Corn drydown signals when to harvest

COLUMBUS – Many agronomists recommend to begin harvesting corn for dry grain storage at about 24 to 25 percent grain moisture.
Allowing corn to field dry below 20 percent risks yield losses from stalk lodging, ear rots, and insect feeding damage, according to Ohio State Extension corn specialist Peter Thomison.
Localized root lodging caused by heavy rain and strong winds earlier this year may also slow harvest and contribute to yield losses.
Losses add up. The loss of one “normal” sized ear per 100 feet of row translates into a loss of more than one bushel/acre.
In fact, an average harvest loss of 2 kernels per square foot is about 1 bu/acre.
According to an Ohio State ag engineering study, most harvest losses occur at the gathering unit, with 80 percent of the machine loss caused by corn never getting into the combine.
Drydown rates. Thomison said corn will normally dry approximately 3/4 to 1 percent per day during favorable drying weather (sunny and breezy) during the early warmer part of the harvest season from mid-September through late September.
By early to mid-October, drydown rates will usually drop to 1/2 to 3/4 percent per day.
By late October to early November, field dry down rates will usually drop to 1/4 to 1/2 percent per day and by mid-November, probably 0 to 1/4 percent per day. By late November, drying rates will be negligible.
Consider GDD. Estimating dry down rates can also be considered in terms of Growing Degree Days (GDDs), Thomison said.
Generally, it takes 30 GDDs to lower grain moisture each point from 30 percent down to 25 percent. Drying from 25 percent to 20 percent requires about 45 GDDs per point of moisture.
In September most of Ohio averages about 10 to 15 GDDs per day, Thomison said.
In October (as things cool down) the rate drops to 5 to 10 GDDs per day.
Some hybrids, however, vary from this pattern of drydown.
Weather impact. Past research evaluating corn drydown provides insight on effects of weather conditions on grain drying, Thomison said.
During a warm, dry fall, grain moisture loss per day ranged from 0.76 to 0.92 percent. During a cool, wet fall, grain moisture loss per day ranged from 0.32 to 0.35 percent.
Grain moisture losses based on GDDs ranged from 24 to 29 GDDs per percentage point of moisture (i.e., a loss of one percentage point of grain moisture per 24 to 29 GDD) under warm dry fall conditions, whereas under cool wet fall conditions, moisture loss ranged from 20 to 22 GDD per percentage point of moisture.
The number of GDDs associated with grain moisture loss was lower under cool, wet conditions than under warm, dry conditions.