Rind Penetration Resistance: What Does It Measure?.

Rind penetration resistance has been used as a proxy measurement for lodging resistance for over thirty years. However, the question of which geometric, material, or physiological features are measured by rind penetration remains unanswered. We hypothesized that the inability of rind penetration resistance to distinguish between maize varieties in late stage breeding trials was because rind puncture resistance is primarily a measurement of rind thickness, not stalk strength. A detailed biomechanical engineering analysis of rind puncture resistance was therefore conducted.  

A set of 1000 mature corn stalks (5 commercial hybrids x 5 planting densities x 2 locations x 2 replicates x 10 stalks sampled per plot) were subjected to a series of tests to assess structural and geometric properties. Detailed information on stalk geometry and tissue density was collected using high resolution x-ray computed tomography. Rind puncture measurements were acquired at each internode below the ear. 

Data revealed that rind puncture resistance does not have a close relationship with factors that are known to strongly influence stalk strength. Rind puncture was only loosely correlated with rind thickness (R² = 0.24) while correlations to tissue density, stalk diameter, and stalk strength were minimal (R² < 0.2). Rind penetration measurements are also dependent upon frictional forces between the impinging needle and the stalk tissue. However, these forces are not known to be related to stalk strength. Numerous geometric measurements including, rind thickness, diameter, and moment of inertia were found to be better predictors of stalk strength than rind puncture (0.4 < R² < 0.75).

In conclusion, RPR is weakly related to rind thickness and rind thickness influences stalk strength through the area moment of inertia. The failure of RPR to discern between subtle differences in lodging propensity appears to be a consequence of this indirect relationship between RPR and stalk strength.