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Near Infrared Reflectance Spectroscopy Evaluation of Ruminal Fermentation and Cellulase Digestion of Diverse Forages

USDA-ARS
Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, 411 Borlang Hall, 1991 Upper Buford Circle, St. Paul, MN 55108
Dep. of Agronomy, Univ. of Missouri, Columbia, MO 65211

^* Corresponding author.

Digestible dry matter determined by in vitro rumen fermentation (IVDDM) is recognized as a superior assay of digestible energy potential of forages, but fungal ceilulase digestion (NDSCS) can a less costly substitute. Our primary objective was to ascertain the relationship between the two methods and whether near infrared reflectance spectroscopy (NIRS) could be used to estimate in vitro digestibility of diverse forages by either method. We also wanted to compare wavelength selection during equation development by NIRS. We analyzed 499 samples among five diverse cultivated forage and weed groups. Digestibility determined by the two in vitro methods was highly correlated; r ranged from 0.98 to 0.83. The NDSCS method provided more complete digestion than IVDDM for all forage groups. The NIRS calibration equations for both methods had coefficients of determination (R²) of 0.86 to 0.97 for four of the five groups. The R² values were higher and the standard errors of analysis (SEA) were lower for IVDDM (SEA of 7.2-31.1 g kg^-1) compared with NDSCS (SEA of 11.8-38.9 g kg^-1). Biases were negligible. Ten of 46 wavelengths used were identical for the best IVDDM and NDSCS equations, and 26 wavelengths were within 20 nm of being identical. Eleven peaks or valleys of the mean NIR spectra for the 499 samples were at or near wavelengths known to be absorbed by feed nutrients or fiber fractions. We conclude that the IVDDM and NDSCS methods usually similarly ranked cultivated forages or weeds for in vitro digestibility and that NIRS can estimate digestibility by either in vitro method. Wavelength selection during NIRS equation development appears to be based on chemical entities that are often commonly digested by rumen microorganisms and cultured cellulase.

Key Words: In vitro digestible dry matter • fungal cellulase solubles • Neutral detergent solubles • NIRS wavelength selection

Received for publication February 23, 1987.

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