INTRODUCTION

Endometritis is one of the most prevalent postpartum uterine disorders in dairy cattle and is a major cause of impaired reproductive efficiency and economic losses. The condition has been consistently associated with reduced conception rates, increased service per conception (SC), and prolonged days open (DO), ultimately compromising herd productivity and profitability [1]. Endometritis occurs in both clinical and subclinical forms. Clinical endometritis is characterized by visible clinical signs, including purulent or mucopurulent uterine discharge, whereas subclinical endometritis is not accompanied by overt clinical manifestations, making diagnosis considerably more challenging under field conditions [2]. Consequently, accurate diagnosis of subclinical uterine inflammation relies primarily on laboratory-based techniques. Endometrial cytology, which quantifies the proportion of polymorphonuclear neutrophils (PMNs) in endometrial epithelial samples, is widely accepted as a reliable, minimally invasive method for diagnosing uterine inflammation in dairy cows [3].

Beyond localized uterine inflammation, systemic metabolic status plays a critical role in postpartum reproductive health. During the transition period, high-producing dairy cows frequently experience negative energy balance due to the substantial increase in energy demand associated with the onset of lactation, coupled with insufficient dry matter intake [4]. This metabolic challenge promotes mobilization of adipose tissue reserves, resulting in increased circulating concentrations of non-esterified fatty acids (NEFAs) and β-hydroxybutyrate (BHB), two well-established indicators of negative energy balance [5]. Elevated NEFAs and BHB concentrations have been associated with impaired immune competence, reduced neutrophil function, and increased susceptibility to postpartum uterine disorders [6]. In addition to metabolic disturbances, systemic inflammatory and endocrine responses provide valuable information regarding uterine health and reproductive status. Acute-phase proteins, particularly haptoglobin (Hp), are widely recognized biomarkers of systemic inflammation in cattle [7]. Likewise, reproductive hormones, including the prostaglandin F₂α metabolite (PGFM) and cortisol, provide important information about uterine activity, endocrine regulation, and physiological stress during the postpartum period [8]. Collectively, these physiological parameters may provide a more comprehensive understanding of the mechanisms linking uterine inflammation with reproductive performance.

Although numerous studies have independently investigated metabolic, inflammatory, biochemical, or hormonal alterations associated with postpartum uterine disorders, these physiological systems are highly interconnected and collectively influence uterine recovery and subsequent fertility. Nevertheless, most previous investigations have focused on a limited number of biomarkers or single time-point measurements, preventing comprehensive evaluation of the dynamic interactions among metabolic imbalance, systemic inflammation, endocrine responses, and reproductive performance. Furthermore, longitudinal studies integrating endometrial cytology with simultaneous assessment of metabolic, acute-phase inflammatory, biochemical, and hormonal biomarkers within the same animals remain scarce. This knowledge gap is particularly important under tropical dairy production systems, where elevated ambient temperature and humidity may exacerbate physiological stress, reduce feed intake, intensify negative energy balance, impair immune function, and adversely affect reproductive efficiency [9, 10]. In Indonesia, Friesian-Holstein dairy cows are routinely exposed to these environmental challenges, which may produce physiological responses that differ from those reported under temperate production systems [11]. Therefore, integrated longitudinal evaluation of multiple physiological biomarkers under tropical conditions is needed to improve understanding of uterine recovery and to identify clinically useful biomarkers associated with reproductive performance.

Therefore, this study aimed to longitudinally evaluate metabolic, inflammatory, biochemical, and hormonal biomarkers associated with uterine health status in Friesian-Holstein dairy cows diagnosed with suspected or subclinical endometritis using endometrial cytology. Furthermore, the study sought to determine the relationships between these physiological biomarkers, uterine recovery status, and key reproductive performance indicators, including estrus intensity, SC, and DO, under tropical Indonesian dairy production conditions. We hypothesized that cows with more severe uterine inflammation would exhibit greater metabolic imbalance, enhanced systemic inflammatory and endocrine responses, poorer uterine recovery, and inferior reproductive performance, with Hp emerging as the biomarker most strongly associated with reproductive outcomes.