Determining the Influence of Particle Size, Diets, Analytical Methods and Feed Form on the Predictability of the Near Infrared Reflectance Spectroscopy

The Near Infrared Reflectance Spectroscopy (NIRS) technique is a rapid and non-destructive technique used to evaluate the chemical composition of complete feed and ingredients. The accuracy of its prediction is not only affected by instrument calibrations but also sample particle size, shape and arrangement. The purpose of this study was to determine the effect of corn particle size, methods of analysis, diets and feed form (mash and pellet) on the accuracy of the NIRS technique using standard calibrations provided with the instrument. In Experiment 1, treatments were arranged in a 4 × 3 × 3 factorial design. The major ingredients in the different diets were i) soybean meal+DDGs, ii) soybean meal+fish meal+DDGs, iii) soybean meal+fish meal+wheat bran, and iv) soybean meal+wheat bran. These were manufactured using different corn particle sizes (400, 600 and 800 µm) to contain a calculated protein content of 20% and subsequently analyzed using three different methods (laboratory, NIRS-ground and NIRSunground). Experiment 2 was a 3 × 2 factorial with three methods of analysis (laboratory, NIRS-ground and NIRS-unground) and two feed forms (mash and pellet). Diets were pelleted and cooled in a counter-flow cooler for 10 minutes. Prior to NIRS analysis, subsample of mash and pellets was ground through a 0.5 mm sieve for the ground treatment. Ground and unground mash and pellet samples for Experiment 1 and 2 were scanned on a Foss NIRS D2500 machine with a wavelength range of 400 to 2,500 nm at a reflectance of log (1/R) at 2 nm intervals for each sample. Laboratory values from wet chemistry analyses were obtained using the Dumas Combustion method and these were compared to results from the NIRS. In Experiment 1, there was no three way interaction. However, a two-way interaction (P ≤ 0.05) was observed for diets x method of analysis and particle size x method of analysis. Crude Protein (CP) content of samples varied when analyzed with NIRS-unground but similar CP was observed for those analyzed with either NIRS-ground or laboratory method. A difference (P ≤ 0.05) in CP content was observed for diets, method of analysis but not for particle sizes. Results of NIRS-ground samples were greater and closer to the expected CP (20%) than NIRS-unground samples. In Experiment 2, an interaction was observed between feed form and method of analysis. The CP content of unground feed samples varied for the feed forms but grinding samples yielded similar results for both NIRS and laboratory analyses. Analyzing unground feed samples using standard calibrations yielded less accurate results compared to samples ground prior to analysis using either NIRS or laboratory methods.