Assessing genotypic variation for nitrogen use efficiency and associated traits in Brazilian maize hybrids grown under low and high nitrogen inputs

Even though nitrogen (N) is the nutrient required in the largest amount for maize production and is one of the major limiting factors of maize production, we were not aware of any information about the N-use efficiency of maize hybrids currently cropped in Brazil. For this, we assessed the genotypic variation of a set of 114 maize hybrids currently commercialized and planted in Brazil for N-use traits and investigated the relationships among these traits. The hybrids were evaluated over two growing seasons (2015/2016 and 2016/2017) and under two contrasting N inputs (high N with 200 kg ha of N, and low N with 20 kg ha of N as urea). The mean genotypic values of N uptake amount traits were reduced by around 20%, grain N concentration (GNC) by 9.51%, and GY by 12.30%, whereas N-utilization efficiency increased by 12.28% due to N stress. We found a large genotypic variation among maize hybrids for GY and most N-use traits, but due to the influence of N stress, we found a lower or non-genotypic variation for most tested traits under N stress. The Brazilian hybrids showed similar performances for N remobilization and N content in grain under low N input (LN), and for N accumulation during the post-silking stage under both N inputs. The h^X¯2 values were much higher under high N input (HN) than under LN for almost all traits, except for stover N concentration. While there was no evidence that GNC was a clear indicator for an improved GY, mainly under LN, breeding for maize hybrids with decreased GNC will lead to greater N-utilization by them. Under LN, the N accumulation until flowering is very important in determining GY highest-yielding hybrids, whereas under HN, the development of the highest-yielding hybrids seems to be dependent on N allocation from stover to the grain and N-utilization. We also concluded that N stress environments must be included in the screening of maize hybrids for a general adaptation to variation in N level. Finally, we have identified the maize hybrids BM709PRO2 (20), P3456H (64), and SHS7939PRO2 (95) as being well adapted under both N conditions; therefore, they have the potential to be grown in a wide range of soil N.