Estimation of feed crude protein concentration and rumen degradability by Fourier-transform infrared spectroscopy

Currently, rapid methods are needed for feed analysis. This study examined the potential of Fourier-transform infrared (FTIR) spectroscopy to predict the nutritional value of a wide range of feeds for ruminants, as an alternative to the in situ technique. Moreover, we investigated whether universal equations could be developed that would allow the low-cost determination of crude protein (CP) concentrations and their kinetics of degradation into the rumen. Protein nutritional values of 663 samples comprising 80 different feed types were determined in terms of concentrations of CP, water-soluble CP (CP_WS), total-tract mobile bag CP digestibility (CP_TTD), and in situ CP degradability, including the rumen soluble fraction (CP_A), the degradable but not soluble fraction (CP_B), rate of CP_B degradation (CP_C), effective degradability (CP_ED), and potential degradability (CP_PD). Infrared spectra of dry samples were collected by attenuated total reflectance from 4000 to 600 cm⁻¹. Models were developed by partial least squares (PLS) regression in a randomly selected subset of samples, and the precision of the equations was confirmed by using an external validation set. Analysis by FTIR spectroscopy was sufficiently sensitive to allow the accurate prediction of sample CP concentration (R² = 0.92) and to classify feeds according to their CP_WS concentrations using universal models (R² = 0.78) that included all sample types. Moreover, substantial improvements in predictions were observed when samples were subdivided in groups. Models for forages led to accurate predictions of CP_WS and fractions CP_A and CP_B (R² > 0.83), whereas models for CP_TTD and CP_ED could be used for screening purposes (R² > 0.67). This study showed that models for protein-rich concentrates alone could also be used for screening according to the feed concentrations of CP_WS, CP_TTD, CP_ED, CP_A, and CP_B, but models for energy-rich concentrates gave relatively poor predictions. The general difficulty observed in predicting CP_C is because of a low correlation between FTIR spectra and the kinetics of CP degradation, which may be the result of large variation in the reference method (i.e., in situ degradation studies) and perhaps also because of the presence of compounds that can modify the CP degradation pattern in the rumen. In conclusion, FTIR spectroscopy should be considered as a low-cost alternative in the feed evaluation industry.