Assessing milk urea nitrogen as an indicator of protein nutrition and nitrogen utilization efficiency: A meta-analysis

Abstract

This meta-analysis aimed to assess the effectiveness of MUN as a tool for evaluating protein feeding and nitrogen (N) use efficiency in dairy cows. In this study, we selected 48 research papers published between January 2004 and April 2024, focusing on studies involving Holstein dairy cows with detailed dietary descriptions and results, including MUN, milk protein percentage, and yield, and dietary data on CP and NFC. We employed generalized linear fixed or mixed-effects models for data analysis, utilizing forest plots to visualize the estimated effects. On average, the cows included in the study were 121 DIM, produced 34.8 kg/d of milk, with milk protein at 3.16% and milk fat at 3.69%. The average MUN levels were 12.5 mg/dL, with urine N and fecal N excretions of 193 g/d and 196 g/d, respectively. The average DMI was 23.2 kg/d, with an N intake of 596 g/d. The dietary composition averaged 16.0% CP, 43.0% NFC, 33.6% NDF, 20.9% ADF, and 1.64 Mcal/kg of NEL. Our analysis revealed a close association among dietary NFC, CP, and MUN concentrations, identifying NFC and CP as key factors affecting MUN levels. When MUN levels ranged from 8 to 16 mg/dL, the dietary NFC/CP ratio was typically between 2.15 and 3.60. Furthermore, MUN exhibited a weak positive correlation with milk yield, milk protein percentage, and milk protein yield, a strong positive correlation with urine N excretion, and a negative correlation with the ratio of milk N to intake N. These findings imply that the dietary NFC/CP ratio significantly affects the MUN concentration. Further, it seems probable that by monitoring MUN, NFC, and CP levels together, dairy producers can achieve better balance of NFC and CP in diets, thereby enabling optimization of feed formulation and enhancement of the management of dairy cows.