Harvest date should be determined by crop maturity, not by the calendar date. Plan to harvest fields with potential lodging or harvest loss problems (such as stalk rot or deer damage) first. All field shelled corn with more than 15% moisture must be dried for safe storage. The ideal kernel moisture level at which to harvest for dry grain storage is 25%. Corn normally dries approximately 0.75%–1% per day during favorable drying weather (sunny and breezy) during the early, warmer part of the harvest season—from mid-September through mid-October in central Ohio. By late October to early to mid-November, field dry-down rates usually drop to probably no more than 0.5% per day. By mid- to late November, the rate drops to 0.25% per day, and after Thanksgiving, drying rates are negligible.
Dry-down rates can also be estimated in terms of GDDs. Generally, it takes 20 to 30 GDDs to lower grain moisture at each point from 30% down to 20%. In September, the accumulation of GDDs averages 10 to 15 per day. In October, the accumulation drops to five to 10 GDDs per day. These estimates are based on generalizations, however, and some hybrids may vary considerably from this pattern of dry-down.
Monitoring harvest losses is an important part of the harvesting process. Ear corn losses from in front of the combine (preharvest losses) should be subtracted from the total harvest loss estimate. The loss of one normal-sized ear per 100 feet of row translates into a loss of more than 1 bushel per acre. An average harvest loss of two kernels per square foot is about 1 bushel per acre. Keep in mind that most harvest losses occur at the gathering unit. An Ohio State study found that approximately 80% of the total machine loss is caused by corn never getting into the combine.
Drought-induced stalk lodging and insect problems reduce the yield potential of many cornfields if harvesting is delayed much beyond maturity. Ear drop damage may be high in some years as a result of extensive European corn borer damage to hybrids without ECB Bt resistance. Estimates of harvest losses based on long-term average data at Purdue indicate that losses increase by 1%–2% for each week of harvest delay. Yield losses associated with harvest delays are magnified at high plant populations (Table 4.10) and hybrid susceptibility to stalk rots. Ear damage by corn borers, western bean cutworm, and other insects may also increase the potential for grain quality problems caused by ear molds.
Shelled grain weights can be adjusted using a grain shrink table (Table 4.18). Shrink represents both the moisture loss and a 0.5% dry matter loss encountered during drying and grain handling. To estimate the amount a given wet weight of corn will lose during the drying process, multiply the wet weight by the shrink factor from the table. For example, assume that 1 ton (2,000 pounds) of shelled grain at 25% moisture will be dried to 15.5% moisture. Drying and handling losses are 2,000 pounds multiplied by 0.1174, or 235 pounds. This results in 2,000 pounds minus 235 pounds, or 1,765 pounds of grain at 15.5% moisture. Monitor debris and cracked corn in the grain as harvesting progresses. Debris and cracked corn lower grain quality and increase the potential for spoilage of stored corn.
Test Weight and Shelled Corn Grades
The test weight of corn determines the weight of a bushel volume (1.244 cubic feet) of grain. Test weights determined on dry (15.5% moisture) corn indicate whether the grain crop reached full maturity. Low test weights indicate immaturity. If the bushel test weight of mature corn is determined at harvest when grain moistures are greater than 15.5%, the test weights will be biased downward. In other words, as corn grain dries, test weight increases. Differences in test weight influence the USDA grading of shelled corn (Table 4.19). The adjustments in test weights do not apply if the grain contains more than 10% broken kernels, was damaged by drought or disease, was harvested when immature, or was dried at air temperatures of 180 F or higher.
Ear Corn
Ear corn can be cribbed safely when the grain moisture is 21% or less. However, with cold weather and narrow (4-foot), well-ventilated cribs, corn may be stored when grain moisture is several percentage points higher. Use Table 4.20 to convert ear corn yields to shelled corn equivalents. For example, 4 tons (8,000 pounds) of ear corn at 21% grain moisture is equivalent to 8,000 divided by 77.6, or 103 bushels of shelled corn.
Corn Silage
Corn harvested for silage yields one-third more feed nutrients per acre than corn harvested for grain. Corn in the full dent stage produces 50% more feed than in the milk stage and 100% more feed than in the silking stage. Corn harvested in the milk or silking stage results in poorer quality silage because of its high moisture content.
One of the most important steps in producing quality corn silage is to harvest at the proper moisture. The storage structure determines the proper moisture level at which to harvest.
Desired moisture levels for different structures are as follows:
• Sealed airtight silos, 55%–60%
• Bag silos, 60%–70%
• Upright silos, 62%–68%
• Trench silos, 65%–70%
Ideally, moisture levels in the silage should be monitored at harvest to prevent harvesting the crop outside the desired range. If moisture testing is not feasible, then estimate the crop moisture by the stage of crop development.
Kernel milk line can serve as an indicator of whole plant moisture levels. As kernels start to dent, a separation between kernel starch and milk can be seen. The firm starch is deposited in the crown (outer) area of the kernel and the milk occupies the basal area of the kernel. This appears as a whitish line separating the two areas. As the crop matures, this kernel milk line moves down the kernel, and the whole plant's moisture declines. When this line reaches the midpoint of the kernel, 90% of the final kernel dry weight has been achieved and silage yields reach a maximum. At this point, the stover part of the plant has good digestibility, and the moisture is usually in the desired range for storage in airtight silos.
A higher moisture level and a slightly earlier harvest are recommended for bunker silos (full dent to one-quarter milk line) and upright conventional silos (one-quarter to one-third milk line). Harvest time can be predicted by monitoring the progression of the milk line. When the milk line reaches the base of the kernel, a black layer forms, and the crop is physiologically mature.
Silage harvest should not be delayed beyond the black layer point because the silage gets too dry, the kernels tend to harden, and the digestibility of the stover declines rapidly. The desired chopping length for corn silage is 5/8 to 3/4 inch. If silage is harvested when the crop moisture is lower than desired, consider chopping finer than normal to promote good packing and to minimize air pockets in the silage. For more information on producing silage, consult North Central Regional (NCR) publication 574 Corn Silage Production: Management and Feeding. Additional information on silage production can be found in the OhioMichigan Silage Performance Test. The test program was started in 2005. Silage performance results from 2005 to 2022 can be found at u.osu.edu/perf/archive.