Originally published in Purdue Pest Management & Crop Production Newsletter (16 August 1996)

Assessing Risk of Fall Frost Damage to Immature Corn

R.L. (Bob) Nielsen , Agronomy Department , Purdue University, West Lafayette, IN 47907-1150
Internet address: rnielsen@purdue.edu

Summer's nearly over, kids are heading back to school, college students are returning to Purdue like the swallows of Capistrano, and the Farm Progress Show is almost around the corner. Can a killing fall frost be far behind? Most of Indiana's corn growers are anxious about the effects of even a normally occurring frost, let alone that of an earlier than normal killing frost. Assessing that frost risk is rapidly becoming the topic of the hour around the coffee cups down at the Chat 'n Chew Cafe. Let me add my two-cents' worth.

First of all, let's understand that the delayed planting and cooler-than-hoped-for summer up to now have already done their dirty deeds to the corn crop. What is important from here on for any given corn field is 1) What is its stage of development? and 2) How many growing degree days (GDDs) are expected to accumulate from now until a killing frost? These two pieces of information (or guesstimates) can help determine the relative risk of a field to a killing fall frost.

Grain fill stages of development can be determined from the descriptions presented in an accompanying article in this week's P&C Newsletter. For any given grain fill stage, the approximate GDD accumulations required to reach kernel black layer (physiological maturity) are listed in Table 1.

Table 1. Approximate GDDs from specific grain fill stages to kernel black layer for corn hybrids that typically require 2700 GDDs from planting to black layer.
Grain fill stage Description GDDs to black layer
R1 Fresh silks 1300
R2 Blister 1040
R3 Milk (roasting ears) 900
R4 Dough 775
R4.5 Late dough/early dent 510
R5 Fully dented 250
R6 Black layer 0
Adapted from Table 3, NCH-40, Growing Season Characteristics and Requirements in the Corn Belt, Ralph E. Neild and James E. Newman, 1986.

So, let's say you've sampled ears from a field-at-risk and have determined the grain fill stage. The next question, based on the grain fill stage of that crop today, is whether you can expect to receive enough GDDs between now and a killing frost to mature the crop safely. One of the few ways to estimate the GDDs remaining in the season is to use historical GDD accumulations.

For example, I sampled some ears from a series of planting date plots at the Purdue Agronomy Research Center near West Lafayette on Tuesday, August 13. The grain fill stages are listed in Table 2. The median frost date (that date by which a killing fall frost will occur half of the time) for westcentral Indiana is about October 13.

Based on historical GDD accumulations and normal weather from now on, I expect this location to receive about 971 GDDs between August 13 (the day I sampled the ears) and October 13 (the median frost date). Given that estimate of GDD accumulation and the grain fill stages I determined for each planting date, I can speculate that plantings later than about June 1 in westcentral Indiana will likely not mature before a killing frost occurs during the second week of October.

Table 2. Grain fill stages of development and frost risk assessment for various planting dates at the Purdue Agronomy Research Center near West Lafayette, IN. August 13, 1996.
Planting date Grain fill stage on Aug 13 Will the grain mature before frost?
April 19 Dough Yes
May 2 Late milk/early dough Yes
May 14 Late milk/early dough Yes
May 17 Milk (roasting ear) Probably yes
May 20 Early milk Probably yes
May 31 Blister Maybe
June 5 Pre-blister Likely not
June 15 Fresh silks Likely not
June 21 15-Leaf stage Likely not

Working with 30-year GDD normals obtained from Ken Scheeringa (Indiana's acting state climatologist), I've created the following table that estimates 1) remaining GDD accumulations and 2) the youngest safe grain fill stage that should mature normally prior to a median frost date for each Crop Reporting District in the state. Since hardcopy readers of the P&C Newsletter will not likely receive this article before Monday, August 19, I've developed the estimates in Table 3 to reflect that date. Follow my earlier example to make frost risk assessments for fields of your own.

Table 3. Estimates of youngest safe grain fill stage relative to fall frost risk in Indiana. Estimates valid for crop development as of August 19, 1996 and median fall frost dates.
Indiana Crop
Reporting District
Median frost date
(50 % probability)
Estimated GDD
remaining from
Aug. 19 to fall frost
youngest safe
grain fill stage
Northwest October 6 731 Dough
Northcentral October 6 708 Dough
Northeast October 6 711 Dough
Westcentral October 13 847 Milk
Central October 13 822 Late milk
Eastcentral October 6 714 Dough
Southwest October 20 1005 Blister
Southcentral October 13 897 Milk
Southeast October 13 907 Milk

Bottom Line. On a statewide basis, the estimates listed in Table 3 don't offer much solace to those individuals whose corn fields are just now in the midst of silking, or perhaps have not even reached the critical pollination stage yet. Fields throughout the eastern third of Indiana, where the acreage of delayed planting was greatest, need to be at least in the milk to dough stage of grain fill development by August 19 in order to have a reasonable chance of maturing before a normally-occurring killing fall frost. An earlier than normal frost event would damage even more acres of immature corn.

Return to the the Chat 'n Chew Cafe.

The Corn Growers Guidebook , a WWW resource for corn management systems in Indiana and the eastern CornBelt.

Purdue University Agronomy Extension WWW Home Page.

Purdue Agronomy On-Line! , Purdue's Agronomy Department WWW Home Page.

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