Effects of supplemental progesterone using a CIDR insert on pregnancy per embryo transfer of dairy heifer recipients of embryos produced in vitro.

The objective of this experiment was to evaluate effects of supplemental progesterone immediately following transfer of frozen-thawed, IVP embryos on P/ET. Holstein heifers (n = 452), allocated to nine transfer groups over time, were assigned to be embryo recipients in a completely randomized study from December 2016 to April 2017. All heifers were randomly assigned to one of two treatments: 1) control (CON; n = 212) with no further treatment, or 2) received a CIDR insert containing progesterone for 12 d, beginning on the day of transfer (D 7) and removed 12 d later on Day 19 (CIDR; n = 228). A subset of heifers were subjected to blood sampling on Day 7 (ET) and Day 19 (CIDR removal) to determine circulating concentrations of progesterone. Pregnancy was initially determined using a serum assay for pregnancy specific protein-B at approximately Day 40 after ET and confirmed a month later using trans-rectal ultrasonography. Overall, P/ET did not differ (P =  0.941) between treatment groups. At the initial pregnancy determination, P/ET differed (P =  0.007) among transfer groups. Concentrations of progesterone tended to be less (P =  0.064) in heifers in the CON group compared to heifers treated with the CIDR (3.6 ± 0.27 compared with 4.4 ± 0.27 ng/mL), and differed between transfer groups (P <  0.001) and days post-estrus (P =  0.019) of the recipients. In summary, while treatment with supplemental progesterone at the time of transfer of IVP embryos using a CIDR increased circulating progesterone, there was no influence on P/ET.