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How Does Cool Cloudy Weather Affect Corn During Grain Fill?

September 26, 2019

Extension Climatologist Al Dutcher in a related CW article notes that the third week of August was exceptionally cloudy in parts of central Nebraska. In addition, August temperatures were cooler than normal in most of Nebraska. (See Figure 1 in this Sept. 12, 2019, CW article.) Dutcher also mentions that we can expect above-normal warm temperatures for the next week. How do these factors affect corn during the grain-fill period (R5)?

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.

Nebraska 2019

There is no doubt that reduced solar radiation levels the third week of August in central Nebraska will reduce yield potential to some extent. Also, corn plants fill ears with available photosynthate in preference to keeping stalk tissue well-nourished and alive.  This can increase stalk deterioration and the potential for stalk rots and lodging, especially at high densities. Be wary of stalk quality issues.

But, in our situation this year, clouds are not necessarily bad. In addition to providing rain, cloudy weather can reduce temperatures and extend the grain-fill period. That extension can increase yields if frost doesn’t occur before maturity (R6).

This is the same kind of issue that corn agronomist and breeder Tom Hoegemeyer discussed a few years ago in a CropWatch story relative to cool temperatures, yield, and maturity: “Cool weather during grain filling slows down development, but respiration slows more than photosynthesis. While somewhat less sugar is made per day in cool environments, less is lost to respiration, and the grain-filling period is extended. This can allow the plants to store relatively more sugar, resulting in heavier kernels and higher yields, especially compared to seasons with high day and night temperatures.”

Whether it is cool or cloudy, corn growth and development slow and the grain-fill period extends. Either of these can result in increased yields if the crop matures naturally, but the risk of frost also increases as crop maturity extends.  A pre-mature frost will also affect test weight. (See Todd Whitney’s CropWatch story, Why Grain Test Weights Matter, 2017.)

Our overall late planting in 2019 exacerbates the issue: USDA NASS reports corn development is not only behind last year but also behind average ― 82% dented compared to 91% in 2018 and 90% for the five-year average, as of September 15. This would be a great year to have a late frost for most of the state.

Use the U2U Corn GDD tool to estimate crop maturities for your part of the state/Corn Belt. Compared to 30-year U2U averages, our 2019 Nebraska corn crop in regions such as south-central Nebraska is now following closely with the long-term average development curves.  The challenge is remembering to adjust our expectations since our 2018 corn crop developed well ahead of long-term averages and this year’s late planting dates.

Dutcher’s forecast of above-normal temperatures for the next week could hasten corn development and reduce the frost risk but yield could be compromised if frost occurs before R6.


Early, E.B., W. O. McIlrath, R. D. Seif, and R. H. Hageman. 1967. Effects of Shade Applied at Different Stages of Plant Development on Corn (Zea mays L.) Production. Crop Science 7: 151 – 156.

Reed, A., G. Singletary, J. Schussler, D. Williamson, and A. Christy. 1988. Shading Effects on Dry Matter and Nitrogen Partitioning, Kernel Number and Yield of Maize. Crop Science 28: 819-825.

Schmidt, W.H. and W.L. Colville. 1967. Yield and Yield Components of Zea mays L. as Influenced by Artificially Induced Shade. Crop Science 7: 137- 140.

Yang, Yunshan, Wenjuan Xu, Peng Hou, Guangzhou Liu, Wanmao Liu, Yonghong Wang, Rulang Zhao, Bo Ming, Ruizhi Xie, Keru Wang, and Shaokun Li. 2019. Improving maize grain yield by matching maize growth and solar radiation. Nature: Scientific Reports.

Source: University of Nebraska CropWatch