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Corn Test Weight

Jan 3, 2020

Agronomy, Crop Management, Grain, Market Update, Soybean, Technology

If you’re involved in agriculture, you weren’t sorry to see 2019 come to a close. But with more than 50% of our corn left in the field heading into 2020, we can’t put 2019 behind us just yet.

Late planting and cooler temperatures during August prevented the corn crop from making physiological maturity. And, coupled with the drying expenses and discounts for low test weight, harvest and marketing decisions have never been more difficult. To dive a little deeper into the problems our corn crop faced in 2019 and will continue to face in 2020, we are breaking down and making sense of the data in this month’s blog.

First let’s talk about what we know about growing corn. We know that growth in a corn crop is largely driven by heat units. We commonly use a growing degree day model (GDUs) to help us understand growth throughout the season. We plant a corn seed, and it takes roughly a certain amount of GDUs to accumulate to reach different growth stages. Corn varieties are tracked by GDUs very closely until mid-pollination, and then estimated to black layer. We select varieties with relative maturities which have a high likelihood of hitting black layer, or physiological maturity, before we get a killing frost. This is the point where there are no longer any resources from the plant moving into the kernels. The heat driven portion of corn growth is something that we have become familiar with. However, we also know that photosynthesis is what drives starch accumulation in our corn kernels which eventually leads to heavier kernels and better test weight. Photosynthesis is driven by solar radiation from sunlight. We decided to look at the solar radiation piece to see if this could have played a role in our light test weight corn.

To start, we gathered data for total accumulated radiation and total accumulated GDUs for our Pillsbury location from the North Dakota Agricultural Weather Network. We used the time period of May 1 through October 31. We used data from the last 11 years and included 2004 as it was also a poor test weight year for our trade area. We also gathered average test weight from our Pillsbury and Page locations for those years along with the USDA average Barnes County corn yields.

Accumulated Solar Radiation and GDU Graph

In the above graphic, we graphed accumulated radiation (sunlight) and GDUs. This really puts our 2019 growing season into perspective. We look at the years 2004 and 2009 which were two of the poorest years for corn production in the last 20 years. Average yields were 90.9 and 109, and average test weights were 48 and 51.6, respectively. When we compare these two poor years to this growing season, we accumulated less solar radiation in 2019 than in either of those two years. This is just one graph and one location, but it really helps complete the story of why we are finding such low test weights this season.

Accumulated Solar Radiation and Test Weight Graph

Next, we threw test weight (represented by the yellow line) and accumulated radiation (blue bars) in together. This graph shows that solar radiation plays an important role in achieving test weight in a corn crop. Test weight seems to correlate fairly closely with the up and down trends in sunlight capture for this area. These two factors turn out to be about 63% correlated, which is a strong number in our situation. The main take away from this model would be that while we know that GDU accumulation is key in corn growth, maybe sunlight capture by a corn canopy plays a bigger role in test weight than we have previously thought.

Graph Showing Barnes County Data from 2019


This final graphic is one that is very specific to our growing season in this area. This model from the Midwestern Regional Climate Center gives us some valuable information for Barnes County during the 2019 growing season. The model is assuming a planting date of May 15, and an average first frost for our area of October 5 (number 1). The black horizontal line (number 2) is representing a typical total number of accumulated GDUs it takes for an 85-day corn hybrid to reach black layer. The purple line indicates 30-year average GDU accumulation throughout the growing season, and the green line shows GDU accumulation throughout the 2019 growing season. According to this model, an 85-day hybrid did not reach physiological maturity this year in our area.

To sum this up, I think it’s important to take a look back at this growing season as a whole. The corn crop came up and we had reasonably good stands. Looking at the model above, we were right on the average with heat accumulation for a fair part of the growing season. This corn crop set itself up to be a monstrous crop. It grew rapidly in the early part of the growing season and put a lot of energy into setting another great yielding crop like we have become used to. Where we really started to lose track of the average is right around August 15. Actual heat accumulation broke away from the average and we just never were able to catch back up. A growing season is only so long, and we are limited to that time between planting and first killing frost to capture all available resources possible to produce a high yielding, good quality crop. This crop took advantage of the resources to make the potential for that great yield and quality. We simply got too cold and captured too little sunlight in late August through September to accumulate enough starch in these kernels to make a good, heavy test weight. We believe the lack of sunlight in August and September caused the low test weight even where corn made black layer before a killing frost. We still have a decent crop out there, but with an average August and September, we would have finished filling the kernels and turned a decent crop into a huge one.

Corn on Grates

We hope these ideas are useful to each of you as we look back on this year and start planning for next year. As always, reach out to any of us here at ACI with any questions or comments. We wish you all a successful rest of harvest and a Happy New Year!

– Cole Ehrlin, Arthur Companies Agronomist

– Justin Knott, Arthur Companies Agronomy Manager