Cool August temperatures across Iowa slow growing degree day (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.

I addressed GDD accumulations in a recent CropWatch blog posting. Growing degree day accumulations clearly 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. 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).

Earlier this season I addressed the potential impact of late corn planting on yields; see Crop Model Output and Field Research Data. The August 12th USDA yield forecast in part reflects this; Iowa’s USDA August forecast yield of 163 bushels per acre is almost 9 percent below 30-year trend-line yields(10 percent below the 30-year trend is “officially” drought). 

Late-season corn development and frost probabilitiesDry matter accumulation and grain moisture during reproductive stages

Let’s address another question here: Will the corn crop mature before frost? My response to this question depends on when the first 28°F or colder frost occurs and the crop’s current development stage. Table 1 presents a timeline of corn development as well as kernel dry matter and moisture content during dent – R5. Physiological maturity (R6) is the point when maximum kernel dry matter occurs – normally around 35 percent grain moisture. Black layer formation occurs a bit later than R6, typically 28 percent ± 4 percent. Contrary to popular thinking, kernels do not lose dry matter after R6.

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

Late-season corn development and frost probabilitiesFreeze dates

Figure 1 shows the most recent 30-year dates for median first fall 28⁰F frost across the Midwest. The median date for portions of NW and NE Iowa ranges from October 1 to 10; that for SE IA range from October 21 to 30. The median first fall 28⁰F frost for rest of the state  ranges between October 11th and 20th (from MRCC).

Mesonet provides tables of probabilities by specific locations for fall frost events with different temperature thresholds. These data are averages since 1951. Figures 2, 3, and 4, display probabilities of temperatures less than 29⁰F for Iowa’s nine crop reporting districts. For example: for SW Iowa(fig 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 Oct. 5.

Late-season corn development and frost probabilitiesThe date with 50 percent probability of less than 29⁰F temperatures ranges with  northern CRD’s occurring earlier than the southern CRD’s – a range of 9 to 12 days earlier in the north in the three central and eastern-most CRDs. The northwest CRD has later frost dates then the west cenral CRD.

Fifty percent probability dates of temperatures below 29⁰F for the western parts of the central and southern CRD’s arrive six days earlier than the eastern parts of those regions. Those dates vary little across northern Iowa CRDs, October 14 to 18.

Late-season corn development and frost probabilitiesWarmer temperatures in the current short-term forecast may help accumulate GDDs 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 season is the timing of the first 28⁰ F frost this fall. A later than normal frost encourages longer seed-fill period and higher yields. An early frost … well let’s hope it doesn’t happen!