This past year across Delmarva, reported yields in irrigated fields underperformed while rainfed fields overperformed. In our irrigated studies, yields ranged from 164 to 255 bushels/acre, while rainfed fields ranged from 126 to 201 bushels/acre. One of our irrigated cover crop studies improved by 30 bushels, but most other studies had a 30 bushel drop compared to 2019, similar to fields across the region. The lower planting populations in rainfed fields may have withstood multiple stressors from high rainfall events (including hurricane Isaias), but the results were still surprising.
We had one corn population study (20 to 40k/acre) set up to test drone imagery at our irrigation research farm (Table 1). The realistic expectation was that yields would drop off before rates hit 40k seeds, but as with most things in 2020, nothing worked out as we expected. The highest absolute yields (202 bu/acre) were at rates of 40k, but yields were not significantly different between 28k and 40k seeds/acre (Table 1). So we wouldn’t necessarily recommend 40k seeds, but there is a linear increase in yield (R2 = 0.81) up to that rate (Figure 1), although a plateau did appear to form at 32k before increasing again.
| Population | Yield (bu/acre) | Stand Counts (plants/acre) | V6 NDVI | R2 NDVI |
| 20000 | 168 d | 20033 g | 0.51 e | 0.92 f |
| 22000 | 174 cd | 21448 fg | 0.52 e | 0.92 ef |
| 24000 | 186 bc | 22559 f | 0.53 de | 0.93 ef |
| 26000 | 184 bc | 26021 e | 0.55 d | 0.93 d |
| 28000 | 192 ab | 27582 e | 0.55 de | 0.93 de |
| 30000 | 190 ab | 29505 d | 0.57 bcd | 0.93 bcd |
| 32000 | 193 ab | 31182 c | 0.57 cd | 0.93 cd |
| 34000 | 193 ab | 32140 c | 0.58 abc | 0.93 ab |
| 36000 | 190 ab | 34317 b | 0.59 ab | 0.93 ab |
| 38000 | 196 ab | 37279 a | 0.59 ab | 0.94 ab |
| 40000 | 202 a | 38585 a | 0.60 a | 0.94 a |
Figure 1: Corn Seed Planting Rate vs Yield.
Stand counts were typically lower than target rates by 500-1000 seeds per acre (Table 1) and also had a linear relationship (R2 = 0.79) with yield (Figure 2). Measurements of NDVI by drone imagery at the V6 and R2 stage also had linear relationships with yield (0.82 and 0.91, respectively), with a higher slope and more separation between NDVI values at the V6 stage. Differences in planting rates were typically indistinguishable at about 4000-6000 seeds/acre (e.g. 40k vs 34k), but any planting rates higher than this could be distinguished.
Figure 2: Yield vs Stand Counts and NDVI (V6 and R2).
Average NDVI values across all seeding rates (Figure 3) had a slight drop after VT (1200 GDD), which coincided with a high rainfall event from Tropical Storm Fay (July 7th). While this may not explain statewide drops in yield, the stress from excessive rainfall at pollination, followed by temperatures above 90°F at the beginning of reproductive stages, may have led to lower overall yields in this study. We will try to repeat this study in 2021 and see what shakes out. The use of drone (or satellite when available) imagery in this study displays the usefulness of an easily obtainable measurement to try and determine yield limiting effects of weather.
Figure 3: NDVI across Growing Degree Days in the Population Study.
