Smoke, Sun & Crop Effects

Overcoming Stresses with Proper Management

By Paul Hermans

If you were to ask my wife, she would tell you that I am addicted to the weather. Every day I will look at my weather app at least five to 10 times, especially during the hot hazy days of summer, checking to see who got rain, how much, and when we might get a nice shower to boost our crops.

The unique part of my weather app is the ability to get alerts of incoming weather patterns. Lately I have been seeing a lot more air-quality index alerts coming across my screens due to the wildfires happening across Canada.

Many growers have been asking what impact this will have on their crops.

Before we look at the crop side of things, I wanted to review the intensity of the wildfires across Canada.

According to the Canadian Interagency Forest Fire Centre, in June there were more than 450 active fires in Canada. Of this, 240 were reported as being out of control.

The total area burned as of the middle of June was 5.2 million hectares, or about 12.8 million acres. To put that in perspective, in 2022 Statistics Canada reported Canadian farmers grew 5.3 million acres of soybeans and 3.6 million acres of corn. Combined, that is 8.9 million acres, which represents 75 per cent of the total burned area.

Back to the crop physiology side of corn and soybeans. If we look at both a corn and a soybean plant, crop development occurs at a faster rate with more heat and sunshine – but to a point.

Corn likes it hot and prefers temperatures in that 28 C to 30 C range. Soybeans, on the other hand, like a bit cooler temperature, topping their growth rate out around 23 C to 25 C.

So, what effect do wildfires have on crops?

The most common one would be a reduction in sunlight intensity and resulting heat, which help a crop develop. As the sunlight reaches earth, a portion is reflected, thus reducing the total amount available to plants.

Soybean Field and Cloudy Sky
   Paul Hermans photo

To track crop development, researchers use Growing Degree Unit (GDU) calculations for corn and soybeans, using a base of 50 F (10 C). Using the following formula (maximum temperature plus minimum temperature) divided by two, minus 50 F will give you GDUs for a given day.

On days with wildfire smoke, I have noticed that temperatures are two to three degrees Celsius cooler, and sometimes as much as five degrees cooler than the previous day. On a GDU calculation, that could be a 10 to 15 per cent drop in a daily accumulation. On a warm July day, we can get around 25 GDU per day (F). If we take an average of four degrees cooler (night and day temperatures), that equates to about five fewer GDU per day. If we had five consecutive days in a row with this smoky haze, it could equate to about one lost growing day.

That is where the typical agronomist response comes in – “it depends.”

For most of our air quality alerts, they have lasted about two days in duration. Luckily, we have not had prolonged exposure with reduced sunlight intensity.

On the other side, for areas in Eastern Canada that are experiencing drought-like conditions, a reduction in solar energy helps the crop development by slowing down plant growth rate and water use/uptake.

The second “it depends” part of the equation relates to when the depletion of sun occurs.

In previous articles we talked about the different growth stages for corn and soybeans. It is most critical for corn to have high solar levels of radiation during the reproductive stages when corn utilizes energy to pack starch into the kernel, setting the plant up “for next year’s prodigy.” Reducing this energy source can have impacts on yield, with fewer kernels around a cob, a reduction in kernels up and down the cob, and/or a reduction in kernel weight.

Growing showing solar radiation deviation by week.
   Solar radiation as it relates to growing stages. -Pioneer Canada photo

For soybeans, the month of August is critical for development. Yes, we need ample solar radiation for soybeans, along with rainfall as well to drive soybean seed set, and most importantly, set size for higher yields.

To mimic a reduction in solar radiation, studies performed in the U.S. using mechanical devices that can be moved into a corn field during various vegetative and reproductive faces to reduce solar intensity were implemented. In 2022 a study was conducted reducing solar radiation by 70 per cent beginning at the V13 stage. Five different timings were applied with a shade curtain over the crop canopy. The following yield results were observed:

Chart showing Shade Reduction and Yield.

Let me quickly point out that these are extreme research conditions. Yes, we can get a yield reduction from less sun. However, when it happens, and for how long, is more important.

Solar radiation numbers this June compared to June 2022 solar radiation in Eastern Ontario were below the 10-year average – no surprise, considering the wildfire smoke, cloudy days, and weather patterns we experienced. July was more promising, leading into corn flowering time.

If we have a good cob-set leading into the later part of grain-fill and experience a lack of sun, this could lead to stalk nutrient depletion as the corn plant uses all available sources to fill the kernels on the cob. Keep an eye on July to September grain-fill periods. Assess stalk quality at harvest time and prioritize fields that have reduced stalk integrity.

In the future, it will be hard to predict wildfire issues in Canada. Utilizing sound agronomy principles will be the best path forward to increasing crop yields. Timely planting dates, seed selection, improved fertility programs and improving soil health will allow for more soil moisture reserves.

The use of fungicides will also allow for improved plant health benefits, ensuring the crop you grow gets to the finish line.

All things we can manage. Control what we can control. Mother Nature is beyond our control.

Weather can be stressful. Overcoming these stresses can be achieved with proper management. Unfortunately, we cannot control the weather – all we can do is put our best foot forward to minimize the severity of what stresses Mother Nature throws at us.

Here’s hoping the next months are full of sun, timely rains, and less smoke.

All the best with the upcoming harvest season. BF

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