The 2023 corn crop as the season winds down

Giovani Preza Fontes and Emerson Nafziger
Department of Crop Sciences, University of Illinois

Rainfall in late June and early July brought relief from very dry conditions, but soils dried out again before August 5-9 rains over most of Illinois recharged soil moisture enough to restore crop prospects for the 2023 season. The August 15 U.S. drought monitor map showed 28% of Illinois to have no drought, 57% to be abnormally dry, and 14% to be in moderate drought. At this point in the season, there’s little concern about abnormal dryness, nor would we expect moderate drought to represent a severe threat to the crop before it reaches maturity. Temperatures have been below normal most of this week, and are expected to be above normal over the next week.

2023 weather and corn yield prospects

The August 1 yield estimate released on August 11 pegged Illinois corn yields at 201 bushels per acre. That follows the statewide yield of 214 bushels in 2022, and is very close to trendline yield for 2023. There are similarities between the two years: 1) early dryness benefited stands and soil oxygen levels both years, resulting in no standing water damage and good root establishment; 2) dryness limited crop height both years, and to some extent, leaf area in upper leaves; 3) dryness was relieved by rainfall in late vegetative stages, greatly enhancing pollination success, especially in 2023, when rainfall arrived 8 or 9 days earlier than in 2022.

Kernel counts are generally higher in 2023 than they were in 2022, but the August 1 yield estimate is 2 bushels less in 2023 than it was in 2022. The final yield in 2022 was 11 bushels higher than the August 1 estimate; can we expect a similar increase in 2023? It’s too early to say, but lower kernel counts in 2022 were offset by high ear counts and above-normal kernel weights in many fields. Ear counts this year may be slightly lower than they were last year, but if kernels can fill to at least normal weights, some increase in statewide yield may be possible.

As we’ve noted before, the crop condition rating in 2023 dropped much more drastically under dry conditions in 2023 than it did in 2022: it reached a low of 60% good + excellent by mid-July 2022, while it dropped to only 26% by June 26 this year. It rebounded to 64% by August 13, and given that it’s based on visible improvement, it may not increase very much more until and unless yields begin to come in higher than expected. Another measure we can use is from the website Crop-CASMA, which shows that the current NDVI (a satellite-based measure of crop canopy color) in Illinois is overall darker green than it was at this time in 2022, especially north of I-70; the southern third of Illinois is lighter green this year than last.

What to expect as grain-filling progresses

The R5 stage is called the “dent” stage, and is the stage during which about 55% of the kernel dry matter accumulates. It typically takes about 575 GDD to go from the end of R4 (the dough stage) to the end of R5 (also known as R6, or “black layer”) when dry matter accumulation ends. Typical GDD accumulation rates during August are around 25 per day, so this stage takes about 23 days, with some variation related to hybrid maturity. A general guideline is that corn reaches maturity about 55-60 days after silking. This year, Illinois corn started silking in late June and reached 50% silking around July 13. Dent stage was reported first (1%) on August 6, and reached 20% on August 13. We expect to reach 50% dent by early next week, and with warm weather on the way, the earliest-planted fields should reach maturity by about August 25. Those planted at the end of April or early May are about 200-250 GDD behind the earliest-planted fields, so should take about 7 or 8 days longer if it stays warm. At this point we do not expect the extended filling period we saw in 2022 when the leaves stayed green past maturity, but we can’t rule that out, especially if cool weather returns before maturity.  Checking the progression of the milk line is another way to monitor the progression of kernel development. Figure 1 shows the ear of early-April-planted corn (A) and corn planted on April 27 (B) near Champaign. The milk line in the early-planted corn (A) had advanced to about ½ the distance from kernel crown to base (indicating stage R5.5); by this stage, kernels have accumulated close to 90% of their total dry matter. Corn planted in late April (B) is at the beginning of R5 stage, with the milk line barely visible. The dent forms as starch hardens in the crown; not all ears show much dent (think popcorn), but if the crown is solid and no longer liquid when pressed, the kernel has reached stage R5.

Figure 1. Corn ears at stage R5 near Champaign, Illinois. The photo of A was taken on August 16, and of B was taken on August 17. GDD from planting to the time of the photo was about 2,475 for A and 2,290 for B.

Fields that have received 3 inches or more of rain in August, that have a good canopy with good color, and where the crop is now in the dent stage should have little difficulty in filling kernels to at least normal weight, even though conditions are expected to remain dry for the coming weeks. Normal temperatures would be better than above-normal temperatures in terms of hourly demand for water. Still, warm days mean fewer days left to maturity, so we can be optimistic. Corn at this stage uses less water as a percentage of evaporation than it did a month ago, as dry matter accumulation slows in the last few weeks of the season.

Estimating corn yield

We often focus on how many kernels each ear has, but the important number is how many kernels there are per acre. With most corn fields in Illinois at the dough (R4) or dent (R5) stage, kernel numbers are fixed, leaving only the accuracy of kernel counts and guesses of final kernel weight as the only sources of variation in estimating yields.

The first step in estimating yields before harvest is to count the number of ears per acre – usually in 1/1000th of an acre (17.4 feet of row in 30-inch rows). If second ears on a plant have kernels, second ears should be included (counting as one ear for that plant.) Pull three or four ears from the section of row used for the ear count, count the number of rows and kernels per row on each ear, then multiply these to get kernels per ear. Average the number of kernels per ear, then multiply that average times the number of ears in the 1/1000th of an acre to calculate the number of kernels per 1/1000th of an acre. For example, a field with 34 ears per 1/1000th acre and an average of 560 kernels per ear has 19,040 kernels in 1/1000th of an acre.

The final step is to divide the kernels per 1/1000th of an acre by the expected (guessed-at) kernel weight per 1/1000th of a bushel. Under typical grain-filling conditions, we can expect about 80,000 kernels per bushel, so this number would be 80. In our example, dividing 19,040 by 80 gives 238 bushels per acre. The closer to maturity the crop is the easier it is to estimate final kernel weight, of course. Under highly favorable conditions during grain-filling, kernels may get larger than normal, and there may be 75,000 or fewer kernels per bushel. In contrast, kernels per bushel may be 90 or 95 thousand in a drought year. A change from 80 to 95 kernels per 1/1000th of a bushel in our example (19,040 kernels per 1,000th of an acre), corresponds to a yield drop from 238 to 200 bushels per acre.