A note on sample preparation in the analysis of nitrate and nitrite in forage

Nitrate content in forage is of interest as it pose a risk of poisoning for ruminants at levels above 1000 mg NO3-N per kg DM (Strickland et al, 1995; Undersander et al, 1999). Analysing nitrate in forages by colorimetric methods have been questioned during decades due to interference of coloured substances in the forage (Wiseman & Jacobson, 1962; Wegner, 1972; Anderson & Case, 1999). Nevertheless, the colorimetric method using flow injection analysis (FIA) with a Cu-Cd reduction column has become a standard method in analysing nitrite and nitrate after reduction to nitrite (MacKown & Weik, 2004). The problem of diverging results in nitrate analysis has often appeared as a result of different nitrate extraction techniques, as opposed to differences due to detection methods after extraction (Anderson & Case, 1999). The sample preparation and analytical method in use at the departments of Soil and Environment, Crop Production Ecology and Animal Nutrition and Management at the Swedish University of Agricultural Science has been water extraction of dried samples followed by the nitrite and nitrate analysis according to the ISO method 13395;1996. As nitrate, and in particular nitrite, are unstable compounds, doubts have been raised concerning the influence of the sample preparation step. The present study was made with the aim to compare result of nitrite and nitrate analyses of dried or un-dried samples and extraction with boiling water or room tempered water