Effect of injectable progesterone after artificial insemination on pregnancy outcomes in high-yielding dairy cows

Context. Adequate progesterone levels are crucial for embryo development and maintenance of pregnancy. Aims. This study aimed to evaluate the effects of injectable progesterone (iP4) administered 14 days after timed artificial insemination (TAI) on conception at day 30, maintained pregnancy at day 60, and pregnancy loss in high-yielding dairy cows. We hypothesized that exogenous iP4 supplementation during the critical period of maternal recognition of pregnancy would enhance conception rates. Methods. A total of 382 lactating Holstein cows were subjected to TAI. On d -11, they received an intravaginal device containing 1.9 g of P4, associated with 10.5 µg of buserelin acetate (GnRH) and 2 mg of estradiol benzoate. On d -4 and -2, the animals received 25 mg of dinoprost tromethamine (PGF2α), and on d -2, the device was removed and 1 mg of estradiol cypionate was administered. On d 0, TAI was performed. Fourteen days after insemination (d 14), the cows were divided into two groups: control (n = 186) and iP4 (n = 196, supplemented with 350 mg iP4). Pregnancy diagnosis were performed on days 30 and 60 post-TAI. The data were analyzed using a mixed-effects model that included fixed effects, random effects, and covariates. Key results. The conception rates at 30 and 60 days post-insemination were significantly higher (P 0.05) in the iP4 group (44.7% and 39.2%, respectively) than in the control group (35.1% and 31.7%, respectively). A significant interaction was observed between iP4 supplementation and body condition score (BCS), in which cows with BCS 2.75 treated with iP4 had a higher conception rate at 30 and 60 days than those in the control group. Pregnancy loss between 30 and 60 days was not affected by treatment. Implication. These findings suggest that iP4 supplementation 14 days after artificial insemination may improve fertility in high-producing dairy cows, but they should be interpreted with caution due to the lack of physiological evaluation of the mechanisms underlying the action of exogenous progesterone.