rates achieved, and also the relative difference between conventional semen and sexed semen. This may indicate that decision rules regarding timing of AI is better suited to sexed semen on some farms compared with others.

2019 dairy cow field trial 2:

A trial to evaluate the effect of timing of AI was conducted using semen from 3 bulls resident at Cogent. Ejaculates were split and processed as either conventional semen (15m sperm per straw) or sexed semen (4m sperm per straw). Enrolled cows were synchronised using a 10-day P4-Ovsynch protocol. 24 herds participated in the study, and each provided cows that were ≤4 parity and ≥50 days in milk on the day of AI. Cows within each herd were grouped based on parity and days in milk, and randomly assigned to one of 3 treatments:

1. Conventional semen at 16 h after the final injection of GnRH (CONT)

2. Sexed semen at 16 h after the final injection of GnRH (SEXED-16)

3. Sexed semen at 22 h after the final injection of GnRH (SEXED-22)

Every herd was allocated an equal ratio of conventional and sexed semen from each of the three bulls. The main findings were as follows:

• Conception rates were less in both sexed semen treatments compared with conventional semen (49.0, 51.3 and 61.1% for SEXED-16, SEXED-22 and

CONT, respectively). The relative conception rate (RCR) was 80.3% and 84.0% for SEXED-16 and SEXED-22, respectively. The two sexed semen treatments did not differ statistically, suggesting that inseminations between 16 to 22h after GnRH is optimum for AI with sexed semen.

• The best third of herds had an average relative conception rate of 100% (range: 91% to 121%), meaning that, on average, the conception rates achieved with sexed semen were equal to those achieved with conventional semen. Conversely, the poorest third of the herds had a mean relative conception rate of 67% (range: 48% to 77%), meaning that, on average, the conception rates achieved with sexed semen were two-thirds as good as those achieved with conventional semen.

• Of note, the herds with the poorest relative conception rates achieved excellent conception rates with conventional semen (see Figure 1 poorest third of herds). This shows that the cows were highly fertile, and synchronisation was performed correctly to facilitate a high fertility ovulation event

2021/2022 heifer trial

A six-d progesterone CO-Synch protocol was used for ovulation synchronization of dairy heifers, with timed AI (TAI) at the same times as (TAI-0) or 8 h (TAI-8) after the second injection of gonadotrophin-releasing hormone (GnRH), which was either 48 h or 56 h after removal of the PRID device.

Conception rate was greater for TAI-8 heifers (59%) compared with TAI-0 heifers (51%), with variable performance between herds.

Conclusions :

While it is clear that good reproductive performance is possible with sexed semen, attention to detail is critical. The most fertile dams (heifers, younger cows that are calved >50 days and in good BCS) and large team of high EBI bulls should be used. Also excellent straw handling on the day of AI is needed. Some guidelines on straw handing are as follows:

• Organise sexed straws into one goblet

• Thaw MAX 2 sexed semen straws at a time

• Thaw straws at 35 to 37 °C for 45 seconds

• Load straws into pre-warmed AI guns, keep warm.

• Deposit semen in uterine body within 5 mins

Acknowledgements:

Financial support from the Dairy Levy Trust, MII, Munster Bovine, Glanbia and CEVA is gratefully acknowledged. We are also grateful to the herdowners that participated, and to ICBF, Munster Bovine, Progressive Genetics, Dovea Genetics, Sexing Technologies and Cogent for facilitating the trials.