The timing of the first 28 degree F frost this fall will affect corn yield potential in Iowa, thanks to later-than-normal crop development. A later-than-normal frost would encourage a longer seed-fill period and higher yields, while an early frost would certainly not be the ideal situation.

Cool August temperatures across Iowa slowed growing degree day, or GDD, accumulations. In addition, Iowa's late corn planting dates this year obviously impacted the crop as well. These two factors affect corn yield potential.

FROST PROBABILITIES: Many Iowa corn and soybean fields were planted late this year due to a wet spring. The timing of the first 28 degree F frost this fall will affect corn yield potential. A later-than-normal frost is what Iowa needs this year, to allow longer kernel-fill period for corn and higher yields. An early frost would play havoc on late developing crops.

Iowa State University Extension corn agronomist Roger Elmore is keeping track of GDD accumulations. Growing degree day accumulations in Iowa lag behind normal. Cool temperatures after silking not only slow GDD accumulation, thus slowing crop development, but also can increase yield potential given specific conditions, he says. The record yields of 2009 resulted from slow GDD accumulation after silking -- coupled with a late frost. On the other hand, warm temperatures after silking in 2010 reduced corn yield potential (See an ICM News article reporting this).

Late-season corn development and first frost probabilities in Iowa

Earlier this season Elmore addressed the potential impact of late corn planting on yields; see Crop Model Output and Field Research Data. The August 12 USDA yield forecast in part reflects this; Iowa's USDA August forecast yield of 163 bushels per acre is almost 9% below 30-year trendline yields (10% below the 30-year trend is "officially" a drought).

Dry matter accumulation and grain moisture during reproductive stages

Let's address another question: Will Iowa's 2013 corn crop mature before frost? "My response to this question depends on when the first 28 degree F or colder frost occurs and the crop's current development stage," says Elmore.~~~PAGE_BREAK_HERE~~~

The table accompanying this article presents a timeline of corn development as well as kernel dry matter and moisture content during dent stage, which is R5. Physiological maturity (R6) is the point when maximum kernel dry matter occurs – normally around 35% grain moisture. Black layer formation occurs a bit later than R6, typically 28%, plus or minus 4%. Contrary to popular thinking, kernels do not lose dry matter after R6.

Get the full-size table at the ISU Extension website.

Based on the data in Table 1, corn in early dent (R5 stage of growth) has about 60% grain moisture, has accumulated about 45% of its dry matter, and needs another 33 days to mature. At three-quarter milk line, 97% of the dry matter is accumulated and it will take about two weeks to mature.

Freeze dates: Figure 1 shows the most recent 30-year dates for median first fall 28 degree F frost across the Midwest. The median date for portions of northwest and northeast Iowa ranges from October 1 to 10; that for southeast Iowa range from October 21 to 30. The median first fall 28 degree F frost for rest of the state ranges between October 11 and 20 (from MRCC).

Figure 1. Median fall 28°F freeze dates based on 1981-2010 averages. From MRCC. Full-size image

Mesonet provides tables of probabilities by specific locations for fall frost events with different temperature thresholds. These data are averages since 1951. Figure 2 and Figure 3 and Figure 4, accompanying this article display probabilities of temperatures less than 29°F for Iowa's nine crop reporting districts. For example: for Southwest Iowa (Figure 2) the average date of the first hard freeze is Oct. 21. In addition, one year in five the freeze may be later than Oct. 28, and one year in 10 it may be Nov. 4 or later. On the other hand, note that one year in 10 the hard freeze is on or before October 5.~~~PAGE_BREAK_HERE~~~

Figure 2. Probability of temperatures less than 29°F for Northwest, West Central & Southwest Iowa. The horizontal yellow line marks 50% probability. The vertical light purple lines point to the dates of the 50% probability of temperatures less than 29°F temperatures for each of the three Crop Reporting Districts. Data from Mesonet. Full-size image

Figure 3. Probability of temperatures less than 29°F for North Central, Central & South Central Iowa. The horizontal yellow line marks 50% probability. The vertical light purple lines point to the dates of the 50% probability of temperatures less than 29°F temperatures for each of the three Crop Reporting Districts. Data from Mesonet. Full-size image

Figure 4. Probability of temperatures less than 29°F for Northeast, East Central & Southeast Iowa. The horizontal yellow line marks 50% probability. The vertical light purple lines point to the dates of the 50% probability of temperatures less than 29°F temperatures for each of the three Crop Reporting Districts. Data from Mesonet. Full-size image

The date with 50% probability of less than 29°F temperatures ranges with northern crop reporting districts occurring earlier than the southern districts – a range of 9 to 12 days earlier in the north in the three central and eastern-most crop reporting districts. The northwest crop reporting district has later frost dates then the west central crop reporting district.

Fifty percent probability dates of temperatures below 29°F for the western parts of the central and southern crop reporting districts arrive six days earlier than the eastern parts of those regions. Those dates vary little across northern Iowa crop reporting districts, October 14 to 18.

Here's how Elmore sums up the situation: Warmer temperatures during the past week across Iowa and in the current short-term forecast for the rest of August have helped accumulate growing degree days faster. However, much of the state remains dry (see drought monitor). Warmer temperatures with dry conditions will stress the crop even more. The critical issue of this whole crop season is the timing of the first 28° F frost this fall. A later than normal frost encourages a longer seed-fill period and higher yields. What if we have an early? An early frost …Well, let's hope it doesn't happen!