Technical note: Use of near infrared reflectance spectroscopy to assess nitrogen and carbon fractions in dairy cow feces

The feasibility of using near infrared spectroscopy (NIRS) for rapid prediction of nitrogen and carbon fractions of dairy cow feces was investigated. Fecal samples were collected from different farms characterized by differences in milk performances and feeding regimes. Dried fecal samples were first analyzed by chemical analysis for fractions of carbon and nitrogen in terms of ammonium-N (NH4-N), water soluble nitrogen (WSN), undigested dietary nitrogen (UDN), bacterial and endogenous debris nitrogen (BEDN), organic matter (OM), total C, neutral detergent fiber (NDFom), acid detergent fiber (ADFom), and lignin(sa). After chemical analysis, the samples were scanned by near infrared reflectance spectroscopy. The fecal spectrum and reference data of the chemical analysis were calibrated and cross validated using the partial least squares regression method. In addition, 24 fecal samples were collected for external validation. The derived coefficient of determination (R2) for calibration and standard error of calibration (SEC) were measured for total N and for NH4-N, WSN, UDN, OM and also for total C as 0.97 and 0.07, 0.92 and 0.02, 0.91 and 0.13, 0.90 and 0.04, 0.95 and 0.51, and 0.97 and 0.29, respectively. The NDFom, ADFom and lignin(sa) showed high R2 values with 0.97, 0.85, and 0.92. However, SEC values were measured as 1.09, 1.49, and 0.77. The co-efficiency of determination in the cross validation (SECV) attained 3.16, 2.16, and 1.24, respectively, which were on a higher level in comparison to the other fractions. The lowest R2 was found for BEDN measured as 0.78, with lower SEC as 0.15 and SECV as 0.17. Ratio of SEC to SD concerning the nitrogen fractions ranged between 0.29 and 0.50 and between 0.15 and 0.45 regarding the carbon fractions. The coefficient of determination for external validation was similar to the result obtained from cross validation for all parameters. This study suggested that NIRS is a suitable method for the determination of carbon and nitrogen fractions in dairy cow feces. The inclusion of a higher number of samples is expected to further improve the calibration equations.