2019 was a unique year in many ways. Record rainfall and unprecedented planting delays occurred broadly. There were many obvious factors influencing final yield such as planting date and drought. Lodging or reduced yields in fields with no apparent stress can be more challenging to understand. Available solar radiation from sunlight, in addition to temperature and precipitation, plays a strong role in corn growth and development. Tracking temperature and precipitation deficits in season are often easier to visualize than seasonal sunlight accumulation. The maps below show how much photosynthetically active radiation (PAR) differed in 2019 from the prior six-year (2013-2018) average.
July 2019 solar radiation deviation from 6 years prior
August 2019 solar radiation deviation from 6 years prior
September 2019 solar radiation deviation from 6 years prior
Sunlight is an essential component in photosynthesis that results in production of carbohydrates used for plant development and grain production. Reductions of plant-available light at key growth stages can have negative impacts on yield and possibly put plants at higher risk for stalk lodging. Low solar radiation has the biggest effect on yield during silking and grain-fill periods. Experiments that intentionally shaded corn to approximately 50% solar radiation reduced yield by 12-20% when shaded during silking and by 19-21% when shaded during grain fill.1.2 Shading during silking results in ear tip-back or fewer kernels per row whereas shading during grain fill results in decreased kernel weight from shallower kernels. Flowering commonly occurs mid- to late July followed by grain fill throughout August in most geographies with normal planting dates. July and August are therefore the most critical months when evaluating seasonal sunlight deficiencies and their impact on yield and stalk strength. If planting was delayed into late May and early June, it may be more applicable to consider August and September solar radiation impact on corn since flowering and grain-fill dates would be later than normal.
Corn can become even further susceptible to lodging if sunlight is limited during the grain-fill period. Limited photosynthesis during grain fill signals the plant to remobilize carbohydrates from the stalk to the ear which weakens the stalk. This problem can be further exacerbated when near-perfect growing conditions during ear-size determination (at V6 for kernel rows/ear and at V18 for kernels/row) and pollination are followed by below-normal solar radiation during grain fill. Favorable growing conditions throughout flowering will set plants up for higher yield potential and create a greater demand for carbohydrates during grain fill than the plant may be able to supply with limited light availability.
The solar radiation maps above illustrate areas that receive both higher and lower than normal daylight accumulation for each month. Below-normal July accumulations in northern Nebraska, northwest Iowa, western Minnesota and nearly all of the Dakotas in 2019 may explain unexpected ear tip-back and reduced yields in these areas. In August, most of the same areas continued to encounter below-normal light accumulation as well as a broader geography that included the remaining parts of Iowa, Nebraska, Kansas, sections of Missouri, western Illinois and northern Oklahoma. Lower light availability in these areas during August could have added increased demand for carbohydrate relocation from stalks to support grain fill resulting in higher potential for stalk lodging, reduced yields or a combination of both. Areas forced to delay planting into June should also consider light-limited geographies within the September map that could be at higher risks due to stress during grain fill. Continue to scout fields throughout harvest and adjust harvest schedules accordingly to help minimize losses.
Contact your local Golden Harvest Seed Advisor with questions or for more agronomic insights.
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