Despite the rapid expansion of in vitro embryo production (IVP) technology for genetic improvement in the cattle industry in the last decades, pregnancy rates by the transfer of IVP embryos are still lower than those derived from in vivo-produced embryos. The objective of this study was to analyse factors affecting pregnancy rates after the transfer of IVP Japanese Black cattle embryos under farm conditions. Holstein heifers (n = 4,475) and cows (n = 8,541) were selected as recipients. Most cows (80%) were managed in tie-stall barns and most heifers (80%) were managed in pens. Embryo transfers were performed for 9 years, from 2004 to 2012. The embryos were produced from oocytes derived from a local abattoir and semen from 14 proven bulls by the Livestock Improvement Association of Japan (Hamano and Kuwayama 1993 Theriogelogy 39, 703–712). The fresh IVP embryos (quality; IETS code 1) that reached the blastocyst stage after 7 to 8 days (insemination = Day 0) were transported by an airplane (2 h) and subsequently by a car (1.5 h). Embryos were non-surgically transferred to each recipient on Day 7 to 9 of their natural oestrous cycle on farms. Pregnancy was diagnosed on Day 40 to 60 after oestrus. Pregnancy results were statistically analysed using the GLM procedures of SAS. The following variables were included in the model: recipient parity (0, 1, 2, or 3), day (7, 8, or 9) of the oestrous cycle at the time of embryo transfer, oestrus behaviour (increased activity observed by farmers), presence of mucus at oestrus, presence of blood after oestrus, and year (1, 2, 3, 4, 5, 6, 7, 8, or 9) and season (April–June as spring, July–September as summer, October–December as fall, or January–March as winter) of embryo transfer. The Bonferroni correction was used to counteract the problem of multiple comparisons. Heifers had significantly higher pregnancy rates than cows (51.0% v. 37.9%, respectively; P < 0.01), and first parity cows had higher pregnancy rates than third parity cows (42.9% v. 35.7%, respectively; P < 0.01). Pregnancy rates were significantly higher in recipients that received an embryo transfer on Day 8 of their oestrous cycle, than on Day 7 (46.6% v. 42.4%, respectively; P < 0.01). Recipients without oestrus behaviour had higher pregnancy rates than those with oestrus behaviour (46.3% v. 43.4%, respectively; P < 0.01). The presence of mucus and/or blood after oestrus and the season of transfer were not found to significantly affect pregnancy rates. The results of this study indicated that performing IVP embryo transfers on Day 8 of a recipient’s oestrous cycle will improve the pregnancy rate, season does not have an effect on pregnancy rate, and the detection of oestrus by monitoring increased activity is not always reliable and instead should be determined by multiple symptoms on farm conditions.
Studies on factors affecting superovulation and embryo transfer in Hungarian Merino ewes
