Determination of urea in compound feed for ruminants: interlaboratory comparison for the validation of the spectrophotometric method in Commission Regulation (EC) No 152/2009

Description of the subject. Urea is a feed additive authorized in ruminant feedingstuff with a maximum content in compound feed of 8,800 mg·kg-1. Commission Regulation (EC) No 152/2009 displays the official method for the determination of urea in feed but does not specify the type of feed for which the method is applicable. Following reports from Member States stating that the official spectrophotometric method is not fit for purpose for other feed than ruminant feed and given the fact that no method performance characteristics are set, the Joint Research Centre organized an interlaboratory exercise for the validation of this method. Objectives. The objective was to carry out an interlaboratory ring trial for the validation of the official spectrophotometric method for the determination of urea in compound feed for ruminants, under specific request from DG SANTE of the European Commission. Method. A collaborative study was carried out on six different test materials prepared from ruminant compound feeds containing urea at various mass fractions and a blank test material. Nineteen laboratories enrolled for the study and eighteen laboratories from ten EU Member States submitted results. Results. The performance characteristics were established, namely, the validation criterion used in this study (RSDR) was in all cases, except in sheep feed at 3,000 mg·kg-1 measured at 420 nm, achieved (i.e. lower than 20%). Slightly better results in terms of precision and trueness were obtained at 435 nm. Conclusions. Commission Regulation (EC) No 152/2009 should be revised as regards the official spectrophotometric method with a restriction of the scope, which is the determination of urea in ruminant compound feedingstuff at authorized level. Additionally, the selected measurement wavelength, which is either 420 nm or 435 nm, has a minor impact on the performance profile of the method.