Relationship between milk urea level, protein feeding and urinary nitrogen excretion in high producing dairy goats

The aim of this study was to evaluate in high producing dairy goats the relationship between milk urea level (MUL) and diet composition, milk yield (MY), intestinal digestible protein (PDI) balance, milk N efficiency and urinary N (UN) excretion. Twenty-one mean treatment data, derived from thirteen diets tested in five experiments conducted on goats kept in metabolic cages to collect separately feces and urine, were used. Diets, orts and milk were analyzed for chemical composition; urine was analyzed for N content. PDI with N as limiting factor for rumen microbial growth (PDIN) was calculated according to the French system as well as PDI requirements. Relationships were developed using regression analysis based on mixed models that account for between-experiment variation. There were significant relationships between MUL (mg/dL) and dietary crude protein (CP) (g/kg DM): MUL = −45.3 + 0.484 CP (R2 = 0.914), and between MUL and PDIN balance (%): MUL = 22.9 + 0.347 PDIN balance (R2 = 0.917). The relationship between MUL and MY was not significant. Milk N efficiency resulted negatively related with MUL and positively related with MY (kg/d): milk N efficiency = 33.8–0.38 MUL + 2.58 MY (R2 = 0.869). UN excretion (g/d) was positively related with MUL:UN excretion (g/d) = 14.4 + 0.348 MUL (R2 = 0.713). The urea concentration of goat's milk can be utilized for a finer tuning of protein feeding, in order to improve milk N efficiency and reduce urinary N excretion.