A simple method for estimating nitrate nitrogen concentrations of standing Sudangrass ( Sorghum sudanense (Piper) Stapf) and Sudan‐type sorghum ( Sorghum bicolor Moench × S. sudanense ) at the heading stage by the nitrate nitrogen concentration of the stem juice and the dry matter ratio

A simple method for estimating the nitrate nitrogen (NO 3 ‐N) concentrations of standing Sudangrass and Sudan‐type sorghum at the heading stage was developed. It has been reported that the NO 3 ‐N concentrations on a dry matter basis of these crops could be estimated from multiplying the concentration factors (CF) by the dilution factors (DF): CF was the NO 3 ‐N concentration of the stem on a fresh matter basis and DF was the dry matter partitioning ratio of the stem divided by the dry matter ratio of the stem. In the present study, the methods for estimating the CF by NO 3 ‐N analysis of the stem juice and the DF by the dry matter ratio of the whole crop were examined. One half of all 68 harvests was used for the CF and DF predictions, and the other half was used to validate these and NO 3 ‐N concentration estimations. The regression equation for estimating the CF from NO 3 ‐N concentrations of the stem juice was: CF (g kg −1 FM) = 0.61 × NO 3 ‐N concentrations of the stem juice at the 25% position (g L −1 ), which means at a position corresponding to 25% of the total stem length from the bottom, with a coefficient of determination ( R 2 ) of 0.908. The quadratic equation obtained from the relationship between the DF and the dry matter ratio of the whole crop (DM%) was: DF = 0.00652DM% 2  − 0.492DM% + 11.29, and R 2  = 0.953. In a comparison between the estimates and the actual CF and DF measurements for the validation samples, the CF and DF could be estimated with a high degree of accuracy from each estimate equation with an R 2 of 0.845 and 0.958, respectively. The NO 3 ‐N concentration estimates, calculated by multiplying the estimated CF by the estimated DF, corresponded to the analytical values by the established method with an R 2 of 0.893.