231 Effect of circulating progesterone concentration on the day of embryo transfer on fetal development and calf birthweight

The establishment of pregnancy in ruminants is dependent on an optimal embryo-maternal interaction, driven mainly by the steroid hormone progesterone (P4). Elevated circulating P4 concentrations in the first week after conception have been associated with accelerated post-hatching conceptus elongation, mediated through earlier changes in the uterine endometrial transcriptome and alterations in the uterine lumen fluid (histotroph) composition. The consequences, if any, of this accelerated growth on the development of the fetus and calf birthweight are not known. Using data generated from two independent studies, the objective was to identify an association between circulating P4 concentration on the day of embryo transfer (ET) and (i) fetal weight at 42 days of gestation (Experiment 1) and (ii) calf birthweight (Experiment 2). In Experiment 1, a previously validated model of asynchronous ET was used, known to alter uterine exposure to P4. Single Day 7–8 in vitro-produced (IVP) blastocysts were transferred into cross-bred beef heifers on Day 7 (n = 26) or Day 9 (n = 25) after the expected ovulation. Heifers were slaughtered at Day 42 of gestation to measure fetal membranes and fetal weight. In Experiment 2, single IVP Aberdeen Angus blastocysts were transferred into synchronised lactating dairy cows (n = 95) on Day 7 after ovulation. Calf birthweight was recorded immediately after birth. In both experiments, a blood sample was collected at the time of ET to measure P4 concentration. The effect of P4 on fetal weight and calf birthweight was determined through a multiple regression model considering P4 as a continuous or categorical variable (classified in quartiles) and controlling for other factors known to affect development, such as sex and gestation length. There was a positive correlation between recipient circulating P4 concentration at ET and fetal weight at Day 42 (R = 0.41, P < 0.05), which was not affected by fetal sex. For calf birthweight, recipients that were in the quartile with the least P4 concentration at Day 7 (5.2 ± 0.7 ng/mL) gave birth to lighter calves (42.0 ± 6.7 kg; P = 0.005) than those in the other quartiles (8.7 ± 2.3 ng/mL P4; 46.6 ± 7.8 kg birthweight). Sex and gestation length affected birthweight, but they did not interact with circulating P4 concentration. In conclusion, circulating P4 concentrations during early pregnancy impacted fetal development following ET in beef recipients and calf birthweight after ET in dairy recipients.

This research was supported by a H2020-MSCA-Individual Fellowship (Proposal 101021311) to MBR and a grant from the Irish Department of Agriculture, Food and the Marine to PL and SB (Proposal 2021R665).