Characterization of conception rate after embryo transfer in comparison with that after artificial insemination in dairy cattle

Artificial insemination and embryo transfer (ET) are important in the reproduction of dairy cows. The conception rate after AI or ET is an essential indicator when selecting appropriate breeding methods. However, information on the environmental factors affecting ET conception rate when compared with AI is limited. We aimed to investigate environmental factors affecting ET conception rate and characterize the differences in environmental factors between AI and ET. Records of the first AI (n = 1,870,143) and ET (n = 29,922) from Holstein nulliparous, primiparous, and multiparous cows in Hokkaido, Japan, were analyzed using separate multivariable logistic regression models. For each breeding method, we grouped primiparous and multiparous cows according to milk yield at peak lactation (PY; <25, 25–30, 30–35, or ≥35 kg in primiparous; <40, 40–45, 45–50, or ≥50 kg in multiparous) and the interval from calving to first AI or to first ET (CFI and CFT, respectively; <60, 60–79, 80–99, or ≥100 d) to evaluate the effects of PY and CFI or CFT on conception rate. The AI conception rate decreased with increasing PY in primiparous and multiparous cows, whereas ET conception rate did not decrease significantly. Additionally, the ET conception rate did not decrease even in primiparous and multiparous cows slightly earlier than 60 d in CFI/CFT compared with those in CFI/CFT after 60 d, which differed from the AI conception rate. Collectively, breeding by ET leads to the avoidance of negative effects of high milk yield and calving on the conception rate, indicating that cows are fertile by ET within 60 d after calving.