127 Pregnancy improvement following transfer of parthenogenetic embryos after artificial insemination in dairy cows is dependent on interval from calving

There is a need to improve conception rates to first service in high-yielding dairy cows, which is currently around 35% based on UK national records. In vitro-produced (IVP) embryos or trophoblastic vesicles transferred one week after AI can improve pregnancy outcomes. However, the former increases twinning rates while the latter are laborious to generate. The production of parthenogenetic embryos (bPaEs), in contrast, is straightforward and can improve conception rates to AI in repeat breeder cows. The current study sought to determine the benefit of prophylactically transferring two bPaE 6 to 8 days following first service insemination on dairy cow fertility. The study was undertaken on two commercial dairy farms in Devon, UK, involving 256 Holstein-Friesian cows. Farm 1, autumn block-calving, grass-based system, milking 2× daily. Farm 2, all year round calving with housed cows milked 3× daily. Minimal voluntary waiting period of 42 days before AI. At AI, cows within each farm were paired according to parity, semen type (sexed vs unsorted), and randomly assigned to one of two treatment groups: AI (n = 128), or AI + bPaE (n = 128). Two IVP frozen–thawed bPaEs were transferred 6 to 8 days following AI. Milk yield and body condition (BCS) were recorded, as was pregnancy establishment, determined by ultrasound at 30 to 40 days gestation, and returns to oestrus. Pregnancy outcomes were analysed by logistic regression, and service intervals using general linear models (fitted terms in final models: “farm,” “days-in-milk; DIM,” “treatment”); data presented as means ± standard error of the mean. Neither milk yield nor BCS affected fertility outcomes. Pregnancy rates to AI alone increased (P = 0.007) with DIM on both farms (≤56 days = 29.0 ± 9.06; ≥70 days = 57.7 ± 6.74). Method of conception interacted with DIM to determine pregnancy rate (P = 0.038) and interservice interval (P = 0.017) for those cows that returned to oestrus. Mean pregnancy rates for bPaE vs AI at ≤63 days (i.e. date of transfer) = 45.8 ± 8.84 vs 27.0 ± 8.22%; and at ≥77 days = 59.6 ± 7.00 vs 62.8 ± 6.25%. Similarly, mean interservice intervals for bPaE vs AI at ≤63 d = 29.3 ± 3.18 vs 33.2 ± 2.56 days; and at ≥77 days = 29.9 ± 2.26 vs 24.9 ± 2.21 days. Transfer of bPaE during the early postpartum period (≤63 DIM), when fertility is low, improves pregnancy rates to AI and does not adversely affect interservice intervals for cows returning to oestrus. In contrast, the transfer of bPaE beyond 77 DIM, when fertility is higher, reduces pregnancy rates to AI and delays returns to oestrus by around one week on average.

Funding for this research was provided by Innovate UK (10002606).