Seed N is important for soybean [Glycine max (L.) Merr] because of the high grain protein concentration. However, the association between N2 fixation and N remobilized from vegetative tissues to seed across different maturity groups (MGs) is not weil understood. We hypothesized that increased amounts of biomass at the beginning of seedfill for latematuring genotypes would provide a larger pool of N that was avaiiabie for remobilization during seedfill and would increase yield relative to earlier-maturing genotypes. Near isolines for maturity (MG IV, V, and VI), a modern breeding line (R01-416F, MG V), and a nonnoduiated soybean genotype (MG VI) were harvested periodically from beginning seedfill to physiological maturity to determine N remobilization and N^ fixation. Nitrogen fixation contributed approximately 90% of the seed N. The MG VI isoline accumulated larger N amounts in vegetative tissues than the MG IV isoline. Maturity isoiines had similar seed N content, but the MG iV genotype had higher N harvest index (2009) than the MG Vi isoline. in 2009, R01-416F had greater yields and N content than the isolines, but there was no indication that N remobilization during seedfill was greater. These data indicate that N^ fixation continued until late seedfill and supplied nearly all of seed N. A large reservoir of N in the vegetative tissue of late MGs was not fully used, and N was not a limitation to yield under these conditions. A.T.

Continued yield increases in modern commercial maize (Zea mays L.) hybrids will require increased plant density, improved nitrogen‐use efficiency, and breeding for a hybrid's potential yield response to this management. The objective of this study was to determine the genetic variation of commercial hybrids in response to plant density and nitrogen (N) fertilizer levels to assist breeding programs to select hybrids with high yield stability or adaptability to crop management. From 2011 to 2014, 101 hybrids were grown in eight different environments at two planting densities (79,000 and 110,000 plants ha‐1) and three N rates (0, 67, and 252 kg N ha‐1). Broad‐sense heritability increased with increased N rate and plant density. Increased plant density altered yield from ‐0.60 Mg ha‐1 to +0.58 Mg ha‐1 under high N conditions, whereas the yield response to increased N ranged from +4.47 to +5.64 Mg ha‐1. Hybrids that combined above‐average yield under unfertilized and low‐N conditions exhibited greater‐than‐average yield stability across environments under high‐N conditions. Hybrid yield stability variance was larger under high‐N than under low‐N conditions because of greater genotype × environment interaction. Hybrids that were adaptable to high plant density and N conditions exhibited greater‐than‐average yield potential and yield variation across environments. Selecting hybrids with both high yield and yield stability may be difficult, as yield under lower N levels and yield increases with high N fertilization were negatively correlated.

Nitrogen use efficiency (NUE) in maize (Zea mays L.) is an important trait to optimize yield with minimal input of nitrogen (N) fertilizer. Expired Plant Variety Protection (ex-PVP) Act-certified germplasm may be an important genetic resource for public breeding sectors. The objectives of this research were to evaluate the genetic variation of N-use traits and to characterize maize ex-PVP inbreds that are adapted to the U.S. Corn Belt for NUE performance. Eighty-nine ex-PVP inbreds (36 stiff stalk synthetic (SSS), and 53 non-stiff stalk synthetic (NSSS)) were genotyped using 26,769 single-nucleotide polymorphisms, then 263 single-cross maize hybrids derived from these inbreds were grown in eight environments from 2011 to 2015 at two N fertilizer rates (0 and 252 kg N ha −1 ) and three replications. Genetic utilization of inherent soil nitrogen and the yield response to N fertilizer were stable across environments and were highly correlated with yield under low and high N conditions, respectively. Cluster analysis identified inbreds with desirable NUE performance. However, only one inbred (PHK56) was ranked in the top 10% for yield under both N-stress and high N conditions. Broad-sense heritability across 12 different N-use traits varied from 0.11 to 0.77, but was not associated with breeding value accuracy. Nitrogen-stress tolerance was negatively correlated with the yield increase from N fertilizer.

No abstract

Maize (Zea mays L) yield increases associated with better usage of N fertilizer, (i.e., increased N use efficiency [NUE]), will require innovative breeding efforts. Genomic selection (GS) for N‐use traits (e.g., uptake or utilization efficiency) may speed up the breeding cycle of programs targeting NUE in maize. We evaluated the GS accuracy of 12 N‐use traits for training populations (TPs) varying in composition (TC) and size, predicted yield performance under different N fertilizer rates, and investigated the usefulness of GS for NUE in maize breeding programs. A total of 552 maize hybrids were planted under low (0 kg N ha−1) and high N fertilizer (252 kg N ha−1) conditions across 10 environments. Training composition scenarios included T0 (hybrids in which none of the parents were included in the random subset of inbreds), T1 (hybrids in which one of their parents were included in the random subset of inbreds), and T2 (hybrids in which both of their parents were included in the random subset of inbreds). Training population sizes ranged from 10 to 40 or 30 to 90 hybrids, depending on the N‐use trait. Across different TC, TP sizes, and N‐use traits, GS accuracy ranged from −0.12 to 0.78 and was greatest with larger TP sizes when both parents of untested hybrids appeared in the training and validation sets (T2 hybrids). Moreover, GS accuracy in response to different TC and TP sizes was dependent on the N‐use trait. Successful breeding for N stress tolerance or improved yield response to N fertilizer level will require selection of specific N‐use traits.