50 Sex ratio at birth in dairy cattle is affected by the in vitro embryo production process

Data from a herd of 3,000 high-producing milking dairy cows under an intensive feeding system was analysed. The IVF laboratory at the farm uses a co-culture system (Vivanco-Mackie; page 30-48 in Proc. 2000 Australian Embryo Transfer Society) and transfers ∼1000 embryos per year. The semen used for IVF and for inseminations (AI) in the herd was female-sexed (F-sexed) and non-sexed (conventional). During the 4 years of evaluation (2016–2019) 1123 calves were born to AI with F-sexed semen and 6,456 to AI with conventional semen; the proportion of males born to AI with F-sexed semen was 9% (91% females), which was not different (P > 0.05) from the expected ratio of 10% males and 90% females. The proportion of male calves born to AI with conventional semen was 53.2% (46.8% females), which agrees with the expected ratios for the population. During the same period of evaluation, 805 calves were born to IVF embryos transferred; 240 calves were born from embryos produced using F-sexed semen, of which 29.6% were males and 70.4% females, which was different (P < 0.05) from the expected ratio of 10% males and 90% females. The proportion of male calves born from embryo transfer of IVF embryos produced with conventional semen was 82.7%, higher (P < 0.05) than the expected 53.2% of males for the population; the proportion of females was 17.3%, significantly lower (P < 0.05) than the expected 46.8% females for the population. For IVF, the proportion of males was significantly higher (P < 0.05) than expected in all cases; for AI, there was no difference (P > 0.05) between the observed sex proportion and the expected values. We conclude that the in vitro embryo production process affects the sex ratio at birth in this population, and some factor in the IVF process is affecting the survival to calving of female embryos and consequently increasing the proportion of male calves born from in vitro-produced embryos. Gutierrez et al. (2001 Reprod. Fertil. Dev. 13, 361–365) found a similar effect in dairy cattle and linked it to glucose imbalance in the media. Larson et al. (2001 PNAS 98, 9677-9682) found that female cattle embryos had a lower ability to progress in development in glucose-rich media, but the mechanism is not yet fully understood. Xu et al. (1992 Mol. Reprod. Dev. 31, 249-252) and others indicate that in vitro-produced male bovine embryos predominate among blastocysts, expanded and hatched stages. Meanwhile, embryos lagging in development are predominantly female. Kun Tan et al. (2016 Reproduction 151, 443-543) found that IVF in mice favours the birth of males because of dysregulated expression of representative dimorphic genes. Further studies in cattle are necessary to determine the cause of the increased proportion of males born from in vitro embryos and to find ways to control it.