Meta-analysis of progesterone supplementation during timed artificial insemination programs in dairy cows

A systematic review of the literature was performed with the objective to evaluate the effects of progesterone supplementation using a single intravaginal insert during timed artificial insemination (AI) programs on fertility in lactating dairy cows. A total of 25 randomized controlled studies including 8,285 supplemented cows and 8,398 untreated controls were included in the meta-analysis. Information regarding the presence of corpus luteum (CL) at the initiation of the synchronization protocol was available for 6,883 supplemented cows and 6,879 untreated controls in 21 experiments. Studies were classified based on service number (first AI vs. resynchronized AI), use of presynchronization (yes vs. no), and insemination of cows in estrus during the synchronization protocol (inseminated in estrus and timed AI vs. timed AI only). Reproductive outcomes of interest were pregnancy per AI (P/AI) measured on d 32 (27 to 42) and 60 (41 to 71) after AI, and pregnancy loss between d 32 and 60 of gestation. Random effects meta-analyses were conducted and treatment effect was summarized into a pooled risk ratio with the Knapp-Hartung modification (RR_K+H). The effect of moderator variables was assessed using meta-regression analyses. Progesterone supplementation increased the risk of pregnancy on d 32 [RR_K+H=1.08; 95% confidence interval (CI)=1.02-1.14] and 60 after AI (RR_K+H=1.10; 95% CI=1.03-1.17). The benefit of progesterone supplementation was observed mainly in cows lacking a CL at the initiation of the timed AI program (d 60: RR_K+H=1.18; 95% CI=1.07-1.30) rather than those with CL (d 60: RR_K+H=1.06; 95% CI=0.99-1.12). Progesterone supplementation benefited P/AI in studies in which all cows were inseminated at timed AI (d 60: RR_K+H=1.20; 95% CI=1.10-1.29), but not in studies in which cows could be inseminated in estrus during the timed AI program (d 60: RR_K+H=1.04; 95% CI=0.92-1.16). Progesterone supplementation tended to reduce the risk of pregnancy loss (RR_K+H=0.84; 95% CI=0.67-1.00). Service number and presynchronization did not influence the effect of progesterone supplementation on fertility. In summary, progesterone supplementation using a single intravaginal insert during the timed AI program increased P/AI mostly in cows without CL and reduced the risk of pregnancy loss in lactating dairy cows. Insemination of cows in estrus during the synchronization protocol eliminated the benefit of supplemental progesterone on P/AI.