Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 12, 2003
Publication Date: January 15, 2004
Citation: Mackown, C.T., Weik, J.C. 2004. Comparison of laboratory and quick-test methods for forage nitrate. Crop Science. 44:218-226. Interpretive Summary: Timely and accurate assessment of nitrate concentrations in forage and water sources can be used to manage potential risks of exposing livestock to excessive nitrate intake. This research compared a standard laboratory nitrate assay protocol for winter wheat forage samples to a novel biochemical linked laboratory and field-test method, and two quick-test methods that use small hand-held instruments. In addition, the accuracy of a nitrate field-extraction method suitable for fresh wheat pasture samples was evaluated and compared with standard laboratory extraction of oven dried and ground wheat samples. The biochemically linked laboratory and field-test methods were the most accurate. Among the quick-test nitrate assays a hand held test strip reflectance meter was easiest to use and only slightly less accurate than the enzyme method. Both of these methods were much less variable and did not overestimate tissue nitrate values, as did a hand held compact nitrate specific electrode meter. The nitrate field-extraction method underestimated tissue nitrate concentration and requires either improvement in extraction efficiency or proper calibration with dry tissues before it will be useful. The results of this research can be used by agronomists, crop consultants, producers, and manufactures seeking information and protocols to assess forage nitrate levels.
Technical Abstract: Producers and consultants could use timely and accurate assessment of forage nitrate levels to take corrective steps to avoid or reduce the risks of excessive nitrate intake by livestock. Forage samples from winter wheat pasture (Triticum aestivum L.) with a wide range of tissue nitrate levels were collected at jointing (grazing) and heading (hay). These samples were used to compare a laboratory flow injection analysis (FIA, Cd reduction) method with novel laboratory and field-test enzyme linked methods, and two hand-held quick-test nitrate assays using an ion specific electrode (ISE-card) and test strip reflectance meter (TSR). Comparisons were performed with hot-water extracts of oven-dried samples and fresh samples macerated in dilute propanol solution. Tissue nitrate mean differences between the other methods and FIA were nearly always greater (13 to 66%, P = 0.05). In part, these differences can be attributed to interferences in the extracts that suppressed the detection of nitrate with FIA and falsely elevated nitrate detection with the ISE-card. Variability and deviation from nitrate values obtained with FIA were greatest with the ISE-card. About 18% less nitrate was extracted from fresh tissue than oven-dried tissue, but the efficiency of extraction from fresh tissue was constant. The cost per analysis and accuracy of nitrate assay methods for field screening of forage nitrate was similar for the F-NaR and TSR methods, but the TSR was easier to use. Initial startup costs of the F-NaR method is substantially reduced, if a portable colorimeter is not used. Finally, interferences in wheat forage extracts leading to slight underestimation of forage nitrate occurred with TSR, but were nearly absent with NaR linked assays of nitrate.