By Alexander Lindsey, Ohio State University Extension
As fall is progressing, crop harvest is also occurring throughout the state. However, many producers are seeing slower than usual drydown in their corn fields this October. This may be in part due to how the weather conditions impacted corn growth and development this year.
In many parts of Ohio in 2020, temperatures were near the long-term average this season. One marked difference though was that precipitation was below normal for much of the season around the state.
In 2018 and 2020, temperatures were very similar to one another in each month, with the exception of May being slightly cooler and September being slightly warmer in 2020. The only month in which 2020 received more precipitation than in 2018 was May. Cool wet conditions resulted in planting dates that extended into the latter part of May for the state (USDA reported 57% corn acres planted on May 17, 2020), but also may have contributed to delayed emergence due to slow heat unit accumulation (only 11% of the corn was emerged on May 17). Wet conditions following planting may have also contributed to poorer root development in 2020. Poor root development may have impacted corn’s ability to access soil nitrogen and soil moisture once precipitation levels dropped as well.
In 2020, rainfall in June-September was substantially less than in 2018. As a result, the corn crop may have experienced a sort of “delayed development” this year. Temperatures were below the long-term average, which may have contributed to the crop not exhibiting strong stress symptomology as one might expect when moisture levels are low. Rather, the crop may have been able to extend its growth phase to utilize the precipitation that did occur in August and September this year.
For example, at the Western Agricultural Research Station in 2020 only 8 days in July registered precipitation greater than 0.05-inch, with four of those experiencing at least 0.25-inch. In August, 10 days were recorded with precipitation greater than 0.05-inch, but only two of these exceeded 0.25-inch.
As long as leaves and stalks above the ear remained intact, the crop may have been able to extend the grainfill period beyond what was expected based on growing degree day accumulation. Corn ears achieve approximately 50% of their grain yield prior to entering the R5 or dent growth stage. An additional 40% of yield is gained during the first half of the R5 growth stage. Given the later rain events paired with moderate temperatures, plants may have extended the R5 phase beyond what occurs in a normal year. Application of a strobilurin fungicide has been shown in past work to delay senescence in some environments, which could also delay corn drydown if conditions were favorable for this to occur.
A similar phenomenon related to an extended corn drydown phase was also observed in 2019 in Indiana and in Michigan where corn drydown was progressing slower than expected given the GDD accumulation. In the case of 2019, this was suspected to be in part to delayed planting. By May 31, 80% of corn acres in Ohio had been planted in 2020 with 55% of the acres emerged. Given later planting progress in Ohio, this may also hold true for 2020 in that black layer was achieved later than usual in the state given the crop was utilizing late-season precipitation to complete the grain fill process.
Once the corn crop reaches physiological maturity (or kernel black layer), grain moisture content is approximately 35%. Under favorable conditions (warm, sunny, and breezy), grain moisture content can decrease by 0.75 to 1.0 percentage point per day. Recent work from Iowa State suggests grain drydown is approximately 0.7% per day in the first 20 days after physiological maturity, but drops to 0.44% per day after that point. In general, accumulation of 20 to 29 GDDs is required for grain moisture to lower 1%. However, as the weather turns cooler and potentially cloudier, the grain moisture content reductions will likely be lower, ranging from 0.0-0.5% per day. If temperatures remain warm in October, it is possible more grain drydown will occur as well. Agronomists recommend starting harvest when grain moisture content drops below 25%, and producers may need to plan on moisture levels being a little higher this year to accommodate a timely crop harvest.