When Should I Start Corn Harvest and What are the Implications?

Corn harvest will be upon us soon, if not already, and anticipation is building. However, we still need to get this corn crop out of the field and safely deliver it to market. The optimal timing to successfully harvest your corn crop may vary depending on several factors.

Field grain drying rate – Corn normally dries about 0.6% per day in Mississippi after it reaches physiological maturity or black layer (when grain moisture is around 30%) down to 15% moisture. However, this drying rate will usually be much faster when grain moisture is high, and then scale back considerably as it drops into the teens, as the disparity between grain moisture and the environment diminish. Thus, you will likely see grain moisture fall about 0.75% per day or more the first two weeks after physiological maturity. The drying rate of mature corn grain is nearly exclusively dependent upon environmental conditions, rather than plant disposition. Therefore, when rainfall persists, along with cloudy days, high humidity and cool temperatures, grain dry-down will slow considerably or be suspended. This is much more likely with a late crop, or when harvest is delayed until late September or thereafter. The “black layer” is an abscission layer that effectively cuts off moisture and nutrient transfer between the plant and the grain, when kernels are physiologically mature and development is complete. As readily apparent during the last few days, corn leaves naturally senescence or die after grain reaches physiological maturity, particularly when temperatures are hot.  Therefore, using a harvest aid will have little or no significant effect on corn grain dry-down rate, since there is no longer any moisture transfer to the grain, and the plants are naturally dying.

Leaves of a Delta corn field naturally senescing shortly after physiological maturity.

Factors that affect harvest efficiency and losses – The best time to begin corn harvest may vary considerably, primarily depending upon how long it might take to harvest your crop. This will depend on how many acres you have relative to your harvesting capability, including combine, trucking, and storage capacities. These factors, along with other crops you grow on your farm, determine the potential harvest duration and the relative risk associated with harvest losses, delays or complications. Those potential risks include various types of stalk lodging and/or grain quality deterioration, which can result from inclimate weather, insect pest damage, and late‐season weed growth. The most significant weather-related threat to Mid-South corn is likely high winds promoting stalk lodging. Two corn kernels per square foot or one dropped ear per 100 feet of row equals about 1 bushel per acre yield loss. The bottom line is that the longer corn stays in the field, the greater the likelihood of substantial field losses and complications.

Moisture Dockage ‐ Corn may be harvested any time after grain reaches physiological maturity, which occurs at around 30% moisture. However, corn may not be safely stored until considerable moisture loss occurs. Thus, grain elevators discount wet corn to account for drying expenses and moisture weight loss during drying. Moisture dockage schedules between elevators may vary, so thoroughly compare rates. Most schedules discount about 2.5‐3.0% per each percent moisture above the standard, and may increase as moisture content rises. Water evaporated during drying (shrinkage) accounts for 1.18% of the dockage per percent moisture. You lose this weight regardless of whether you sell wet grain to the elevator, dry it mechanically or let the grain field dry. Thus, you should subtract this value from the dockage rate to show your realized or “actual” dockage. Furthermore, corn delivered to market at less than the standard moisture weighs less, so you are essentially docking yourself (1.18% per percent moisture) if you do so. Thus, you should generally strive to finish harvest before grain moisture falls below 15%.

The black layer separates the kernel from the cob, making drying rate dependent on ambient conditions.

Grain Management during and after Harvest – Although harvesting corn earlier will likely reduce potential field and harvest losses, it does require considerably more effort to properly handle and store grain in our climate. Grain is a living organism which may spoil quickly when the grain moisture and temperatures are high. For example, temperatures during harvest in the South will be well above 80 deg F, which are, of course, a stark contrast to the Corn Belt. Furthermore, moist grain and high temperatures also cause fungal growth to flourish, potentially causing aflatoxin to escalate rapidly in improperly stored grain. If you are hauling high moisture corn directly to market, it is vital to deliver it absolutely as quickly as possible for these reasons. Do not store grain in trucks, combines, bins, or any non-aerated site for more than 4 to 6 hours. Swift and proper handling is also critical if you intend to dry your own corn. Corn should be dried to less than 15% moisture with 24 hours after harvest. High capacity, continuous flow driers are generally capable of rapidly drying corn to 15 percent moisture or less, but in-bin drying systems are generally not, and thus, present serious management challenges. Dried corn should also be aerated and cooled shortly thereafter as well. Fungal growth and associated aflatoxin development becomes dormant when grain moisture drops to about 12 percent, especially when grain temperatures decline to around 55 deg F, so this should be your goal for long-term storage through the fall and winter.