Relationships of both urine nitrogen output and plasma urea concentration with rumen protein balance in lactating goats

Abstract

The updated INRA system incorporates the notion of rumen protein balance (RPB, g CP/kg DM) in an attempt to better fit feed evaluation, animal requirements and responses to diets. RPB is the difference between crude protein intake and entrance at the duodenum (excluding NH3) which is not very easy to measure in vivo. The aim of this study was to test the strength of the relationships between dietary RPB values obtained from the diet composition and criteria easier to measure, such as N urinary outputs or plasma urea concentration, both at a treatment level and at an individual level in a large dataset obtained from lactating goats fed different types of diets. RPB was a better predictor of urinary N output and plasma urea concentration than was the former French system for protein digestible in the intestine. Within experiments, RPB was highly and positively correlated with urinary nitrogen outputs and plasma urea concentration. RPB can be used to estimate the urinary N output in lactating dairy goats, but this study also showed that some of the residual variance was due to variability among animals.