Historical increases of maize leaf area index in the US Corn Belt due primarily to plant density increases

Context

Leaf area index (LAI) and leaf area distribution within the maize plant are important traits used to explain and predict light interception and thus crop productivity.

Objectives

Here we investigate breeding and plant density effects of leaf area traits. Our objectives are to 1) quantify maize breeding impacts on leaf area distribution and determine bell-shape coefficients used in crop modeling, 2) dissect the contribution of breeding from plant density, and 3) explore the relationship between LAI and crop yields.

Methods

We studied 18 hybrids released between 1983 and 2017 at two density treatments: current (8.5 pl m⁻²) and historical increasing density (from 4.6 to 8.5 pl m⁻²) in Iowa, USA.

Results

Results indicated that concurrent changes in hybrids and increases in plant density have increased LAI from 3.4 (in 1983) to 5.9 m² m⁻² (in 2017), with the highest LAI increases (>50 %) to be realized in the middle canopy. At historical increasing in plant density treatment, the LAI increased by 1.6 % year⁻¹, but the individual plant leaf area decreased by 0.33 % year⁻¹ from 1983 to 2017. This trade-off indicates that new hybrids are more tolerant to higher plant populations than old hybrids. At current plant density treatment, the year of hybrid release did not affect LAI or individual plant leaf area. New hybrids had 5 % narrower leaf area distributions, 23 % higher optimum LAI values (5.2 vs 4.2 m² m⁻²) and 19 % higher grain yields compared to old hybrids.

Conclusions

The main reason for the increase in maize LAI in the US Corn Belt is plant density. However, an increase in LAI does not necessarily translate to higher grain yields as new hybrids had significantly higher grain yields than older hybrids at similar LAI values. Present results contribute to our understanding of maize canopy architecture and allow us to better calibrate crop models to accurately estimate LAI and grain yield.