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Last week I shared my views of the risk of frost damage to immature corn throughout Indiana. This week's article is an update valid for August 26. I've repeated much of the text so that you don't have to dig out last week's newsletter from the trash barrel. If you want to cut to the chase, look at Table 3.
Review of Last Week: 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 last week's P&C Newsletter (8/16/96). 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.
My Own Example: As I did last week I sampled some ears from a series of planting date plots at the Purdue Agronomy Research Center near West Lafayette on Sunday, August 18. 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 867 GDDs between August 18 (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 one I made last week.
| 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 18, 1996. | ||
| Planting date | Grain fill stage on August 18 | Will the grain mature before frost? |
|---|---|---|
| April 19 | Late dough/early dent | Yes |
| May 2 | Late dough/early dent | Yes |
| May 14 | Dough | Yes |
| May 17 | Late milk/early dough | Probably yes |
| May 20 | Milk (roasting ear) | Probably yes |
| May 31 | Early milk | Maybe |
| June 5 | Early milk | Likely not |
| June 15 | Pre-blister | Likely not |
| June 21 | Fresh silks | 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 26, 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.
If you compare the values in Table 3 with those in last week's article, you will notice a dramatic reduction in expected GDD. This is to be expected as the summer begins to cool off on its way towards fall. Fewer GDD per day will accumulate as the days shorten and temperatures decline.
| Table 3. Estimates of youngest safe grain fill stage relative to fall frost risk in Indiana. Estimates valid for crop development as of August 26, 1996 and median fall frost dates. | |||
| Indiana Crop Reporting District |
Median frost date (50 % probability) |
Estimated GDD remaining from Aug. 26 to fall frost |
Approximate youngest safe grain fill stage |
|---|---|---|---|
| Northwest | October 6 | 599 | Late dough |
| Northcentral | October 6 | 580 | Late dough |
| Northeast | October 6 | 583 | Late dough |
| Westcentral | October 13 | 708 | Dough |
| Central | October 13 | 687 | Dough |
| Eastcentral | October 6 | 586 | Late dough |
| Southwest | October 20 | 854 | Milk |
| Southcentral | October 13 | 752 | Dough |
| Southeast | October 13 | 761 | Dough |
Bottom Line. Fields throughout the eastern third of Indiana, where the acreage of delayed planting was greatest, need to be at least in the dough stage of grain fill development by August 26 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. 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.
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