Maize breeding strategies for increased grain yield and nitrogen use efficiency

Context

Breeding and higher plant densities over the last 40 years have increased maize yield and biomass production in the US Corn Belt. However, the effects of planting density versus genetic changes over time on plant and organ nitrogen (N) concentrations and uptake remain poorly understood.

Objectives

This study aimed to (i) quantify changes (genetic gains) in N concentration and uptake across different plant organs at the R2 and R6 stages, (ii) determine whether maize breeding and increasing plant density have altered N dilution patterns, and (iii) examine changes in the yield to crop N uptake relationship to guide future breeding strategies to increase yield and nitrogen utilization efficiency (NUE).

Methods

We conducted seven field experiments over two years across the US Corn Belt, evaluating 18 maize hybrids released between 1983 and 2017. The hybrids were grown under varying plant densities to reflect historical and current management practices. Organ dry matter and N concentrations were measured at R2 and R6 growth stages. The N utilization efficiency (NUE) was calculated as yield divided with crop N uptake at R6 stage.

Results

Breeding and plant density have increased crop N uptake by 45 kg N ha⁻¹ over the past four decades. Breeding accounted for 54 % of this increase and plant density for the remaining 46 %. Breeding primarily increased ear N uptake while plant density improved leaf N uptake. The increase in reproductive organ N uptake was higher than that of vegetative organs, resulting in a higher nitrogen harvest index (NHI) in newer hybrids. Whole-plant and organ level N concentrations declined over time, which was driven by dry matter increases rather than changes in the rate of N dilution. Modern hybrids exhibit greater efficiency in converting N into yield, resulting in a 25 % increase in NUE over the last 40 years. This resulted from increasing crop N uptake by 17 %, NHI by 5 %, and decreasing grain N concentration by 15 % over the last decades. Present grain N concentration and NUE estimates are below crop physiological limits, suggesting that there remains genetic potential to further increase both yield and NUE through targeted breeding.

Implications

These findings offer valuable insights for understanding historical changes in maize N uptake and N allocation to organs, which could help design future breeding targets to further increase yield and NUE.