PS9-23. Pregnancy Losses Associated with Hormonal Factors in Beef Cows: A Systematic Review and Meta-analysis.

Abstract Gestational losses represent a major challenge for cow–calf production systems because they compromise reproductive efficiency and cause economic losses to the livestock industry. Most of these losses occur during the embryonic or early fetal period and therefore represent reproductive failures that are often difficult to detect. In this context, due to the limited number of studies relating gestational losses to hormonal alterations in beef cows, the objective of this study was to compile and quantitatively analyze, through a systematic review and meta-analysis, the available data associating these factors at different stages of bovine gestation. Additionally, hormonal activity was evaluated among different categories and subspecies of beef females to identify groups with a greater predisposition to hormonal dysfunctions associated with pregnancy loss. More than 1,000 articles were identified in the Scopus, Web of Science, and PubMed databases and screened according to the PRISMA protocol. The inclusion criteria were: i) beef cows or heifers; ii) early pregnancy diagnosis by ultrasonography between d 28 and 32; iii) studies reporting the day or range of pregnancy diagnosis, subspecies, country, parity, and reproductive method; and iv) the presence of hormonal data, including progesterone, estradiol, GnRH, eCG, PGF2α, or pregnancy-associated glycoprotein (PAG) biomarkers. Of the identified articles, 30 studies met the inclusion criteria, totaling data from 48,643 beef females. The meta-analysis followed the methodology described by Borenstein, and statistical analyses were performed using Stata version 14.0 (StataCorp, Texas, USA). A nonparametric formula was used to estimate variance because most studies did not report standard errors or standard deviations. Among the 48,643 females evaluated, 20% experienced pregnancy loss (95% CI: 0.14–0.26), with high heterogeneity among studies (I² = 99%). Among the insemination protocols evaluated, those combining GnRH and progesterone showed lower pregnancy loss (approximately 17%; 95% CI: 0.03–0.31), whereas protocols based on progesterone and estradiol, with or without eCG, showed higher losses of approximately 23% (95% CI: 0.08–0.37) and 24% (95% CI: 0.16–0.33), respectively. The stage of pregnancy loss significantly affected the results (P = 0.004), with early embryonic losses being more frequent (30%; 95% CI: 0.18–0.42) than late embryonic and early fetal losses (11%; 95% CI: 0.01–0.20). Overall, the results of this meta-analysis indicate that gestational losses associated with hormonal factors in beef cattle vary widely among studies and production systems, and even when similar hormonal protocols are used, differences are still observed. In general, the average estimated loss was approximately 20%, and no clear statistical effect of the hormonal protocol type was identified, although numerical differences were observed among groups. Moreover, losses were greater during the early embryonic stage compared with later stages of gestation, indicating that early pregnancy remains the most critical period for pregnancy maintenance in beef cows.