Main Street Data reports elevated risk of corn yield loss from early frost, due to late planting of the 2019 corn crop where flooding and waterlogged soils prevented timely sowing. In these flooded areas it was not uncommon to see fields silking in late July and early August, which means maturity won’t be reached until mid- to late September. Moreover, except for a few hot days in July, temperatures have been relatively mild, and current forecasts indicate continued fair weather for much of the remaining grain-filling period. As a result, the rate of crop development is average or somewhat slower than normal, which adds to the risk of frost damage. Greatest risk will occur in much of Nebraska, South Dakota, Minnesota, and Iowa.

How weather impacts corn yield

In modern corn hybrids, grain forms during a 50-55 day period after silking. Highest yields are obtained when this grain-filling period is extended by fair weather with warm, bright days in the low- to mid-80s, and cool nights in the mid-60s. This temperature regime extends the period of grain filling, resulting in fully developed ears with larger than normal seed size and higher than expected yields. Hot weather shortens grain-fill duration and reduces seed size, causing small ears and below-average yields (I discussed this phenomenon in a recent Main Street Data post).

How to estimate corn yield loss from early frost

A killing frost occurs when night temperatures reach 32F for more than 4 hours. A temperature of 28F can kill a corn crop in less than 1 hour. When leaf tissues freeze, the cells responsible for photosynthesis rupture, which terminates capture of light energy that drives grain filling.

The hit to yields depends on the rate of crop growth during the grain-filling period, the proportion of leaf canopy killed, and the amount of grain-filling time lost. For example, a high yielding corn crop expected to yield 250 bu/ac accumulates over 6 bu/ac of grain yield per day during grain-filling. In such a crop, a killing frost one week before black-layer maturity1 would reduce yields by more than 40 bu/ac if the entire leaf canopy was killed. Grain quality is also diminished. A frost that results in partial canopy die-off, or a crop with lower expected yields would result in smaller yield losses as shown in the table below.

The next 30-45 days are critical for late-planted fields

Although current forecasts indicate fair weather over the next two weeks, grain-filling in many late-planted fields will continue for another 30-45 days, which means there is still time for a yield-reducing heat wave. So, let’s hope for warm, bright weather from now until maturity without excessive heat or frost. We deserve a break after the historic floods and excessive rainfall that disrupted planting and crop establishment in so much of the Corn Belt this year.

1 Use of websites sponsored by ag-related companies does not indicate endorsement of their products. Instead, the webpage was selected because it provided the best publicly available information on the topic.


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