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  • Writer's pictureRachel

Will a Cloudy August Impact Corn Yields?

Updated: Sep 10, 2019


Over the last month or so, we’ve experienced a startling amount of cloudy and wet weather. And while this can have advantages (always good to have rain during grain fill…) the amount of cloud cover can be a detriment to overall yield. This blog post will cover the need for a certain amount of sunshine each year, the amount of sun we’ve had lately, and any possible yield impacts.

GDU’s, Solar Radiation, LAI, and Photosynthesis.

There are several different terms we need to talk about before we get into the yield impacts of our current weather. The first is GDU’s. Growing Degree Units are typically the gauge we use to discuss how plants progress through developmental stages. Different amounts of GDU’s will signal reaching tasseling or silking or physiological maturity. And while this generally is well correlated with the amount of sunlight we are receiving, there are some cases where this is not true. We may have high enough temperatures to gain the daily average GDU’s typical for the month but not have a lot of sunlight. Essentially, cloudy, but warm. The plant will keep maturing, but the lack of sunlight leads to some unfortunate consequences.

Solar radiation is the amount of sunlight reaching the plant canopy. Plants can only absorb a portion of this light; this is called PAR or photosynthetically active radiation. Typically, this range falls between 400-700 nm on the spectral range. The light absorbed in this range is usually 45-50% percent of the total radiation. PAR will decrease proportionally with an increase in cloud cover.

Figure courtesy of Pioneer.

The amount of radiation a plant can absorb is also directly correlated with the amount of leaf area present on a plant. This is called a plant’s leaf area index or LAI. When a plant is at full canopy, the plant’s LAI is maximized as well as its ability to collect sun. When a plant is at full canopy, any significant reduction in solar radiation can be yield limiting.

So, getting back to basics here. What does a plant do with all that collected sunlight? Sunlight is used as an energy source in a process we know as photosynthesis. Plants are able to collect this light and use it to generate carbohydrates and oxygen out of carbon dioxide and water. Plants will later use these stored carbohydrates to power cell function to complete various cellular processes and grow. Aren’t plants amazing?? Anyway, as you can see, if less sunlight is intercepted by the leaves, the less photosynthesis that will happen, the less energy that can be stored, and finally at this time of year, the less energy that can be devoted to grain fill.

Importance of Grain Weights

A decrease in solar radiation will have different impacts during different portions of the growing season. During vegetative stages, this will result in a smaller statured plant. During pollination and early milk, it will likely result in a reduction in the number of kernels. At this point in the growing season, the largest impact will be on kernel weights. (Some reduction in kernel count is also possible.) Essentially, with less than ideal weather conditions, less starch is deposited in each kernel.

Another way to think of it is as the value we use when estimating grain yield. Formulas generally take into consideration number of ears per acre, rows per ear, kernels per row, and finally weight per kernel. While all the other factors are static, (we can count the exact number of rows on an ear, or ears in an acre, or kernels in a row) the weight of kernels is largely variable, and not something that we can easily determine until after harvest is complete. So generally, we assume somewhere between 70,000 to 100,000 kernels per bushel to get an estimate. But what a difference that value can make! Consider a field with an ear count of 31,000 ears per acre and 550 kernels per ear. Using a weight of 70,000 kernels per bushel, our estimate would be 243 bushels per acre. But change that weight to 90,000 kernels per bushel, our yield estimate is now 189 bushels per acre. Pretty big difference. (By the way, calculating a high and low kernel count gives you a good range of potential yield. This is a good way to estimate yield--rather than just one estimate, you can give yourself a more potentially accurate yield range.)

Average Sunny Days vs 2019

So, where do we sit for 2019? Pulling some data for August from the NWS out of Kansas City, we found this. In August we had 3 days with less than 30% cloud cover (defined as sunny), 20 days with 40-70% cloud cover (defined as partly cloudy), and 8 days with more than 80% cloud cover (defined as cloudy). However, high temperatures were fairly similar to normal with highs in the mid 80’s. How does that ratio of cloudy days compare to normal? Over the last 23 years, the month of August shows an average of 12 cloudy days, 11 partly cloudy days and 8 clear days.

Interestingly, there were both less cloudy days, and less sunny days than the average. However, the number of partly cloudy days was significantly higher. Will this shift in distribution impact yield? That’s a hard question to answer.

Past Studies and Yield Impact

First of all, the amount of yield impact is very dependent on the amount of shading the plant experiences. The more limiting the sunlight, the less photosynthesis occurs. As plants need energy, they will start pulling carbohydrates out of the stalks, leading to stalk quality issues like lodging and stalk rots.

Several studies have tried to imitate amount of cloud cover and its impact on yield. One study shaded corn plants for 50% of the time for 2-4 weeks. Another tried shading for 55% of the time for 3 weeks after silking. This resulted in a 21% yield reduction. (The yield reduction was not as dramatic when shaded during or prior to silking.) This higher yield loss comes from that decrease in kernel weight that we discussed previously. Other studies did not show as dramatic of a response.

Interestingly, some studies showed that corn planted at a higher population did not show as much yield impact from a reduction in sunlight. This is possibly due to the fact high population plants are already used to some level of shading from their neighbors and can adapt to a slightly lower amount of photosynthetic activity. Anyone have any fields with multiple populations that want to test this theory out?

Unfortunately, you’ve read to this part of the article without a great answer as to the yield impact we might see. Our cropping systems are complex enough that it is difficult to attribute “lost yield” to any one specific factor. Keep your eyes open this harvest season to see if you can identify any difference in grain weights, population, and yield impacts from a cloudier than normal month.


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