A. Hajebrahimi - MSc, Department of Eco-physiology, Faculty of Agriculture, Tabriz Univ., Tabriz, Iran.
S. Nasrollahzadeh - Assoc. Prof., Department of Eco-physiology, Faculty of Agriculture, Tabriz Univ., Tabriz, Iran.
J. Shafag-Kolvanagh - Assis. Prof., Department of Eco-physiology, Faculty of Agriculture, Tabriz Univ. Tabriz, Iran.
P. Aghaei-Gharachorlou - MSc, Department of Eco-physiology, Faculty of Agriculture, Tabriz Univ., Tabriz, Iran.

In order to evaluate the influence of nitrogen (N) rate and weed interference durations on weed dry weight, grain yield, biological yield and harvest index a set of experiments were conducted in the Research Station of the University of Tabriz, Iran, in 2009.The experiment was carried out as split-plot, based on randomized complete block design with three replications. Nitrogen levels (0, 70 and 140 kg/ha) were allocated to main plots and 10 weed interference periods (two sets) to subplots. In the first set, the plots were kept weed-free (from beginning of the growing season) for 14, 28, 42 and 56 days after corn emergence and weeds were allowed to infest the crop, later. In the second set, weed were allowed to infest the crop in the corresponding period and subsequently kept weed free. Results showed that, with increasing weed interference period, grain yield, biological yield and harvest index decreased and weed dry weight increased. Full-season weed infestation decreased grain yield, biological yield. The constructed models showed that in order to prevent a grains yield loss of 5%, weeds must be removed from the field after 9.4, 13.26 and 15.59 days after emergence (DAE), respectively. Corn required 95.33, 87.79 and 80.5 days weed free period to prevent a grain yield loss of 5% under the written nitrogen rates, respectively. An increase in the nitrogen levels enhanced corn tolerance to weed infestation. Furthermore, reductions in the nitrogen level before corn planting may cause the need for more immediate and long period weed control.

CornCritical periodNitrogenWeed interferenceYield