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)

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 45 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.