Fertility of sex-sorted sperm in seasonal-calving pasture-based dairy herds

Use of X-sorted sperm (SS) increases the proportion of female dairy offspring from artificial insemination (AI). However, SS generally achieves poorer field fertility than conventional, unsorted, sperm (CONV). During the last years, improvements in the sex-sorting technology have aimed to close the gap in field fertility between SS and CONV. Given the potentially severe impact of poor fertility in a seasonal pasture-based system such as that operated in Ireland, there was a need to conduct field studies to evaluate the fertility of fresh and SS under Irish conditions. In the first study (Chapter 2), we investigated the phenotypic fertility performance of dairy heifers and lactating cows inseminated after spontaneous oestrus with fresh (1 or 2 million sperm dose) or frozen (2 million sperm dose) SexedULTRA SS compared with fresh CONV (3 million sperm dose) in seasonal-calving pasture-based dairy herds. SS achieved relative pregnancy per AI (P/AI) ranging from 78.4 to 88.9% of those achieved with CONV. P/AI achieved with frozen SS did not differ from that achieved with fresh SS. Doubling the concentration of sperm in fresh SS straws did not improve P/AI. There was a significant bull effect on P/AI in both lactating cows and heifers. P/AI of lactating cows inseminated with fresh sperm (CONV and SS) varied between bulls as dispatch-to-AI interval increased. Among young cows (parity = 2), the following cow characteristics were associated with greater likelihood of pregnancy establishment after insemination with SS: BCS = 3, DIM > 60 d, and fertility sub-index of the EBI > €100. In the second study (Chapter 3), we investigated the phenotypic fertility performance of lactating dairy cows inseminated with frozen SexedULTRA-4M SS (4 million sperm per straw) compared with frozen CONV (15 million sperm per straw) in seasonal-calving pasture-based dairy herds. Overall, P/AI was greater for cows inseminated with frozen CONV than for those inseminated with frozen SS (59.9% vs. 45.5%; 76.0% relative to frozen CONV). The effect of sex-sorting on P/AI was highly variable between bulls, with greater variation amongst bulls whose ejaculates were shipped to the sorting centre compared with resident bulls. Frozen SS resulted in greater relative P/AI in cows with high and average fertility potential (defined by parity = 2, DIM at AI = 70, EBI fertility sub-index > €60) than in cows with low fertility potential. In 33.1% of the enrolled herds, the P/AI achieved with frozen SS was = 90% of the P/AI achieved with frozen CONV; this was primarily a reflection of herds that had excellent performance with frozen SS, but relatively poor with frozen CONV. In the third study (Chapter 4), we compared perinatal and postnatal mortality as well as productive and reproductive performance of offspring derived from AI with SS or CONV processed from the same ejaculates. Calf survival during the first year of age was not different between animals derived from AI with SS or CONV. No differences existed due to sex-sorting in survival, and reproductive or lactation performance of female offspring. Carcass conformation and fat scores of steers, and carcass weight of young bulls were not affected by sperm treatment. Overall, CONV steers were slaughtered earlier and had greater carcass weight than SS steers, but the distribution of slaughter age did not differ between CONV and SS steers when the analysis was restricted to herds that reared steers of both types. In conclusion, SS achieved lower P/AI than CONV in seasonal-calving pasture-based heifers and lactating cows inseminated after spontaneous oestrus. However, P/AI achieved with SS was dependent on the bull, fertility potential of the cow, and herd. Female offspring derived from AI with SS or CONV did not differ for any of the performance characteristics studied. Further research is needed to clarify whether there is a true negative effect on body weight gain of steers due to the sex-sorting process.