Variability among sources and laboratories in analyses of wheat middlings

A cooperative research study was conducted by members of a regional committee (North Central Regional Committee on Swine Nutrition [NCR-42]) to assess the variability in nutrient composition (DM, CP, Ca, P, Se, NDF, and amino acids) of 14 sources of wheat middlings from 13 states (mostly in the Midwest). A second objective was to assess the analytical variability in nutrient assays among 20 laboratories (labs; 14 experiment station labs and six commercial labs). Wheat middlings were obtained from each participating station's feed mill. The bulk density of the middlings ranged from 289 to 365 g/L. The number of labs that analyzed samples were as follows: DM and CP, 20; Ca, 16; P, 15; Se, 7; NDF, 10; and amino acids, 9. Each lab used its own analytical procedures. The middlings averaged 89.6% DM, 16.2% CP, .12% Ca, .97% P, 36.9% NDF, .53 mg/kg Se, .66% lysine,. 19% tryptophan, .54% threonine, .25% methionine, .34% cystine, .50% isoleucine, and .73% valine. As expected, there was considerable variation in nutrient composition among the 14 sources (P [is less than] .01), especially for Ca (.08 to .30%) and Se (.05 to 1.07 mg/kg). 'Heavy' middlings (high bulk density, [is greater than or equal to] 335 g/L), having a greater proportion of flour attached to the bran, were lower in CP, lysine, P, and NDF than 'light' middlings ([is less than or equal to] 310 g/L), having cleaner bran, resulting in negative correlations between bulk density and CP (r = -.61), lysine (r = -.59), P (r = -.54), and NDF (r = -.81). Each l-percentage-point increase in CP in the wheat middlings was associated with .0235 ([r.sup.2] = .61) and 2.1 ([r.sup.2] = .39)-percentage-point increases in lysine and NDF, respectively. Lysine content was associated with NDF, CP, and bulk density of wheat middlings ([r.sup.2] = .88). There was considerable variation among laboratories (P [is less than] .01) in analysis of all nutrients. The CV among sources (100 [multiplied by] [[Sigma].sub.S]/mean)was greater than among labs (100 [multiplied by] [[Sigma].sub.L]/mean) for CP, Ca, P, Se, and NDF, but the CV among labs was greater than that among sources for DM and all of the amino acids except lysine and phenylalanine. Key Words: Accuracy, Chemical Analysis, Wheat Milling Residues