Repeatability and reproducibility of the Cornell Net Carbohydrate and Protein System analytical determinations

The increasing use in Italy of the Cornell Net Carbohydrate and Protein System (CNCPS) led researchers of five Italian universities to carry out a collaborative study to evaluate the precision of the CNCPS chemical analysis and derivate fractions. Each laboratory conducted in duplicate the chemical analyses according to the Weende (dry matter; crude protein; ether extract; crude fibre; ash), Van Soest (neutral and acid detergent fibre, NDF and ADF; acid detergent lignin; ADL) and CNCPS (soluble proteins, SP; non-protein nitrogen, NPN; neutral and acid detergent insoluble protein, NDIP and ADIP; starch, ST) schemes on the same five feeds (barley meal, wheat straw, maize silage, dried lucerne and field beans). Anomalous analytical data were identified and corrected by the box-plot graphic tool before the calculation of the CNCPS protein (B-1, B-2 and B-3) and carbohydrate (A, B-2 and C) fractions. Finally, repeatability (chemical analysis) and reproducibility (chemical analysis and fractions) were calculated and expressed as relative values (repeatability and reproducibility standard deviation as percentage of the corresponding mean, RSDr and RSDR, respectively). Chemical analyses of the Weende scheme, together with NDF, ADF and ST analyses, have satisfactory repeatability (0.3-6.2%) and reproducibility (0.3-11.2%) values. On the contrary the ADL, NPN, NDIP and ADIP analyses showed high variability, both within and between laboratories (RSDr and RSDR between 20 and 45%). The SP analysis had an intermediate value of precision (RSDr=10.6%; RSDR=16.4%). Finally, since different combinations of several chemical analyses with scarce (ADL, NPN, NDIP, ADIP, SP) or average precision (e. g. NDF and starch) are used to calculate CNCPS fractions (excluding B-2 protein fraction), also the reproducibilities of these fractions are poor and range from 10 to 20%.