Varietal differences in nitrate nitrogen concentration of Sudangrass (Sorghum sudanense (Piper) stapf)

Field experiments were conducted in 1998 and 2000 to examine varietal differences in nitrate nitrogen (nitrate-N) concentrations among 13 Sudangrass (Sorghum sudanense (Piper) Stapf) commercial varieties under heavy nitrogen application. Significant differences in nitrate-N concentrations among the 13 varieties were observed. The maximum nitrate-N concentrations were 1.9 and 3.2 times larger than the minimum concentrations in 1998 and 2000, respectively. The varietal differences in the nitrate-N concentrations of Sudangrass were examined using the method developed by Harada et al. (1998, 2000). That is, the nitrate-N concentration of a whole plant on a dry matter basis can be estimated by multiplying the concentration factor by the dilution factor. The concentration factor is the nitrate-N concentration of the stem on a fresh matter basis and the dilution factor is the dry matter partitioning ratio of the stem divided by the dry matter ratio of the stem. Two late-flowering varieties (late varieties) had higher dilution factors than early flowering varieties (early varieties), resulting in higher nitrate-N concentrations than the early varieties. Among the early varieties, nitrate-N concentrations were strongly correlated with concentration factors and dilution factors with correlation coefficients of 0.971 and 0.963, respectively. Dilution factors were highly correlated with the dry matter ratio of the stem and the dry matter partitioning ratio of the stem with correlation coefficients of −0.887 and 0.615, respectively. Varieties with low nitrate-N concentrations, had characteristics to reduce nitrate-N, that is, a lower concentration factor and dry matter partitioning ratio of the stem, and a higher dry matter ratio of the stem. There was a positive correlation between nitrate-N concentrations and total nitrogen (total-N) concentrations. However, nitrogen uptake of Sudangrass was correlated not with the total-N concentration, but with the dry matter yield. Therefore, a variety with a high dry matter yield has a high nitrogen uptake, even if it has a low nitrate-N concentration. HS9401 is the most useful variety for reducing nitrate-N concentrations and enhancing nitrogen uptake among those tested