Corn maturity may be delayed by late planting and/or below normal summer temperatures. When slow corn development continues into the fall, corn grain may be significantly wetter at harvest. This can result in higher drying costs, mechanical damage to grain, and if a killing frost occurs before corn reaches maturity, yield reductions. This article discusses the possible impacts of cool temperatures and an early freeze on corn development, grain yield, field drydown, harvest, artificial drying and storage.
Because growing degree unit (GDU) accumulation in early to mid-May is similar to GDU accumulation in late September when corn is maturing, each day of planting delay could result in a commensurate 1-day delay in maturity. However, corn is able to adjust to late planting by reducing its total GDU requirement slightly, by about 5 GDUs for each day planting is delayed beyond May 1. This means that corn maturity is usually delayed by only about 1 day for each 1.5 days of planting delay.
“Cool” or “moderate” summer temperatures are rarely more than 1 or 2 degrees below normal when considering the entire summer period. Such conditions would result in a deficit of 90 to 180 GDUs that has to be made up in late summer/early fall. This would result in about a 1- to 2-week delay in corn maturity in the central Corn Belt, and up to 3 weeks in northern corn-growing areas.
During the ear-fill stage of corn development, kernels progressively gain in “dry weight” as starch accumulates and displaces moisture in the kernel. Beginning at the “dent” stage (R5), a line of demarcation is visible between the hard, structural starch deposited in the crown of the kernel, and the milky content of the rest of the kernel (toward the tip). This border is known as the “milk line” (Figure 1).
Corn physiological maturity is complete when an abscission layer (“black layer”) forms at the tip of the kernel, halting further nutrient transport into the kernel and marking the end of yield accumulation (Figure 2).
As corn reaches the R6 stage, moisture content of the kernel is at about 30% to 35%. At this point, grain quality can still be reduced due to combining, drying and handling of wet grain, but the crop is no longer at risk of yield loss due to frost.
The impact on corn yield from an early freeze is dependent on stage of corn growth, low temperature reached, duration of the low temperature period, and other factors (Lauer, 2004). A freeze event with temperatures below 32 F for several hours would likely kill all the leaves and may stop ear development entirely. Should this occur, growers need to determine the ear development stage at the time of the freeze to estimate percent yield loss (Table 1 and Figure 3).
Table 1. Potential grain yield losses after frost.
Corn leaf tissue can be killed by a few hours near 32 F, and in even less time at temperatures below 32 F. At temperatures between 32 to 40 F, the extent of damage may vary considerably, depending on microclimate effects, the aspect of the field slope, and whether or not atmospheric conditions favor a radiation frost. In such cases, it is possible that only upper leaves in the canopy would be killed, while leaves lower in the canopy survive and remain photosynthetically active. If the leaf tissue is killed, it will be evident in 1 to 2 days as a water-soaked appearance, which will eventually turn brown. Therefore, it is best to wait 5 to 7 days before making an assessment of percentage leaf damage for purposes of estimating yield reduction.
Figure 3. Kernel growth stages and approximate grain moisture, GDUs to maturity (black layer or "no milk line"), and yield loss from a hard, killing frost that stops kernel development.
The period from black layer to harvest is defined as the "drydown" period. Kernel moisture loss during the drydown period is entirely due to evaporative moisture loss affected by air temperature, relative humidity and wind. When corn reaches maturity late in the season, field drydown is slower due to cooler air temperatures. For example, according to Ohio State University Extension, corn drying rates of 1% per day in September will usually drop to 1/2% to 3/4% by early to mid-October, 1/4% to 1/2% per day by late October to early November, and only 1/4% or less by mid-November (Thomison, 2011).
Corteva Agriscience research indicates that it takes approximately 15 to 20 GDUs to lower grain moisture each point from 30% down to 25%, 20 to 25 GDUs per point of drydown from 25% to 22%, and 25 to 30 GDUs per point from 22% to 20% (Corteva Agriscience, unpublished). If a hard freeze occurs that stops corn development prior to maturity, these field drying rates may be affected. For example, corn frosted as early as the dough stage may require 4 to 9 extra days to reach the same harvest moisture as corn not frosted (Maier and Parsons, 1996).
Grain moisture at harvest affects the time and cost required to dry the grain to acceptable storage moisture levels, as well as grain quality. Wet grain can incur damage during combining, handling and drying. If grain quality is significantly reduced during harvest and drying, allowable storage time is also reduced, dockage may result, and losses of fines and broken kernels can trim bushels of saleable grain.
In seasons with delayed corn crop development, many growers will have to deal with wetter than normal grain at harvest. Several steps can be taken prior to harvest to make this job go more smoothly (Lauer 2009).
Combine Adjustments: Grain above 30% moisture can be difficult to remove from the cob and is easily cracked and damaged by overthreshing in the cylinder or rotor of the combine. Cylinder/rotor speed and concave clearance are the adjustments most critical to reduce grain damage and threshing losses. At high grain moisture growers may have to strike a balance between damaged grain and higher than normal grain loss from unshelled cobs.
With very wet grain, some ag engineers suggest beginning harvest with combine settings that would likely underthresh a typical, lower moisture crop (Brook and Harrigan, 1997):
Properly drying very wet, lower quality corn is essential to avoid further quality reductions. Growers should screen lower quality grain prior to drying, using a rotary screen, gravity screen or perforated auger housing section. This will help prevent foreign material and broken kernel fragments (or "fines") from blocking air flow essential to uniform grain drying and storage. Next, growers should plan to dry lower quality grain 1 or 2 points lower than the normal 14% to 15% often recommended for long-term storage. This is because of greater variations of moisture content within the grain mass and increased physical kernel damage and broken cobs, which could magnify mold problems.
According to extension specialists at North Dakota State University, energy efficiency is increased at maximum temperatures in high temperature drying systems, but these temperatures could scorch very wet or immature kernels. In addition, high temperature drying causes stress cracks in the kernel, which allows more breakage during handling and storage. The amount of stress cracking depends on initial grain moisture, rate of moisture removal, maximum grain temperature reached in the dryer, and rate of grain cooling. Therefore, drying temperatures need to be limited on corn of 25% to 30% moisture content (or higher).
With natural-air or low-temperature drying systems it will be difficult to adequately dry corn wetter than 26% grain moisture. The maximum moisture content for natural air drying of corn is 21% using an airflow rate of at least 1 cubic foot per minute per bushel of corn (Hellevang, 2009).
Consider these investments to help manage harvest, drying and storing wet, lower-quality grain:
The University of Wisconsin gives these additional grain drying tips (Lauer, 2009):
To reduce drying time and speed harvest, some growers have discussed partially drying and aerating corn while holding it for further drying after completion of harvest. This strategy requires skill and intensive management, especially with low-quality grain. For more tips on grain drying to maximize grain quality, see Appendix I.
Low test weight, lower quality grain is harder to store because it is breakage-prone and subject to mold and "hot spot" occurrence in the bin. Because the storage life of this grain may be only half that of normal corn at the same moisture content, consider selling this grain early rather than storing long term.
To minimize storage problems, begin by screen-cleaning grain before binning to remove as much of the fine material, cob pieces and broken kernels as possible. After filling, "core" the bin (remove up to 10% of the total bin capacity) to eliminate broken kernels and fines that accumulate in the center. Next, level the grain in the bin to minimize moisture accumulation at the top of the grain. Finally, cool grain as soon as it is dry to within 10 degrees of air temperature, and continue to aerate for 10 to 14 days to ensure grain moisture "equilibrium" has been achieved.
Monitoring lower quality grain on a twice-monthly basis is essential to ensure that grain condition is maintained. For more tips on grain storage and monitoring procedures, see Appendix I and Appendix II.
When growers have fields of wet or immature corn in October, deciding when to start combining is difficult. Experiences during several late harvest years suggest that excessive delays may not be a good idea, for these reasons:
For these reasons, timely harvest is usually advantageous, even though drying costs may be increased.
Written by Steve Butzen, Agronomy Information Consultant, Pioneer, Johnston, Iowa.
Continuous Flow Grain Dryers¹
Natural Air In-Bin
In-Bin Continuous Flow
Cooling Grain to Proper Storage Temperatures
Reviewed August 2019
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