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Using near infrared spectroscopy versus in vitro procedures for predicting dry matter digestibility.

A. L. Kerwin



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Using near infrared spectroscopy versus in vitro procedures for predicting dry matter digestibility.
A. L. Kerwin*1, K. M. Glosson2, K. P. Zanzalari2, T. R. Overton1. 1Department of Animal Science, Cornell University Ithaca, NY, 2Phibro Animal Health Corp Teaneck, NJ.

Total-tract dry matter digestibility (TTDMD) of a dairy cow ration can be determined using undigested neutral detergent fiber at 240 h as an internal marker. The objective of this study was to compare TTDMD calculated from neutral detergent fiber digestibility at 240 h (NDFD240) analyzed by near-infrared spectroscopy (NIR) and in vitro methods. Fecal grab samples (n = 13) were collected for 2 consecutive days on 8 prepartum Holstein cows. Total mixed ration (TMR) samples were collected the day before and the first day of fecal sample collection to reflect the manure that was sampled. Corresponding orts samples were collected for each cow. All TMR, orts, and fecal samples were analyzed for dry matter and NDFD240 by NIR and in vitro procedures at a commercial laboratory (Cumberland Valley Analytical Services, Waynesboro, PA). Individual cow intakes were recorded and TTDMD were calculated using NDFD240 values obtained by NIR and in vitro methods. The TTDMD were calculated using both methods and were compared in SAS v.9.4 using a paired T-Test (PROC TTEST) and Lin's concordance correlation coefficient (CCC) was calculated. The CORR procedure was used to assess the correlation between the means and the differences of the 2 methods. The CCC yielded poor agreement (CCC = 0.40; 0 = no agreement, 1 = perfect agreement) between the 2 methods. The paired T-Test revealed that there was not a mean bias between the 2 methods (mean difference = −0.04; P = 0.92); however, the correlation between the bias (the difference between the methods) and the magnitude (mean of the methods) was significant (r = −0.43; P < 0.0001) indicating that the difference between the 2 methods is not random as the magnitude changes. Although NIR is a cost-effective and faster method, these results indicate that in vitro wet chemistry methods should continue to be considered the gold standard method for measuring NDFD240 when calculating TTDMD.

Keywords: dry matter digestibility, method comparison, neutral detergent fiber digestibility.