DefinePK

Abstract

Heat stress is still a significant cause of limited animal fertility, mainly because of disruption of endocrine function and early embryonic mortality. This study aimed to assess the sensitivity of serum progesterone as an indicator of reproductive performance in dairy cows under different thermal loads. A prospective cohort study of six commercial farms included 520 cows and 775 inseminations during the hot season conducted at University of Agriculture, Faisalabad, Pakistan. Environmental heat load was evaluated using the Temperature–Humidity Index (THI), and mid-luteal serum progesterone concentrations were determined using a validated immunoassay. Reproduction parameters such as pregnancy rate, pregnancy loss/60 days, and time to first insemination up to 150 days were systematically recorded. Severe THI (≥78) was significantly associated with the lowest progesterone level (3.42 ± 1.5 ng/mL) when compared to moderate ones (4.28 ± 1.6 ng/mL). The conception rate decreased from 44.1% in moderate heat to 28.7% in severe heat, and the pregnancy loss increased from 6.1 to11.6%. Using mixed-effects modeling, it has been demonstrated that every one ng/mL increase in progesterone increased the odds of conception by 32%. However, this benefit was diminished under severe heat stress. Receiver operating characteristic analysis showed moderate power of prediction (AUC 0.66), with the best cut-off value over which coitus occurs at 3.0 ng/mL of conceiving. Therefore, these results demonstrate that serum progesterone is a valuable biomarker that mediates the effect of heat stress on fertility and provides diagnostic and management implications for enhancing reproductive efficiency in dairy herds exposed to climate-driven thermic challenges