Evaluation of Corpus Luteum and Plasma Progesterone the Day before Embryo Transfer as an Index for Recipient Selection in Dairy Cows

SIMPLE SUMMARY: The use of embryo transfer (ET) in dairy cows helps to improve herd genetics. In order to improve pregnancy rates after ET, the selection of a recipient as well as a high-graded embryo is important. A larger CL and higher P4 on the day of ET represent high reproductive performance in cows. In this study, we evaluated CL size and P4 level the day before ET and their effects on the pregnancy rate. The recipients’ parity and the seasons in which ET was performed had a significant effect on the pregnancy rate, but the synchronization methods and the pre-ET evaluation values of CL size and P4 level could not predict the pregnancy rate. Embryo implantation and pregnancy maintenance depend on high P4 concentration according to the function of the CL, but in the mid-luteal phase, in which ET is performed, development variation makes it difficult to evaluate those values. Intensive evaluation of the variables involved in ET in dairy cows will lead to an improvement in the pregnancy rate and productivity. ABSTRACT: The aim of the present study was to evaluate the effects of synchronization method, season, parity, corpus luteum (CL) size, and progesterone (P4) levels on the pregnancy rate after bovine embryo transfer (ET). Among 165 recipient candidates who received 1 of 2s estrus synchronization treatments, 96 heifers and 43 cows were selected through rectal examination and used as recipients. The day before ET, the CL size and plasma P4 concentration were evaluated. The CL sizes and plasma P4 levels were not different between the selected and unselected candidates, and the pregnancy rates with the two synchronization methods were not different. However, the pregnancy rates were higher in heifers than in lactating cows, and also higher after ET performed from September to February than from March to August (p < 0.05). The recipients with a CL larger than 1.5 cm showed statistically higher pregnancy rates, and although there was no statistical significance, the pregnancy rate was higher when the plasma P4 levels were between 2.0 and 4.0 ng/mL. Exposure to a stressful environment and repeated manipulations can reduce the success rate of ET, and recipient selection with an optimal CL size and P4 level can increase the success rate of ET.