PREDICTION OF ORGANIC-MATTER DIGESTIBILITY IN-VIVO OF GRASS-SILAGE BYNEAR-INFRARED REFLECTANCE SPECTROSCOPY - EFFECT OF CALIBRATION METHOD, RESIDUAL MOISTURE AND PARTICLE-SIZE

The results of a study into the effects of calibration method, residua l moisture content and sample particle size on the prediction of the o rganic matter digestibility in vivo of grass silages by near infrared reflectance spectroscopy are reported. A total of 182 grass silages wi th organic matter digestibility values measured with sheep were divide d into calibration (103) and validation sets (79) on the basis of thei r spectral characteristics. Calibrations were developed using modified stepwise regression, modified partial least squares and principal com ponents regression techniques with and without a scatter correction te chnique to reduce particle size effects. In addition, the use of a noi se repeatability file during the calibration process was evaluated for its ability to reduce the effects of variation in sample residual moi sture content. The highest predictive ability for organic matter diges tibility was obtained using a second order derivative modified partial least squares regression with the use of both scatter correction and noise repeatability procedures. Validation statistics obtained were R( 2) = 0.82, standard error of prediction 2.35 and slope 1.01. The best calibration was also shown to have acceptable analytical repeatability (2.44) and not to distinguish between silages made in clamps or as bi g bales wrapped in plastic.