Evaluation of refractive index for measuring urinary nitrogen concentration in a sensor worn by grazing female cattle

Urinary nitrogen (N) is the predominant source of N loss from grazed pasture systems. However, few tools exist to accurately measure real-time excretion of urinary-N under grazing. For research purposes, a urine sensor has been developed to measure the volume and N concentration of individual urination events from female cattle in the field. The objective of this paper was to establish that the sensor’s refractive index (RI) value could be used to estimate urinary-N concentration. Individual urine samples (168 in total) were collected from 18 cows that were fed monocultures (ryegrass, clover or turnips) or from cattle that grazed conventional mixed pasture (ryegrass/white clover) in the field. Regression analysis of urine sensor RI value and urinary-N and potassium (K) concentrations yielded strong linear relationships (P<0.001) for all feed types. However, individual regression lines varied between feed types, particularly for turnips. We conclude the use of RI in the sensor provides an accurate estimate of urinary-N but site-specific calibration of the sensor is warranted due to different feed types affecting urine K:N ratio and, by inference, other aspects of composition.