No such thing as normal

By Roy A. Ulrich, Technical Agronomist, Dekalb & Asgrow

There is no such thing as normal when it comes to a growing season. However, they are usually marked by a pattern or trends that dictate the year and the outcomes. The 2023 growing season seems like it will be marked by periods of extremes and quick changes in the weather and growing conditions.

For many areas of the state the growing season started off cool and slow, but dry, with many growers having a large portion of acres planted over several weeks of time before the first plants had even accumulated enough heat units to emerge. Then, late spring and early summer turned into a drought from the middle of May through the middle to the end June. The drought was quickly erased for many with a few very large rain events which took the soils from dry and hard to saturated and deprived of oxygen. This very erratic weather pattern makes predicting the outcome of this year’s crop extremely difficult. However, there are certain aspects to the equation for yield that simply don’t change, and we are going to examine one of those variables and how it may affect this year’s corn yield.

Let’s look at the yield calculation in corn:

Yield bu/ac = ears per acre * kernel rows per ear * kernel length per ear/ # of kernels per bushel. This equation is often used for yield estimates and depending on the accuracy and amount of data utilized to run this calculation, the outcome can be very accurate when compared to the outcome for a field recorded by the combine or scale tickets. At this point in the growing season all the values in the numerator of this equation are set. The only value left of influence in this equation is the denominator, but it can have a big impact on corn yields.

For this, we are going to usea field that we diligently scouted and pulled representative ear samples for multiple locations of the field and came up with an average of 35,000 ears per acre averaging 17.2 kernel rows around and 33.2 kernels long. So, the equation looks like this 35,000*17.2*33.2/ # of kernels per bushel. Most consultants or scouts are going to use 85,000-90,000 for the devisor in this equation. Meaning, on average, it takes 85,000 to 90,000 kernels to make one bushel of corn. So, 35,000*17.2*33.2/90,000= 222.07 bu/ac for our example field. Now try using the same information, but changing the number of kernels it takes to make a bushel of corn. This time let’s use a kernel size of 110,000 kernels per bushel. 35,000*17.2*33.2/110,000=181.69 bu/ac.

Next, let’s use a kernel size of 65,000 kernels/bushel: 35,000*17.2*33.2/65,000=307.48 bu/ac. That’s a change of 125.79 bu/ac by only changing one of the factors of the yield equation. How realistic is it to see that big of a range in kernel size? In the drought of 2012, I was using kernel sizes of 110,000+ for accurate yields and in 2022 I used 65,000 kernels per bushel in some cases for accurate yields. So, while these are the extremes, it is possible to have this range of kernel sizes. The largest percentage of kernel weight is added during the R5 or dent stage growth stage of corn when 55% of the kernel dry matter is accumulated. Once the plant reaches the R5 growth stage the total number of kernels produced will not change however the total size and weight of those kernels can change drastically depending on the weather and growing conditions of the crop.

What can we do from a management standpoint during the R5 growth stage to drive the denominator in the yield calculation lower to increase yield? Specifically, during the R5 stage there isn’t much we can do as it is approximately the last 30 days of grain fill and a nutrient application realistically won’t have enough to time get into the plant into the ear to make a positive change. From a fungicide standpoint, unless a very aggressive disease like southern rust or tar spot infects a plant at the very beginning of dent, there isn’t enough time for a disease to cause enough leaf area loss to negatively affect yield. However, having a disease-free plant with no nutrient deficiencies heading into the last 30 days of grain fill sets up a system to maximize kernel size and weight.   

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