8 Relationship between oestrus manifestation and pregnancy rates on recipients used in an invitro-produced embryo transfer program
Authors: Pasqual B. M. , Correa E. E. , da Silva E. P. , Sermersheim M. K. , Naves L. M. , Sell S. G. , Lindsey B. R. , Rubessa M. , Wheeler M. B. 
Published: December 2019

The main objective of an embryo transfer (ET) program is to increase the genetic value of the offspring produced in a given herd. Nutrition, management, and efficiency in the detection of oestrus are among the factors that affect the use of this technology. Protocols that induce oestrus or ovulation allow practitioners to transfer embryos in a predetermined time without oestrus detection (Bó et al. 2012 Reprod. Fertil. Dev. 24, 272-277; https://doi.org/10.1071/RD11918). Although these protocols have been performed adequately for several years, recent attention has been directed to the effect of oestrus expression and circulating oestradiol concentrations during the growth of the preovulatory follicle on embryo development and pregnancy. Oestrus expression is positively associated with high pregnancies per ET (P/ET) and reduced pregnancy losses in recipients receiving invitro-produced (IVP) bovine embryos (Bó and Cedeño 2018 Anim. Reprod. 15, 224-230; https://doi.org/10.21451/1984-3143-AR2018-0060). The present study was designed to evaluate the relationship between time of oestrus manifestation and P/ET on recipients involved in an IVP ET program. A total of 189 multiparous Aberdeen Angus and Girolando cows were used as recipients. The protocol consisted of an intravaginal progesterone (P4) implant (Eazi-Breed CIDR, Zoetis) and gonadorelin (100μg) treatment on Day 0 and dinoprost (25mg) treatment, placement of an Estrotect patch to improve oestrus observation, and CIDR removal on Day 7. Oestrus observation occurred every 12h from 36 to 96h after CIDR removal, and ET was performed on Day 17 and 18. Fresh IVP embryos were transferred after being cultured in standard synthetic oviductal fluid with amino acids with 3% bovine serum (Rubessa et al. 2011 Theriogenology 76, 1347-1355; https://doi.org/10.1016/j.theriogenology.2011.06.004). Only quality 1 and 2 embryos (IETS) were transferred. Embryo stages 4, 5, 6, and 7 (IETS) were randomly assigned to the recipients. No comparison between embryo grade and oestrus observation was evaluated. The cows were sorted according to oestrus observation in the following groups: 36, 48, 60, 72, 84, and 96h after CIDR removal. Pregnancy diagnosis was conducted 25 days after ET by ultrasonography. A chi-squared test with Fisher correction was used to evaluate the data set. Differences were considered significant if P&lt;0.05. Out of the 189 cows submitted to the synchronization protocol, 160 were detected in oestrus (84.66%), 17 (8.99%) did not show oestrus, and 12 (6.35%) lost their Estrotect patches. Considering all recipients (with or without oestrus expression), oestrus manifestation was greater within the 48- to 60-h interval (40/189; 21.16% for both). The overall P/ET was 41.27% (78/189), and there was no statistical difference among groups (time of oestrus after P4 implant removal) for P/ET. Pregnancies per ET of the recipients that did not show oestrus was 35.29% (6/17), and P/ET for the ones that lost their Estrotect was 33.33% (4/12). Therefore, we concluded that all recipients showing oestrus within an interval from 36 to 96h after P4 implant removal may be used in an ET program using IVF embryos without affecting overall P/ET.