Nitrogen efficiency indexes association with nitrogen recovery, utilization, and use efficiency in spring barley at various nitrogen application rates

The nitrogen use efficiency (NUE) is the ratio of the N uptake by the plant to the total amount of applied N fertilizer. The optimization of NUE is essential for sustainable agriculture, especially in the breeding programs to improve the yield of barley ( Hordeum vulgare  L.). We investigated the NUE variation in 32 barley genotypes and to examine the response relationship between phenotypic traits and N level to determine the traits essential for the selection of breeding programs to optimize the NUE. The NUE was investigated in 32 barley genotypes for 2 yr under field conditions using three N levels (60, 120, and 180 kg N ha −1 ). Growth, physiological and agronomic traits, and N efficiency‐related parameters were determined. The parameters investigated were significantly affected by N application and its interactions with traits. Plant height, biological yield, spike length, and grain number (GN) were increased at higher N levels, but the 1,000‐grain weight decreased. The lowest grain yield was 2,000.7 kg ha −1  at 60 kg N ha −1 , and the highest was 8,607.9 kg ha −1  at 120 kg N ha −1 . An increase in the N rate from 60 to 120 kg N ha −1  was associated with a higher total chlorophyll content (CHL SPAD ) (from 19.04 to 23.13), but no significant change in CHL was seen at 180 kg N ha −1 . The agronomic nitrogen use efficiency (ANUE), fertilizer N recovery efficiency and the nitrogen utilization efficiency were both reduced with an increase of the N application rate. A path analysis revealed that GN was the most essential trait for NUE breeding program selection due to its high direct effect (0.72) on ANUE. A linear discriminant analysis showed that CHL at 60 d after sowing was the best trait for distinguishing low from high N levels.