Originally published in Purdue Pest Management & Crop Production Newsletter (30 August 1996)

Still on a Collision Course With Fall Frost....

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

As of August 25, the Indiana Agricultural Statistics Service estimated that only 55% of the state's corn acreage was at the dough stage of grain development or farther, meaning that 45% of the crop was younger than dough stage. Given our discussions the past couple of weeks, it becomes obvious that some of Indiana's late-developing corn crop continues its collision course with a fall frost. This article provides updated estimates of GDD remaining in the season and the associated youngest safe stage of grain development.

As with the previous articles, Table 1 lists the approximate GDD accumulations required to reach kernel black layer (physiological maturity) from any given grain fill stage (see descriptions of grain fill stages in P&C Newsletter, 8/16/96).

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.

My Own Example: Similar to previous weeks, I sampled some ears from a series of planting date plots at the Purdue Agronomy Research Center near West Lafayette on Monday, August 26. 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 708 GDDs between August 26 (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. This assessment is similar to the ones I've made the last two weeks, indicating that the later planting dates are not significantly 'catching up' due to the recent warm weather.

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 26, 1996.
Planting date Grain fill stage on Aug 26 Will the grain mature before frost?
April 19 Late dough/early dent Yes
May 2 Late dough/early dent Yes
May 14 Late dough/early dent Yes
May 17 Mid-dough (a few dents) Probably yes
May 20 Mid-dough (a few dents) Probably yes
May 31 Late milk/early dough Maybe
June 5 Late milk/early dough Maybe
June 15 Early milk Likely not
June 21 Blister 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, September 2, 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 September 2, 1996 and median fall frost dates.
Indiana Crop
Reporting District
Median frost date
(50 % probability)
Estimated GDD remaining
from Sep. 2 to fall frost
Approximate youngest safe
grain fill stage
Northwest October 6 475 Early dent
Northcentral October 6 460 Early dent
Northeast October 6 462 Early dent
Westcentral October 13 577 Late dough
Central October 13 559 Late dough
Eastcentral October 6 464 Early dent
Southwest October 20 710 Early dough
Southcentral October 13 615 Late dough
Southeast October 13 622 Late dough

Bottom Line. Fields throughout northern and eastcentral Indiana, where the acreage of delayed planting was greatest, need to be at least in the early dent stage of grain fill development by September 2 in order to have a reasonable chance of maturing before a normally-occurring killing fall frost. Late-planted fields in southeast Indiana should be at least to the late dough stage of development by September 2. An earlier than normal frost event would damage even more acres of immature corn. Cooler than normal temperatures from here on will further delay the grain maturation process and also increase the risk of a field for fall frost damage.

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.

End of Document