عنوان مقاله [English]
Nitrogen is one of the most important nutrients and plays a major role in crop yield improvement. Currently, under the global pressure to increase grain yield while simultaneously minimizing environmental risks, the nitrogen use efficiency has become a challenging issue. Corn (Zea mays L.) nitrogen use is of continued interest due to agronomic performance and environmental issues. 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) and nitrogen partial factor productivity are also issues of importance. The aim of the present study was to evaluate all these indices for corn cultivars released in Iran during the last 40 years to find the trend changes during the course of their release.
Materials and Methods
In order to evaluate the trend of changes of nitrogen use efficiency for corn cultivars during the course of their release an experiment was conducted in growing 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 (CRBD). Treatments included eight different levels of nitrogen (nitrogen applied from urea source (46%) N: 0, 100, 200 and 300 kg N ha-1, animal manure: 0, 10, 20 and 30 ton ha-1 corresponding to 0, 100, 200 and 300 kg N ha-1 respectively). Nitrogen fertilizer was allocated to the main plots and six cultivars of corn, (704 and 647 released as very old cultivars, 500 as an old cultivars and 705, 706 and 260 as new ones were allocated to sub plots. Sub plots were 2.25×3 m with three replications.
Results and Discussion
Results showed that grain, stover and biological yields, harvest index, nitrogen use efficiency, nitrogen uptake efficiency and nitrogen utilization efficiency were affected significantly by cultivars and fertilizer levels. Maximum and minimum stover yield was obtained with 704 (very old hybrid) and 260 (new hybrid), respectively. In contrast with these results, showed that the new cultivars have significantly higher yield than old corn cultivars. Although corn cultivars had significant effects on nitrogen harvest index, this was not affected by fertilizer levels and types. With increasing in nitrogen and manure fertilizer levels, grain and biological yields increased significantly. Although in most fertilizer levels maximum grain yields were observed in the 704 cultivar, but with increase in nitrogen amount from 0 to 300 kg, the most grain yield increases were obtained 117 and 103% in the 500 and 705 cultivars, respectively. On the other hand, in compared to manure fertilizer, nitrogen increased almost yield traits, so that, with exception of 500 and 647 cultivars, in other cultivars manure fertilizer decreased nitrogen use efficiency in compared to equal nitrogen fertilizer significantly. Maximum and minimum nitrogen use efficiency and nitrogen harvest index were obtained with 260 and 704 cultivars. However, two mentioned cultivars showed significant differences, while other cultivars had no significant differences. When different levels of manure fertilizer were applied, statistically similar effects were obtained on corn yield and nitrogen efficiency indices, as compared to equal levels of nitrogen fertilizer. With evaluation of different corn hybrids and nitrogen fertilizer levels reported a negative linear correlation between nitrogen use efficiency and nitrogen levels. With increasing nitrogen fertilizer levels, nitrogen use and utilization efficiencies and partial factor productivity of nitrogen decreased significantly, but with increasing manure levels this trend was observed only for nitrogen use efficiency and partial factor productivity of nitrogen.
In general, no consistent trends were observed between new and old hybrids and also between nitrogen fertilizer levels and types in terms of nitrogen efficiency indices. Furthermore, yield characteristics responses of corn cultivars to fertilizer levels and types had no significant trends. According to these results, it could not be state certainly that in the last four decades, corn cultivars breeding in Iran leading to improved nitrogen efficiency indices and therefore efficient use of fertilizers.