Management of nitrogen fertilizer application rates based on soil nitrogen fertility with the goal of lowering nitrate nitrogen concentrations in Sudangrass (Sorghum sudanense (Piper) Stapf)

Abstract

High levels of nitrate nitrogen (NO3-N) in forage crops can poison ruminants. To promote stable production of Sudangrass (Sorghum sudanense (Piper) Stapf) with safe NO3-N levels, we investigated the influence of soil nitrogen (N) on Sudangrass yield and on NO3-N concentrations, and we propose a fertilizer management strategy based on soil N levels for a regime based on the application of cattle manure composted with sawdust. We used the available N content determined by incubation of fresh soil as an indicator of soil N. Both the yield and NO3-N concentration of Sudangrass increased with increasing available N. The NO3-N concentrations in the first grass harvest remained lower than 2 g kg−1 (dry-matter basis), which represents the critical level to avoid nitrate poisoning of ruminants in Japan, with an available N content of 40 mg kg−1 (dry-soil basis), the level where yield remained approximately stable without additional N fertilizer. The NO3-N concentration increased linearly with increasing available N and exceeded the critical level when available N exceeded 50 mg kg−1. Under low levels of N (less than 30 mg kg−1), the target yields for the first grass harvest, with safe levels of NO3-N, could be obtained with the application of 10 g N m−2 in fertilizer, lower than the recommended rate of 15 g N m−2. The NO3-N concentration was higher in the second grass harvest under the same levels of soil N with an available N of 40 mg kg−1 or more. The NO3-N concentrations in the second harvest exceeded the critical level when the available N was 40 mg kg−1. These results indicate that available N content is an effective indicator for the stable production of Sudangrass with acceptably low NO3-N concentrations.