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  • Writer's pictureRachel

GDU Accumulation Impacts on Crop Maturity

With our high frequency of rain in July, we’ve had quite a few overcast days. We’ve had several people ask if this reduction in solar incidence has, or is having an impact on our crop maturity. We hope to address that question as well as talk about what is going on in the plant at these different maturities and what we can expect in terms of photosynthesis at this point.

The first thing we should discuss is overall plant stress. For most plants in an upland position, we’ve had a fairly low stress year. Adequate moisture, moderate temperatures, and I think a key point, during our stretches of heat stress, no lack of moisture. We have seen a fair amount of disease pressure, but the vast majority of those have been controlled and that stressor eliminated with a well timed fungicide application. Because of these low stress levels, as a whole our crop is set up for an excellent finish to the growing season.

So what about the GDU’s? Its important to remember that corn needs a set number of GDU’s to reach maturity. This is independent of the number of days. The time between stages is purely dependent on amount of GDU acquired per day. GDU’s are calculated as (Max Air temp+ Min Air Temp)/2 -50. (Our max air temp is capped at 86 degrees and minimum at 50 degrees.) Based on this calculation, it takes roughly 1660 GDU’s to get to R2 or blister stage. To get to dough, it takes approximately 1925 GDU’s. To reach full maturity, it needs about 2700 GDU’s.

If we think about our latest weather, we are probably not reaching our max GDU’s per day. Let’s look at an average late July/early August day. High of 91, low of 68. If we adjust for our max temperature cap, that puts our GDU per day at 27. Then let’s compare to this past Tuesday. High of 75, and low of 49. We adjust our low threshold to 50, and we have a GDU accumulation of 12.6. However, if we look at figure 1, we can see our GDU accumulation over the past month has been surprisingly pretty typical. We did see a decrease in GDU’s over the past 5 days, but expect to see a rebound over the next few days. Overall, from July 15th until now, we have accumulated 501 GDU’s. If we make the assumption that corn was at R1 on July 15th, and has since accumulated 501 GDU’s, we are sitting at around 1901 GDU’s which puts us around milk, to maybe very early dough.

Looking at Figure 2 we can see our total GDU accumulated during this time is just slightly lower than what we typically expect. We had lower accumulated GDU during this same timeframe in 2018, but higher in 2019, 2017, 2016, and 2015.

One thing that is more difficult to quantify and measure the impact of is a reduction in solar radiation. While temperatures have remained near normal, our frequency of rainfall has meant more overcast skies. Cloudier conditions mean less photosynthesis which means less energy to complete necessary plant functions. It is possible that this reduction can impact our yield negatively. Studies using shade cloth found that when shaded after R1, reduced yields more than shade prior to R1. Another study showed that the effect of shading was more pronounced on high populations than low populations. But before we get concerned about possible yield losses, we also have to remember that some of these cooler cloudy days means an extended growing season, and more time for the plant to compensate and increase yields. The University of Nebraska Crop Watch has an excellent article from last year when we experienced a similar cloudy period in August.

So where are we currently sitting for our crop maturities? Jeremy took us through the range of corn maturities we have going on in NE Kansas right now.

If I had to create a summary for our current situation, I would say that our GDU’s are not as behind normal as we might have thought, and that it is too soon to know the impact on a cloudy July and early August. We are well set up for an excellent finish to the growing season, and we may just get there yet.


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