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Cloud Cover and Corn Growth

Dec 18, 2019

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.


Clouds — The Good and the Bad

First, we usually consider clouds a positive, they bring rain. But they also reduce solar radiation and unfortunately the portion of the radiation spectrum that affects photosynthesis ― photosynthetically active radiation, or PAR ― is also reduced. This PAR reduction can reduce yield.

Previous Research with Shade Cloth

Many researchers have investigated the effects of lower solar radiation on corn using shade cloth of different densities. These can effectively block solar radiation by 10% to 90% or more (e.g., Schmidt and Colville 1967, and Reed et al. 1988). Invariably they found that shading the crop two to three weeks after silking (R1) reduced yields more than shading before R1. Most also find that hybrids differ in their responses to shading. Very few researchers used shade cloth during the grain-fill period, which would be similar to the reduced solar radiation period central Nebraska experienced the third week of August.

Early et al., 1967, was one of the few; they shaded plants around the “reproductive phase” for 21 days as well as during the “vegetative stage” for 54 days and the “maturation phases” for 63 days. Shading during reproductive stages reduced plant yields the most, but 30% shading during the maturation stages ― what we consider the seed set and grain-fill periods (R2-R6) ― not only reduced yield per plant 25% to 30% but also reduced kernels per plant and the amount of protein per plant.  

Researchers in a new study shaded plants from silking to maturity (R1-R6) (Yang et al., 2019). They also found reductions in yield and biomass with more shading resulting in more losses. Shading reduced yields more with higher plant populations than with lower populations.

GDD and Light Interception

It is well known that corn yield declines with a delay in planting, however, it is not well known the reason for the yield decline. We analyzed GDD and light interception during the 2019 grain fill period and found that the reduction in light interception to be more important than the GDD accumulation in yield decline with delayed planting (Figure 4A). For example, planting on June 6 resulted in 20% less light interception and in 12% less GDD acc umulation (Figure 4B). The same is also true for soybean.

Figure Above. Panel A: Seven-day running average of temperature and radiation in central Iowa in 2019. Horizontal lines indicate the length of the silking to maturity period for different planting dates (April 16, 26, May 6, 16, 26 and June 6 and 26). Panel B: Growing degree day (GOD) accumulation and radiation interception during the grain fill period of crops planted on different days in 2019. The analysis refers to a 111-d hybrid planted in central Iow a.
In terms of soybean yield, the 2019 field trials averaged 61.6 bu/ac (range 50 to 69) with a moisture con tent of 13% (range 11 to 16%). At the state level, the NASS October 2019 foreca st has soybean yield at 53 bu/ac. The same forecast indicates 22% less pods com pared to 2018 and lYX1 less pods compared to the 2016 record yielding year.

How to Prepare for Next Year

The 2019 cropping year was the most challenging over the last five years in Iowa. Yet, crop yields were at good levels; 2% and 5% below 2018 for corn and soybean crops, respectively. This suggests that Iowa agriculture with modern cultivars, advanced farm equipment, and adaptive management practices can buffer against weather variability. Reach out to your CFE Agronomist for tips and discounts leading up to this next growing season. 

"Did Solar Radiation Affect Corn Yield or Standability" - Syngenta
"2019 Cropping year in review: Soils, crops, and weather" - 2019 Integrated Crop Management Conference - Iowa State University


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