|Hall, Mary Beth|
Submitted to: Journal Dairy Science Supplement
Publication Type: Abstract Only
Publication Acceptance Date: 2/25/2011
Publication Date: 7/10/2011
Citation: Hall, M., Mertens, D.R. 2011. A ring test of in vitro neutral detergent fiber digestibility: analytical variability and sample ranking. Journal of Dairy Science. 94(E-Supplement 1):766. Interpretive Summary:
Technical Abstract: In vitro neutral detergent fiber (NDF) digestibility (NDFD) is an empirical measurement used to describe fermentability of NDF by rumen microbes. Variability is inherent in assays and affects the precision that can be expected for replicated samples. The study objective was to evaluate variability within and among laboratories (labs) of 30-h NDFD values measured in repeated runs. Subsamples of alfalfa (n=4), corn silage (n=5), and grass (n=5) ground to pass a 6-mm screen were sent to 10 labs on 3 occasions over a 12-month period. Subsamples passed a test for homogeneity. Labs ground the samples and ran 2 or 3 replicates of each sample within run, and analyzed 2 or 3 sets of samples. A lab that did not provide in-run replicate data was not included in evaluation of standard deviations (SD). Mean and SD for sample within run within lab were calculated. Factors in the statistical model were lab, run within lab, sample, and lab by sample. All factors affected NDFD (P<0.01 for all) and within-run SD of NDFD (P<0.03 for all). The lab by sample effect suggests against a simple lab bias. Labs used 2 NDFD procedures: 8 labs used a procedure similar to Goering and Van Soest, 1970 (GVS) using fermentation vessels or filter bags, and 2 used a procedure with pre-incubated inoculum (PInc). Among GVS labs, NDFD results were affected by all factors (P<0.01 for all; mean 48.5%, range 42.7 to 53.2%, SED = 0.98). For PInc, mean NDFD was 30.7% (range: 31.8 to 29.6%); GVS and PInc NDFD differed (P<0.01, SED=0.95). Mean within-run SD were 1.9% (range: 0.5 to 3.4%) for GVS and 2.6% (range 1.8 to 3.4%) for PInc. The mean SD for all labs of 2.0% indicates that 95% of results for a sample within a run would be within a range of 8.0% NDFD. Labs ranked samples similarly within forage type. Spearman correlation coefficients between average rankings and those reported by labs were 0.83 for alfalfa, 0.70 for corn silage, and 0.90 for grass (P<0.01 for all). It is concluded that across all labs an average precision of 8% NDFD can be expected for a single analysis within run. Differences between GVS and PInc suggest using results in contexts appropriate to each procedure.