Secondary Corpora Lutea Induced by hCG Treatment Enhanced Demi‐Embryo Survival in Lactating High‐Yielding Dairy Cows

Abstract

Using a novel in vivo model considering a low developmental competence embryo (demi-embryo) and a subnormal fertility recipient (lactating high-yielding dairy cow), this experiment evaluated the effect of human chorionic gonadotrophin (hCG) treatment at embryo transfer (ET) on embryonic size at implantation, embryonic survival and recipient plasma progesterone (P4 ) and bovine pregnancy-specific protein B (PSPB) concentrations until day 63 of pregnancy. Embryos were bisected and each pair of demi-embryos was bilaterally transferred to recipients (n=61) on day 7 of the oestrous cycle. At ET recipients were randomly assigned to treatment with 1500IU hCG or to untreated controls. Higher (p<0.01) pregnancy rates on days 25, 42 and 63, and embryo survival rate on day 63 were observed in hCG-treated cows with secondary CL than in hCG-treated cows without secondary CL and in untreated cows. Pregnancy rates and embryo survival rate were similar in hCG-treated cows without secondary CL and untreated cows. Embryonic size on day 42 was not affected by treatment with hCG, presence of secondary CL and type of pregnancy (single vs twin). Presence of secondary CL increased (p<0.05) plasma P4 concentrations of pregnant cows on days 14, 19 and 25 but not thereafter and of non-pregnant cows on days 14-21. Treatment with hCG and presence of secondary CL had no effect on plasma PSPB concentrations, which were higher (p<0.05) in twin than in single pregnancies. In conclusion, secondary CL induced by hCG treatment at ET significantly increased plasma P4 concentrations, the survival rate of demi-embryos and the pregnancy rate of high-yielding lactating dairy cows. Embryos were rescued beyond maternal recognition of pregnancy, but later embryonic survival, growth until implantation and placental PSPB secretion until day 63 of pregnancy were not affected by treatment or presence of secondary CL.