Effects of Nitrogen Rates on Critical Period of Natural Weed Interference on Barley Yield (Hordeum vulgare L. var. Makoei)

Effects of nitrogen rates on critical period of natural weed interference and yield of barley (Hordeum vulgare L.) were studied during 2006 and 2007 at the Agricultural Research Station, University of Tabriz, Iran. The experiment was carried out as split-plot, based on randomized complete block design with three replications. Nitrogen levels (0, 40, 60 and 120 kg/ha) were allocated to the main plots and 10 natural weed interference periods (two sets) were assigned to subplots. In the first set, the plots were kept weed-free up to Em (Emergence - 1st leaf emerged), El5 (5th leaf unfolded), Ti (Tillering - Main shoot and 3 tillers), Se (Stem elongation - 5th node detectable) and Ea em (Ear emergence - Emergence of head complete) growth stages (GS), corresponding to 10, 15, 23, 35 and 59 Zadok’s scales, respectively, by hand weeding. In the second set, plots were left weed infested up to the corresponding periods and subsequently kept weed free. Weed density was 161 plants per square meter in the control plots. Results showed that grain yield decreased with increasing weed interfrence duration. Full-season weed infestation decreased barley grain yield 42.51% as compared with the control. The constructed models indicated that in order to prevent a 5% grain yield loss, weeds at 0, 40, 80 and 120kg N/ha must be removed from the fields after 20.5, 29.6, 31.9 and 30.5 days from emergence (DAE), respectively. Barley required 130.5, 106.7, 105.2 and 109.8 days weed free period to prevent a 5% grain yield loss under given nitrogen rates, respectively. An increase in the nitrogen levels enhanced barley tolerance to weed infestation. The growth and grain yield of barley were also increased by nitrogen application in the sustainable level, however, weed dry weight was not affected. The absence or more reductions in nitrogen use may warrant more intensive weed management. Furthermore, more reductions in the nitrogen level before barley planting may cause the need for more immediate and long period weed control