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.