Effects of hormonal interventions on fertility outcomes in dairy cattle exposed to heat stress: a meta-analysis

Normalization of the deranged hormonal milieu using hormonal therapy has been recommended as a strategy to improve the fertility of heat-stressed dairy cows, yet evidence of its effectiveness remains elusive and inconsistent. We conducted a meta-analysis to assess the effect of hormonal interventions in improving fertility in dairy cows exposed to heat stress. Ninety-eight trials reported across 32 studies were included in the meta-analysis. GnRH and progesterone, but not hCG, increased the chances of conception when administered around or post-insemination (RR = 1.26, P = 0.0001; I 2 = 37%, RR 1.98, P < 0.00001; I 2 = 35%, and RR = 1.06, P = 0.65; I 2 = 4%, respectively). The risk of pregnancy loss was significantly reduced following post-estrus or AI treatment with GnRH or hCG (RR 0.47, P = 0.004; I 2 = 53%), although with limited evidence. No evidence of the benefits of timed artificial insemination on conception rates in heat-stressed dairy cattle was found (RR = 1.04, P = 0.73). Subgroup analysis showed that conception rates following GnRH, hCG, and progesterone treatments varied with factors such as body condition score and presence of reproductive diseases at the time of treatment. Heterogeneity was generally low or moderate across the main analyses. We conclude that hormonal treatment may improve the fertility of dairy cattle in a manner that varies with the nature of the treatment and the physiological status of the animals at the time of application.