Use of near-infrared reflectance spectroscopy for the estimation of the microbial portion of non-ammonia-nitrogen in the duodenum of dairy cows

The analytical potential of near-infrared reflectance spectroscopy (NIRS) was tested for predicting the microbial portion of the total N at the duodenum of lactating cows. Samples and reference data were obtained from animals fitted with rumen cannulae and proximally positioned duodenum cannulae and fed 40 different diets. From a total of 158 individual experiments, the microbial portion of the total N at the duodenum as well as the total flow of microbial-N were satisfactorily estimated by NIRS as compared to 15N measurements which served as reference. The standard error of predicting the microbial portion of the total N at the duodenum amounted to 3.1% in absolute terms and 4.5% in relative terms. Calculated values for the daily rate of microbial protein synthesis per unit intake of metabolisable energy and daily flow rate of microbial-N at the duodenum showed no relevant difference between the two methods of assessment. The results indicate that NIRS is an interesting tool for estimating the microbial portion of the total N at the duodenum rapidly and at low cost.