Sap analysis for diagnosis of nitrate accumulation in cereal forages1

Abstract

Development of a quantitative, preharvest quiektest for NO3 levels in cereal forages would improve crop management options to avoid NO3 toxicity in livestock. Our objective was to determine if concentrations of NO3 in sap expressed from oat (Avenasativa) and barley (Hordeum vulgare) are correlated with those in dry tissue of simultaneously harvested hay, and to test the reliability of the Cardy portable NO3 meter for sap analysis in these species. In 1993, whole plant samples were gathered from plots fertilized with variable nitrogen (N) rates at four environments in Montana, and were analyzed for NO3 concentration in lower‐internode sap and in whole plant dry matter. In 1994 and 1995, the study was repeated at two environments. The sampling technique included three subsamples from each plot for sap analysis, followed immediately by harvest of the entire plot for hay, and further subsampling for dry matter NO3 analysis after drying. Linear correlations between dry matter and sap NO3 concentrations were found across species at each environment in 1993 with r values of 0.64 to 0.81. No relationship was established for oat at one environment. Locations differed in the coefficient of correlation, indicating environmental influences on the relationship and/ or variability due to sampling technique. In 1994 and 1995, each species fit a separate linear correlation across site‐years with r values of 0.89 (oat) and 0.87 (barley). The consistency across site‐years (1994–1995) indicates that the variability in preliminary results was overcome with sampling technique. We propose a quantitative quiektest for NO3 levels in cereal forages using conditional predictions of dry matter NO3 based on observed values of sap NO3. Since sap NO3 readings with the Cardy portable nitrate meter were well correlated (r=0.93) with Accumet ISE readings across critical ranges, quiektest procedures are practical.