Nitrogen application effects on forage sorghum production and nitrate concentration

Forage sorghum (Sorghum bicolor (L.) Moench) is an important annual forage crop but prone to high nitrate concentration which can cause toxicity when fed to cattle (Bos taurus and Bos indicus). Two field experiments were conducted over six site-years across Kansas to determine the optimum nitrogen (N) rate for no-till forage sorghum dry matter (DM) yield and investigate the effect of N fertilization on sorghum forage nitrate content. A quadratic model described the relationship between sorghum DM and N rate across the combined site-years. Maximum DM yield of 6530 kg ha−1 was produced with N application rate of 100 kg N ha−1. The economic optimum N rate ranged from 55 to 70 kg N ha−1 depending on sorghum hay price and N fertilizer costs. Crude protein concentration increased with N fertilizer application but N rates beyond 70 kg N ha−1 resulted in forage nitrate concentrations greater than safe limit of 3000 mg kg−1. Nitrogen uptake increased with N fertilizer application but nitrogen use efficiency and N recovery decreased with increasing N fertilizer rates. In conclusion, forage sorghum required 55–70 kg N ha−1 to produce an economic optimum DM yields with safe nitrate concentration.