Luteal Presence and Ovarian Response at the Beginning of a Timed Artificial Insemination Protocol for Lactating Dairy Cows Affect Fertility: A Meta-Analysis

SIMPLE SUMMARY: The widespread adoption of synchronization protocols for timed artificial insemination (AI) in the dairy industry has improved reproductive performance. The most common timed AI protocol is a combination of two hormones (i.e., gonadotropin-releasing hormone and prostaglandin F 2α). Primarily, these protocols increase the insemination risk without affecting the risk of conception. Based on an advanced understanding of the underlying physiology of timed AI protocols, modifications have been developed in order to increase the conception risk. We performed a meta-analytical assessment to evaluate the effect of luteal presence and ovulatory response at the beginning of a timed AI protocol. There was a clear benefit on conception risk for cows starting a timed AI protocol with a functional corpus luteum (+10.5 percentage units) and cows ovulating at the beginning of a timed AI protocol (+11.0 percentage units). Our work highlights the importance of prerequisites for achieving excellent fertility results in timed AI protocols for lactating dairy cows. ABSTRACT: Progesterone (P4) concentration during follicular growth has a major impact on fertility response in timed artificial insemination (TAI) protocols. Luteal presence at the beginning of a TAI protocol and ovarian response after the first gonadotropin-releasing hormone (GnRH) injection (G1) affect P4 concentration and subsequently pregnancy per artificial insemination (P/AI). A systematic review of the literature and meta-analytical assessment was performed with the objective of evaluating the magnitude of the effect of luteal presence and ovarian response at the beginning of a TAI protocol on P/AI in lactating dairy cows. We considered only studies using synchronisation protocols consisting of GnRH and prostaglandin F (2α). The time interval between G1 and prostaglandin F (2α) (PGF (2α)) had to range from 5 to 7 d. The time interval between the PGF (2α) injection and G2 had to range from 48 to 72 h. We used 28 controlled experiments from 27 published manuscripts including 16,489 cows with the objective of evaluating the effect size of having a functional corpus luteum (CL) at G1 on P/AI. Information regarding ovulatory response after G1 was available for 5676 cows. In a subset of cows (n = 4291), information was available for luteal presence and ovulatory response at the initiation of the TAI protocol. A functional CL at G1 increased (p < 0.001) the relative risk of conceiving (RR (relative risk) = 1.32; 95% CI = 1.21–1.45) in lactating dairy cows. Ovulation after G1 increased (p < 0.001) the relative risk of conceiving (RR = 1.29; 95% CI = 1.20–1.38) in lactating dairy cows. The effect of ovulatory response on P/AI after G1 was affected by luteal presence at G1. In summary, there was a clear benefit on P/AI for cows starting a TAI protocol with a functional CL (+10.5 percentage units) and cows ovulating at the beginning of a TAI protocol (+11.0 percentage units).