Nitrogen use efficiency and harvest index in black seed (Nigella sativa L.) at different weed competition durations

The competition for nitrogen is an effective factor can be affected crop- weed interactions. In order to determine of black seed nitrogen use efficiency and harvest index at different weed competition durations, a field experiment was conducted at Agricultural Research Station, Ferdowsi University of Mashhad, Iran, during 2009- 2010. The experiment was Randomized Complete Block Design (RCBD) with 12 treatments and 3 replications. Two sets of treatments consist of weed-infested and weed-free periods were used. At the first set of treatments, weeds compete with black seed up to 0, 14, 28, 42, 56 and 70 day after emergence (weed-infested periods). At weed free treatments, plots were kept free of weeds up to the mentioned stages. Results indicated that different weed interference durations had significant effects on nitrogen contents of seed, stubble and biomass of black seed. Nitrogen harvest index was significantly affected by different weed competition durations. In this experiment, uptake (recovery), physiological (internal) and use (agronomic) efficiency of nitrogen in black seed decreased with decreasing weedfree and increasing weed-infested periods. However, these competition periods had more effect on nitrogen uptake efficiency than physiological efficiency of nitrogen.