Kentucky Corn: Impact of July, Aug. Weather on Yields

Rainfall and temperatures influences corn yield throughout its lifecycle, but especially during the seed fill period. Dr. Bob Nielsen at Purdue recently wrote an excellent article on temperatures, rainfall and corn yields for Indiana.

He compared yield departures from the trendline to temperature and rainfall departures from normal. He found that wetter and cooler weather correlated with higher corn yields in Indiana. We used similar methods for Kentucky corn and made the following conclusions.

  1. The combination of rainfall and temperature variation explains about 72% of the yield variation in Kentucky
  2. Total rainfall does not equal distribution of rainfall.
  3. Monthly average temperatures do not account for ranges and timing of temperature.

Now, let’s walk through how we made those comparisons.

We began with Kentucky average corn yields from 1997 to 2017 (21 seasons) from USDA-NASS.  Corn yield increased roughly 2.4 bushels per acre per year over that time (Figure 1). Corn yield in 1997 was 8% below trendline while corn yield in 2017 was 11% above trendline. The yield departure from trend was mostly within about 20%. The exception was 2012, which was 54% below trend yield. We will discuss 2012 a little later.

 Figure 1. Kentucky Corn Yield, Resulting Trendline and Deviations from Trendline for Each Season

Figure 1. Kentucky Corn Yield, Resulting Trendline and Deviations from Trendline for Each Season. Click Image to Enlarge

Using the percentages of trendline yield allows us to understand better the rainfall and temperature effect on corn yield. Since trendline yields increase each year, the yield in 2017 should be 50.4 bushels per acre better than the yield from 1997 (21 years x 2.4 bushels per acre per year).

So, a 10-bushel yield difference in 1997 is a larger percentage change than a 10-bushel difference in 2017. Percentage of trendline yield better reflects the magnitude of yield gain or loss for a season.

Rainfall Effect

Wetter seasons typically resulted in higher corn yields (Figure 2). Drier weather in July and August usually resulted in lower corn yields. For the 13 seasons that had above normal rainfall, 10 had above normal yield. For 8 seasons below normal in rainfall, 7 were below normal in yield.

There were exceptions to this. In 2016, July and August were nearly 5 inches above normal but yield was right on trend. In 2012, the July-August rainfall was slightly above normal, but yields were 54% below trend! July was extremely dry early, yet rains the final week brought July totals to above normal.

The 2012 season is a clear example that the total rainfall does not account for the distribution of rainfall. Much of the corn in 2012 was damaged, severely, before those late rains in July occurred. The 2012 season was extremely hot, which we will discuss more in the temperature section.

Because 2012 was such an oddity, we did not use it to calculate the comparisons between yield and rainfall.

With 2012 removed, the r-square of the line that fits the comparison of yield and rainfall was 0.421 (Figure 2), suggesting that July and August precipitation accounted for about 42% of the yield deviation from trend.

 Figure 2. Corn Yield versus Precipitation

Figure 2. Corn Yield versus Precipitation. Click Image to Enlarge

Temperature Effect

July and August temperatures accounted for nearly 47% of the yield deviation when 2012 was excluded (Figure 3). Most years when temperatures are below normal, yields are above normal. Out of the 11 seasons where temperatures were below normal, yields were normal or above normal 9 times. For the 10 seasons above normal in temperature, 8 were below normal for yield.

The majority of seasons above normal in temperature had below normal yields. There are exceptions. In 2001, the two months were slightly above normal for temperature and 16.5% above normal for yield. July 2001 was below normal in temperature.

The heat occurred later in August. Rainfall was above normal, as well, in 2001. Another extreme was 2012 where temperatures were 4.30 degrees above normal in July and 0.6 degrees below normal in August. Just as monthly totals do not capture the timing of rainfall events, monthly averages do not capture the range or timing of temperatures.

Those factors are important, especially during pollination and seed set.

 Figure 3. Kentucky Corn Yield versus Temperature

Figure 3. Kentucky Corn Yield versus Temperature. Click Image to Enlarge

Rainfall and Temperature Combined Effect

Up to this point, we see that a positive rainfall departure helps corn yield and a negative temperature departure helps corn yields. These figures examine rainfall and temperature, separately. Now, we can combine temperatures and rainfall to measure the combined effect corn yield.

We can take the rainfall departure (where positive is beneficial) and subtract the temperature departure (where negative is beneficial) to get a combined departure index (Figure 3). When we remove the 2012 season, combination of rainfall and temperature departures accounts for nearly 72% of the yield deviation in Kentucky!

 Figure 4. Kentucky Corn Yield versus Rainfall-Temperature Index

Figure 4. Kentucky Corn Yield versus Rainfall-Temperature Index. Click Image to Enlarge

Relating this to 2018

According to the UK Ag Weather Center and NOAA, the July and August 2018 rainfall was 0.39 inches above normal and temperature was 1.16 degrees below normal. So, the weather from the past two months suggests that the 2018 Kentucky corn yield will be above trend.

The USDA-NASS places the September forecast for 2018 Kentucky corn at 177 bushels per acre, which is 8.5% above trend of 163.2 bushels per acre. The trendline in Figure 3 suggests a corn yield of 5.5% above trend or 172.2 bushels per acre. My guess is we will be closer to 168. That is simply a guess, for entertainment purposes, only. Feel free to criticize once it is wrong.

Thanks to Izabela Cardoso for collecting the data and to Bob Nielsen for the idea.

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