Rumen-Protected Niacin Supplementation Modulates Uterine Arterial Hemodynamics and Endometrial Microbiome Functional Potential in Postpartum Japanese Black Cattle

This study examined whether rumen-protected niacin (RPN) supplementation is associated with uterine hemodynamics and the postpartum uterine environment in Japanese Black cattle. Experiment 1 used a crossover design to evaluate acute uterine vascular responses to RPN in synchronized heifers (n = 6). Experiment 2 was an exploratory evaluation of postpartum cows (n = 6) during long-term RPN supplementation using uterine artery Doppler measurements, uterine inflammatory indicators, and endometrial microbiome analysis. In Experiment 1, high-dose RPN administration was associated with changes in uterine hemodynamic parameters (p < 0.05) and higher baseline-adjusted whole-blood NAD⁺ concentrations across the post-administration period (p < 0.001). In Experiment 2, RPN supplementation was associated with time-dependent differences in uterine hemodynamic parameters, whereas uterine inflammatory indicators were not significantly affected. Although broad treatment-associated differences in overall endometrial microbial diversity and community structure were not detected, exploratory differences in specific bacterial taxa were observed between treatment groups, and predicted metabolic pathways showed treatment-dependent temporal changes. Exploratory integrated analysis further identified concurrent within-animal patterns across uterine hemodynamics, microbial community structure, and predicted microbial functional potential. These exploratory findings suggest that RPN may modulate uterine hemodynamics and be associated with selective changes in the postpartum uterine environment, providing a basis for further investigation of the relationships among nutritional intervention, uterine hemodynamics, the endometrial microbial ecosystem, and subsequent reproductive physiology.