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