Yield increase in the second half of the last century was attributed to breeding new high yielding cultivars and use of chemical inputs, particularly nitrogenous fertilizer, where contribution of fertilizers in yield improvement has been reported in the range of 30 to 50 percent. In fact, genetic potential of high yielding cultivars is achieved using sufficient nitrogen supply because canopy development, radiation capture photosynthesis and radiant use efficiency is associated with the amount of nitrogen uptake. In recent years, enhancement of inputs efficiency and in particular chemical fertilizers have been the concern of many scientists, though nitrogen use efficiency has not been the goal of plant breeder in a broader sense. Nitrogen Use Efficiency (NUE), Nitrogen Uptake Efficiency (NUpE) and Nitrogen Utilization Efficiency (NUtE) are the main indices normally used to investigate the nitrogen efficiency of plants. However, Nitrogen Harvest Index (NHI), Nitrogen Partial Factor Productivity are also evaluated for this purpose. The aim of the present study was to evaluate all these indices for wheat cultivars released in Iran during the last 60 years to find the trend of changes during the course of their release.
In order to evaluate the trend of changes of nitrogen efficiency for wheat cultivars during the course of their release, an experiment was conducted in the growth season of 2014- 2015 in the experiment farm of faculty of agriculture, Ferdowsi University of Mashhad, Iran. The experiment was based on split plot with complete randomized Block design. In this study, treatments included 12 different levels of nitrogen (nitrogen applied from urea 46% N: 0, 100, 200 and 300 kg N ha-1, animal manure: 0, 10, 20 and 30 ton animal manure ha-1 which respectively equal with 0, 100, 200 and 300 kg N ha-1, and the combination of both chemical and manure fertilizer with 0:0, 50:5, 100:10 and 150:15 kg ha-1: ton ha-1 ratios, respectively). These were allocated to the main plots and six wheat cultivars of wheat, Roshan and Tabasi released as very old cultivars, Falat as an old cultivars and Parsi, Sirvan and Sepahan as new ones were allocated to sub plots. Sub plots were 1.5×2 m with three replications.
Results showed that there was no difference between cultivars in terms of economic yield, nitrogen use efficiency, nitrogen uptake efficiency and nitrogen partial factor productivity except for those of Sirvan which were lower than the others. However, there was difference between cultivars in terms of nitrogen utilization efficiency, nitrogen harvest index, straw yield and biological yield; though no particular then between old and new cultivars in this report. Without considering the type of fertilizers, by increasing nitrogen level from 100 to 300 kg N ha-1 nitrogen efficiency indices decreased significantly, so that almost maximum nitrogen efficiency indices obtained with 100 kg N ha-1. In agreement with these results, indicated that nitrogen utilization efficiency and nitrogen harvest index differ between new and old and also between new wheat cultivars, significantly. In contrast with these results, showed that although the new cultivars have significantly higher nitrogen use efficiency than old wheat cultivars, but no significant differences in nitrogen utilization and uptake efficiencies and nitrogen harvest index were observed between under study cultivars. In this study, in all nitrogen levels, and cultivars, nitrogen utilization and uptake efficiencies contribute 37 and 79% in nitrogen use efficiency changes, respectively.
With an increase in fertilizer levels (chemical, manure and combination of both), efficiency indices were reduced, but these reductions were lower when chemical and manure fertilizers were combined and hence more efficient fertilizer use. Results of this study indicated that in recent decades, breeding of wheat cultivar, could not improve nitrogen efficiency indices and subsequently efficient use of fertilizers in Iran.