INFLUENCE OF NITRATE AND AMMONIUM NUTRITION ON THE GROWTH OF GIANT FOXTAIL (SETARIA-FABERI)

The objective of this study was to evaluate growth and seed production of giant foxtail under different N sources (NO3 and NH4) and N fertil izer application rates. Nitrate and NH4 fertilizers plus nitrification inhibitor were applied at 56, 112, and 225 kg N ha(-1) under field co nditions, and in the greenhouse four N rates (1, 5, 10, and 25 mM N) w ere applied in three NO3:NH4 ratios (100:0, 50:50, 0:100). Growth of g iant foxtail was affected by N rates under both greenhouse and held co nditions. In 1993, abundant rainfall in May and June allowed a rapid a nd earlier uptake of N by giant foxtail, resulting in larger plants wi th greater N accumulation and higher numbers of heads and seeds than i n 1994. Total dry weight increased with increasing N rates; however, s eed production reached a maximum at approximately 150 kg N ha(-1). Nit rogen translocation efficiency decreased with increasing N rates. Gian t foxtail did not show any preference to N form; however seed producti on was reduced when the high N rate was applied as NH4 compared to NO3 . These results suggest that NH4 fertilizer applications with a long-t erm nitrification inhibitor could reduce the seed production of giant foxtail and its contribution to the soil seedbank for subsequent growi ng seasons.