Nitrogen Application Effects on Forage Sorghum Dry Matter Production and Nitrate Concentration.

Forage sorghum (Sorghum bicolor (L.) Moench) is an important supplemental forage crop but prone to high nitrate concentration which causes toxicity when fed to cattle (Bos taurus). Two field experiments were conducted over six site-years across Kansas to determine the optimum N rate for no-till forage sorghum and also investigate the effect of N fertilization on sorghum hay nitrate content. Nitrogen fertilizer rates ranged from 0 to 112 kg ha^-1 in Exp.1 (eastern Kansas) and 0 to 140 kg ha^-1 in Exp.2 (western Kansas). Results in Exp.1 showed total DM produced increased linearly from 4743 kg ha^-1 with 0 N to 7439 kg ha^-1 when 112 kg N ha^-1 was applied. In Exp.2, site-year × N rate interaction had effect on DM yield. In site-years when DM yield responded positively to N fertilizer application, forage DM produced with 56 kg N ha^-1 was similar to yields of greater N rates. A quadratic plateau model described the relationship between forage DM and N rate across the six site-years, with maximum DM yield of 6133 kg ha^-1 at an N rate of 85 kg ha^-1. The economic optimum N rate averaged 60 kg N ha^-1, but varied depending on hay value and N fertilizer price. Crude protein concentration increased with N fertilizer at each site-year but N rates beyond 56 kg N ha^-1 resulted in unsafe forage nitrate concentrations greater than 3000 mg kg^-1. Nitrogen use efficiency and N recovery decreased with increasing N fertilizer rates. In conclusion, forage sorghum required 60 kg N ha^-1 to produce economic optimum DM yields with lower nitrate concentration.