Predicting Nitrogen Content in the Northern Mixed-Grass Prairie

Forage quality and quantity are important factors affecting livestock production from grazing lands. ““Greenness”” has been proposed as an indicator of herbage quality in semiarid environments, particularly nitrogen (N) content. The objective of this study was to assess the potential of estimating N content of forage using dead:green ratios and accumulated growing-degree-days (AGDD). Standing crop samples were collected April through October over 3 years from each of 3 replicated grazing regimes on a silty range site in eastern Montana. Samples were sorted into live, current dead, and old dead components, then dried, ground, and analyzed for N content. The AGDD for base temperature 45°°F (7.2°°C) was calculated from National Oceanic and Atmospheric Administration (NOAA) reported monthly average temperatures for Miles City. An equation to predict percent N in the total standing crop from percent dead forage and AGDD was developed using multiple linear regression. This equation accounted for 75.9%% of variation in percent N, and prediction error variance was 0.026. To validate this equation, data were obtained from samples collected from April through September in an independent study of 8 areas on silty and clay-pan range sites grazed during autumn and winter. Samples from these sites were treated and analyzed in the same manner as those used to develop the equation. The developed equation was used to predict percent N for the validation data. The resulting correlation between predicted and actual values was 0.79. The regression coefficient for actual values on predicted values was 0.995 ±± 0.125. The intercept did not differ from 0.0 (P == 0.22), and prediction error variance was 0.042. This equation has utility for predicting N level of forage from Northern Great Plains rangelands.