Jarrod O. Miller, Extension Agronomist, jarrod@udel.edu and James Adkins, Irrigation Engineer, adkins@udel.edu
A full soybean canopy is important for maximum sunlight interception for growth, while also shading out weeds. For canopy development, planting populations and row spacings are methods which you control, while stress (deer, rainfall, etc.) are often out of your hands. Based on last year’s population x row spacing x irrigation study, we used drone imagery to examine when canopy closure may occur.
The value we used to measure canopy is “NDVI”, which can range from 0.20 to 0.95 in most corn or soybean fields (with 0.95 representing “full” canopy for each crop). In simple terms, NDVI measures “greenness”.
For soybeans, NDVI increased with population in June and July, indicating that higher populations were covering the soil faster (Figure 1). By August though, all populations had the same NDVI, therefore similar canopies. Interestingly, the higher populations dropped faster and had lower NDVI after August.
For row spacing, 15” rows maintained higher canopy coverage between July and September (Figure 2), while irrigation had better canopy between July and October (Figure 3). While the differences in NDVI where very small for row spacing, they were still significantly different. Over an entire field this may represent a loss of sunlight that was hitting the soil instead of plant leaves. For irrigated plots, it is obvious the fields remained greener through October, senescing slower and probably maintaining yield.
While we found no difference in yields last season with 60k vs 180k seeds, there were obvious differences in early season soil cover. Canopy coverage should be considered when lowering populations or choosing row spacing. Shading out weeds is an important management technique, and while lowering populations had no effect in August, it had an effect earlier in the season.
Figure 1: NDVI (normalized difference vegetation index) for each population treatment across the 2022 growing season. NDVI will range from 0.20 to 0.95 most years in corn and soybeans, peaking during reproductive stages. Columns with different letters are significantly different within that month.
Figure 2: NDVI of row spacing (15 and 30 inches) across all populations. Columns with different letters are significantly different within that month.
Figure 3: NDVI by irrigation treatment over the growing season. Columns with different letters are significantly different within that month.