Near Infrared Reflectance Spectroscopy: A Survey of Wavelength Selection To Determine Dry Matter Digestibility

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Abstract
This study examined the usage of wavelengths for determining DM digestibility by near infrared reflectance spectroscopy. Samples of legumes, grasses, and mixtures of forages and concentrates with known in vitro or in vivo DM digestibilities were scanned. The samples and analysis equations were from many different studies and locations. Wavelengths chosen for calibration equations in each study were obtained from published and unpublished reports. Frequency analysis was employed to determine which wavelengths were selected within specific forage types and across all forage types. Many of the wavelengths chosen to determine digestibility were 1600 to 1900 and 2200 to 2300 nm, regardless of forage type. The 1600- to 1700-nm region is associated with overtones of C-H, C-N, and N-H groups, which are related to fiber and protein, and the 2200- to 2300-nm area is associated primarily with C-H groups (fiber). There were slight differences in frequency of wavelength selection between legumes and grasses within these two predominant areas. Forage and concentrate mixtures utilized wavelengths in the 1500-nm area more than the other forage types. This area is associated with O-H and N-H groups (water, starch, and protein) in grains. Published wavelength data from near infrared reflectance spectroscopy determinations, combined with the frequency of wavelength occurrence for digestibility determinations, may help ascertain the relationship of plant chemistry measured by this method and digestion by animals.