Milk urea nitrogen and plasma urea nitrogen of dairy cows grazing: Review

Abstract

The objective of this review was to describe the main factors that influence the production of dairy cows submitted to grazing and the importance of the evaluation of milk and plasma urea nitrogen for animal performance. Pastures are of fundamental importance in Brazilian cattle raising, guaranteeing low production costs since it is the most economical and practical way to produce and offer food for the herd. However, despite the great diversity of grasses with high-quality forage potential, several regions in Brazil present low productivity. In recent years, due to advances in genetic improvement in dairy cows the consumption of protein sources has become a limiting factor for milk production, especially when reared on grazing. The concentration of milk urea nitrogen (MUN) and plasma urea nitrogen (PUN) can be used to monitor crude protein intake, which should be as close as possible to the required requirements of the nutrient by the animal. When MUN values are high (>18 mg/dL) may indicate excess crude protein (CP) in the diet, both the rumen degradable protein fraction (RDP) and the rumen undegradable protein (RUP), ruminal fermentation rate of the non-fibrous carbohydrates fraction (NFC), or protein:energy ratio increased. Low values of MUN (