Another Bin-Busting Corn Year In Illinois Approaches Maturity
DR. GIOVANI PREZA FONTES AND DR. EMERSON NAFZIGER
URBANA, ILLINOIS
The Illinois corn crop got off to a good start in 2025, with good stands and good canopy color, aided by warm (after a cool spell in late May) and dry conditions in June. Still, crop ratings have been average so far, similar to those in the previous five years except for 2023, when very low June ratings rebounded after rainfall. On August 10, only 63% of the crop was rated as good or excellent (G+E), which is lower than in any year since 2021, except for 2023, when it was 61% and was still recovering. In contrast, Iowa corn has been above 80% G+E all season, reaching 85% on August 10.
It was no surprise to see the 2025 corn yield projected at 222 bushels for Iowa, but it was surprising to see 221 bushels projected for Illinois. In 2024, the August 1 estimate for Illinois was 225, but the final yield was 217, a record for Illinois and for all states where most corn is rainfed. The US yield is predicted to be 188.8 bushels per acre, 9.5 bushels (5.3%) higher than the previous yield record, set in 2024. So, while 2025 is set to be a great year for corn, we will consider here what it will take to achieve such high yields and what might prevent them from reaching such high levels, at least in some places.
2025 weather
Warm (1 to 4 degrees above average) and dry (1 to 3 inches less than normal rainfall over approximately 65% of the state) June weather helped the crop get off to a good start. That followed the pattern that we have seen in each of the previous three seasons over much of Illinois: good stands; little or no standing water along with warm soils to encourage root growth; and, with good mineralization and little loss of soil N, outstanding canopy color. The rainfall in late June or July needed to end the dryness fell over much of Illinois in 2025, but missed a few places, including much of Champaign County.
Statewide temperatures averaged 2 to 6 degrees above normal in July, with very high growing degree day accumulations due mostly to warm nights – daytime highs usually reached the 86-degree cutoff, above which GDDs don’t increase with temperature. Precipitation totals in July ranged from only 2 to 3 inches in southern Illinois to more than 12 inches in parts of west-central and northwestern Illinois. While not nearly as well-watered as Iowa, most of Illinois has adequate soil moisture now, although soils are starting to dry out, with below-normal rainfall over nearly all of the state so far in August.
A key question now is whether the amount of soil moisture in the field is enough to finish filling kernels by the time the crop matures. In each of the past four years, the period from early August to early September was dry or very dry in much of Illinois, but yields were mostly good to very good. It’s likely that having a good first half of the season, with well-developed root systems, helps the crop maintain a good canopy and facilitates water and nutrient uptake to complete grain fill. Unlike in previous years, it has been dry since May in some places this year: in Champaign, it has rained more than one inch only twice, on May 20 (1.22”), and on June 4 (1.07”); the total since May 1 is 7.76 inches. We would estimate that the crop has taken up 16 to 18 inches of water by the time it reaches stage R4, and needs 4 to 6 inches more to get to physiological maturity (black layer.) As soon as leaves begin to show “wrinkling” in the afternoon heat, daily yield additions are decreasing. As long as there is a green canopy, rainfall will restore productivity, but not enough to recover the yield lost previously.
Corn progress
Warm weather since early June has kept corn development ahead of schedule. Silking began on June 22 and reached 50% by July 14. While late-June heat and drought stress initially raised concerns about potential pollination problems in early-planted fields, pollination was successful in most fields. We’ve received a few reports of fields exhibiting wrapped tassels, and while the extent and potential yield impact of this unusual phenomenon are not yet known, there is some indication that pollination was adequate in some, perhaps most, of these fields. Where it’s been dry (for example, in Champaign), we see a considerable amount of ear tip dieback. Silking was heavy, so this likely came from abortion of tip kernels after pollination. Dry conditions likely triggered much of this, but because kernel row numbers seem to be above normal in some fields this year, it may also have resulted from simply having too many kernels for the plant to support. If kernel numbers per ear are at least 500, kernel numbers per acre should be high enough for good yields.
As of August 10, 68% of the Illinois corn crop was at the dough stage (near the 5-year average of 66%), while 19% was dented—a sharp increase from 1% the previous week and well ahead of the 11% average over the last five years. One of the key features of the 2025 season has been the consistently good canopy conditions in most fields, as measured by NDVI readings. Having a uniform and dark green canopy color in early July that lasts into mid-August is a strong sign that the crop is well-positioned for grain filling. Fields in many parts of Illinois currently hold the five or so inches of water needed to finish the crop. If temperatures remain above normal, we can expect the early-planted corn crop to start maturing in late August, slightly ahead of the normal schedule. Cool temperatures will slow that progress, but will help the crop extract water if rains continue to hold off.
What to expect as grain-filling progresses
The R5, or “dent”, stage marks a critical point in yield determination, as roughly 55% of the kernel’s dry matter is accumulated during this period. It typically takes about 575 GDD for the corn crop to transition from the beginning of R5 to R6 (black layer), when dry matter accumulation ends. In August, Illinois averages around 25 GDD per day, meaning that the R5 stage usually lasts about 23 days, with some variation related to weather and hybrid maturity. A general guideline is that corn reaches maturity 55-60 days after silking.
Monitoring the milk line progression is a useful way to track progress toward maturity, once the plant reaches stage R5 (dent). Figure 1 shows ears from a planting date trial at the Orr Agricultural Research and Demonstration Center (Pike County) using a 112-d relative maturity hybrid (~2,700 GDD to black layer). Approximately 78 GDD was accumulated between April 1 and 15, and as of August 12, corn planted on those days is reaching R5.5 (when the milk line is almost halfway from the kernel crown to the base), indicating almost 90% of the dry matter is complete. Corn planted on May 1 had accumulated 2,230 GDD and was at the beginning of R5 (milk line was just visible), with only about 45% of the kernel dry matter accumulated. Corn planted on May 15 had accumulated 1980 GDD and was at R4 (dough stage). Denting occurs as starch hardens in the crown; some hybrids show less visible denting, but if the crown is solid rather than liquid when pressed, the kernel has reached R5.
Loss of photosynthetic activity, due to leaf area loss, leaf disease, lack of adequate water, or pale leaves due to lack of adequate nitrogen, will limit how much sugar reaches kernels and is deposited there. If grain-filling ends prematurely, we have often worried about loss of stalk integrity as most available sugar is directed to the kernels. Current hybrids that develop under good conditions, like most of this year’s crop has, typically are able to deposit enough lignin in the stalk in mid-season to keep plants standing even if stalk sugar levels are low by maturity. In dry areas where the crop came under visible stress in late June or early July, stalks may not have the strength to remain standing if strong winds occur. ∆
DR. GIOVANI PREZA FONTES AND DR. EMERSON NAFZIGER
UNIVERSITY OF ILLINOIS