Differential seasonal response of milk urea nitrogen between dual-purpose Romanian cattle breeds under year-round housing

Abstract

Milk urea nitrogen (MUN) is a critical biomarker for nitrogen efficiency in dairy systems, yet limited data exists for Eastern European dual-purpose breeds. This study evaluated MUN variation patterns in Romanian Brown and Romanian Simmental cattle to optimize nitrogen management and support EU environmental goals. We analyzed 4,311 test-day records from 544 cows (102 Romanian Brown, 442 Romanian Simmental) collected between 2020-2025 in year-round housed free-stall systems. Linear mixed models with REML estimation assessed effects of breed, season, lactation stage, parity, and their interactions on MUN concentration. Mean MUN was 10.93±5.50 mg/dl with significant seasonal variation (p<0.001), showing spring peaks (12.91 mg/dl) and winter lows (8.62 mg/dl) despite continuous indoor housing. A significant breed × season interaction (p<0.001) revealed differential responses: Romanian Brown exhibited greater seasonal amplitude (5.16 mg/dl range) versus Romanian Simmental (3.51 mg/dl). The model explained 33.9% of total variance with moderate repeatability (ICC=0.1145). Dual-purpose Romanian cattle show pronounced seasonal MUN patterns with breed-specific responses, indicating opportunities for targeted nutritional management and genetic selection to improve nitrogen efficiency while meeting environmental regulations.