Relationships between dietary factors and nitrogen partitioning to milk and urine in temperate grazing dairy cattle

Abstract

ABSTRACT Urinary nitrogen (UN) excretion and milk production traits of 35 groups of 15 grazing cows each were measured over two years. Urine volume and N concentration were measured with urine sensors and daily UN excretion was calculated for four consecutive days. Milk yield, composition and cow liveweight (LW) were used to estimate daily dry matter intakes (DMI) based on back-calculated animal energy requirements and feed metabolisable energy (ME). Different N fractions in the diet were estimated using laboratory data and protein digestion equations. Mean estimates of N intake and UN excretion were 460 and 227 g N/d, respectively. Urinary-N represented 52% of the N consumed, which aligns with indoor N balance studies. Urinary N excretion was weakly correlated (r = 0.29) with dietary N intake, but moderately correlated (r = 0.63–0.67) with diet N concentration, diet N:ME ratio, and diet effective rumen degradable protein (ERDP). The ERDP balance had moderate to strong correlations with N utilisation efficiency (r = −0.89) and the UN expressed relative to N intake (r = 0.59) and N in milk (r = 0.78). These relationships illustrate the potential of urine sensors and energy-based estimations of intake to assess the influence of dietary management strategies to mitigate UN excretion from grazing animals.