Late-season corn development and frost probabilities
click image to zoom 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).
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.
click image to zoomFigure 2. Probability of temperatures less than 29⁰F for NW, WC & SW Iowa. The horizontal yellow line marks 50 percent probability. The vertical light purple lines point to the dates of the 50 percent probability of temperatures less than 29⁰F temeprtures for each of the three Crop Reporting Districts. Data from Mesonet. Freeze dates
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