Embryo transfer (ET) in cattle can be an important tool for genetic modification [1] and can improve pregnancy rates compared with AI when fertility is low, such as during heat stress [2] and [3] or in repeat breeder cows [4]. In addition, production of embryos in vitro is an efficient way to utilize X-sorted sperm, because one straw of semen can produce more embryos than with AI.
There are few reports as to whether the use of X-sorted sperm for IVF results in production of embryos with altered competence for establishment of pregnancy after transfer to recipients. Reduced competence is possible because embryos produced with X-sorted sperm have been reported to have altered steady-state levels of specific mRNA [5] and ultrastructural characteristics [6]. However, in one study, there was no difference in pregnancy rates after transfer of embryos produced with X-sorted sperm compared with embryos produced with conventional sperm [7]. Also, characteristics of calves produced by AI with X-sorted sperm did not differ from calves produced with nonsorted semen [8].
The major aims of this study were to determine the efficacy of establishing pregnancy in lactating cows on commercial dairy farms using transfer of an in vitro-produced embryo compared with AI, and to test whether pregnancy success was reduced in cows receiving an embryo produced using X-sorted sperm versus cows receiving an embryo produced with conventional sperm. So that findings could be readily applied, commercially available oocytes, semen, ET and AI services, and ovulation synchronization procedures were used. Additional aims were to evaluate whether characteristics of calves born as the result of ET with X-sorted or conventional sperm were altered compared with calves born after AI.