Quantifying the Effects of Planting Density in Maize: A Structural Engineering Investigation.

Maize varieties that are tolerant of high planting densities are frequently eliminated from breeding studies due to unsatisfactory lodging scores. If the lodging propensity of such varieties were improved further increases in yield could be realized. A detailed structural engineering analysis of the problem of stalk lodging as it relates to increased planting densities was therefore conducted.

An international observational study of lodged corn plants revealed that regardless of planting density, stalks consistently break within three centimeters of a node line and fall in the direction of the minor stalk diameter (90° from the leaf groove). A detailed morphology study involving micro CT scanning of 1000 stalks (5 hybrids, 5 planting densities, 2 locations, 2 replicates, 10 stalks/replicate) demonstrated that at low planting densities stalk cross-sectional shape is optimal for resisting bending loads (i.e. approximately circular).  However, at higher planting densities not only is the overall girth of the stalk reduced but the cross-sectional shape of stalks becomes more elliptical, thus reducing structural strength even further.  Rind thickness and flexural stiffness were also significantly reduced at high planting densities. Interestingly we found that tissue density increased with planting density.  This may be a compensatory mechanism to counteract reduced girth and rind thickness. Plant to plant variability within each hybrid did not change with planting density.

Perhaps one of the greatest impediments to development of maize varieties that are tolerant of high population stress is the lack of a reliable method to quantify lodging susceptibility. Morphological data indicated that the area moment of inertia (an engineering quantity related to cross-sectional shape) predicted 76% of the variability in stalk strength. Rind puncture resistance measurements gathered from the same stalks only predicted 18% of the variation in strength. Area moment of inertia predictions were not negatively affected by hybrid, planting density or location.