Nitrate Accumulation in Crabgrass as Impacted by Nitrogen Fertilization Rate and Source

Crabgrass (Digitaria spp.) could provide high quality summer grazing in the southeastern U.S. However, little information is available on the tendency of crabgrass to accumulate nitrate. This study evaluated the effect of N fertilizer rate and source on the nitrate concentration of crabgrass forage. Plots were established in late spring of 2001‐2003 near Blackstone, VA. Seven N rates ranging from 0 to 300 lb/acre were applied in 50 lb intervals at seeding as ammonium nitrate or broiler litter. Nitrate concentrations in the forage increased with N rate (P < 0.01) and were greater during periods of moisture stress. First harvest nitrate accumulated to dangerous levels (> 5000 ppm) when more than 84 and 143 lb of N per acre was applied as ammonium nitrate in 2001 and 2002, respectively. In 2002, nitrate concentrations in second harvest forage fertilized with ammonium nitrate ranged from 1000 to 20000 ppm and increased with N rate (P < 0.001). In contrast, nitrate concentrations in first and second harvest forage fertilized with broiler litter never exceeded the generally safe range (P < 0.01). Final harvest nitrate concentrations never exceeded the generally safe range regardless of N source.