The current forecast for Indiana suggests that a moderate frost event could occur later this week across northern areas of the state. Low temperatures are forecasted to reach the low 30's. Considering that only about half of Indiana's corn crop is physiologically mature (Indiana Weekly Weather & Crops, Sep 18 ), what effects does an early frost have on immature corn? The following thoughts are paraphrased from an excellent publication on the topic, NCH-57, Handling Corn Damaged by Autumn Frost, available from your local Purdue Cooperative Extension Service office.
A frost incident that only damages the corn plants' leaves affects yield potential less than a true killing frost that obliterates the leaves, stalk, and husks. Considerable whole plant damage will occur when temperatures fall below 32 degrees F for 4 to 5 hours or below 28 degrees for even a few minutes. Less damaging frost can occur at temperatures greater than 32 degrees when conditions are optimum (clear skies, low humidity, no wind) for rapid radiational cooling of the leaves.
A killing frost that occurs before normal black layer formation will often cause premature black layer development , resulting in incomplete grain fill and lightweight, chaffy grain. Grain moisture will be greater than 35%, requiring substantial field drydown before harvest.
Yield losses from total plant death prior to kernel black layer are estimated to be 55, 41, and 12% for soft dough , full dent, and half-milk line stages of development, respectively.
Yield losses from death of leaves only (not stalk) prior to kernel black layer are estimated to be 35, 27, and 6% for soft dough, full dent , and half-milk line stages of development, respectively. Yield losses are less when only leaves are killed because the stalk remains capable of remobilizing carbohydrates from the stalk tissue to the developing ear for some time after the damage occurred.
Given that about half of Indiana's corn crop has is physiologically mature and about half has reached at least the dent stage of development (Indiana Weekly Weather & Crops, Sep 18 ), the yield losses due to a killing fall frost later this week (about Sep 22) would be no greater than 27 % to 41 % for individual fields.
Frosted grain will dry fairly normally, after an initial delay in moisture loss. Remember that even if a corn crop barely reaches black layer before a killing frost occurs, the grain moisture will still be 30 to 35%. Some field drydown will need to occur before the corn can be safely harvested. Drying rates in the field typically drop to 1/2 to 3/4 percentage points per day in early October, so field-drying grain from 35% to 25% could require 2 to 3 additional weeks.