The effect of different levels of N fertilizer on yield and yield components of maize (Zea mays L.) under competition with different densities redroot pigweed (Amaranthus retroflexus L.) and millet (Panicum miliaceum L.)

In order to study the competition effects of redroot pigweed (Amaranthus retroflexus) and millet (Panicum miliaceum) on maize performance under different rates of N fertilizer, a field experiment was conducted in 2009 in the research fields of Tarbiat Modares University in Tehran as a randomized complete block design with factorial arrangement of treatments and three replications. Accordingly, three factors included nitrogen fertilizer (75% optimum or 138 kg N ha-1, optimum or 184 kg N ha-1 and 125% optimum or 230 kg N ha-1), weed species (redroot pigweed and millet) and weed densities (5 and 25 plants m-1 for redroot pigweed and 7.5 and 37.5 plants m-1 for millet) consisted. Results showed that the highest grain yield (921.89 g m-2) was achieved in treatment 230 kg N ha-1 while the lowest yields (466.72 g m-2) belonged to treatment millet fertilized with 138 kg N ha-1. High densities of millet and redroot pigweed resulted in significant reductions in number of grains per row and the ear length. Differences in N application rates cause significant variations in biological yield and some of the yield components of maize. Overall, results indicated that in fields where a nitrophile species is the dominant, increasing N application rate beyond the optimum not only does not increase maize grain yield but also reduces its yield and causes pollution of environment.