Comparison of Laboratory and Quick‐Test Methods for Forage Nitrate

Nitrate contained in forage may pose performance and health risks to ruminants. With timely and accurate assessment of forage nitrate levels, steps to reduce the risks of excessive nitrate intake by livestock can be applied. Traditionally, plant nitrate is measured in the laboratory from finely ground oven‐dried tissue, which is slower than nitrate quick‐test assays of plant sap. However, use of plant sap nitrate requires calibration to nitrate in dried samples. Winter wheat forage (Triticum aestivum L.) collected at jointing and heading from fields fertilized with 56 to 235 kg N ha−1 was used to compare a laboratory flow injection analysis (FIA) method (Cu‐Cd reduction column) with nontraditional laboratory microplate (M‐NaR) and field‐test (F‐NaR) enzyme linked kits (nitrate reductase, E.C. 1.6.6.1), and two hand‐held quick‐test nitrate assays using a card mounted ion specific electrode (ISE‐card) and test strip reflectance meter (TSR). Hot‐water extracts of oven‐dried samples and fresh samples macerated in propanol solution with a high‐speed hand‐held blender were prepared. Compared with FIA, mean differences in tissue nitrate were nearly always greater (13–66%, P = 0.05) with the other methods. For dried samples, these differences were due partially to extract interferences that suppressed detection of nitrate with FIA and falsely elevated nitrate detection with the ISE‐card. Interferences caused only a slight underestimation of forage nitrate with TSR, and were nearly absent with the M‐NaR assay. The ISE‐card was the most variable and deviated the most from the FIA. Nitrate extraction over a nearly four‐fold range was 18% less from fresh than oven‐dried tissue. The quick‐test consumable cost per nitrate assay was similar for F‐NaR and TSR methods, but the TSR was easier to use. Because a hand‐held meter is not required with F‐NaR, initial startup cost can be reduced. Both TSR and F‐NaR performed well for quick‐tests of tissue nitrate.