Does the Correlation Between Spring and Summer Weather Explain the Puzzling Results on Late Planting and Corn Yield? • farmdoc daily

In a recent farmdoc daily article (May 9, 2022), corn planting date results from agronomic field trials in Illinois were compared to state-level estimates of late planting impacts. The field trials indicated that the yield penalty for planting past mid-May is non-linear and substantial, with reductions as large as 10 to 20 percent. By contrast, the state-level regression estimates indicated the impact of late planting is linear and rather modest, causing the Illinois corn yield to deviate above or below trend by no more than two bushels in most years. Several explanations for the different types of studies discussed in the article from May 9th. One possibility not explored in that article was a negative correlation between spring and summer weather. If this is the case, cool, wet springs would tend to be followed by better than average summer weather. As a result, the negative impact of late planting on corn yields could be offset by good summer growing season. The purpose of this article is to investigate the correlation between spring and summer weather in Illinois to determine if this explains the puzzling results across different types of planting date studies.

Analysis

In Illinois, we start to have a variety of agricultural plants. This data was presented earlier farmdoc daily articles (April 14, 2022; May 9, 2022). Figure 1 presents the results from 42 central and northern Illinois field trial locations over 2007-2021. Data from these experimental field trials reveal two key patterns. First, there is a relatively wide window for planting corn in Illinois and expecting “normal” yields. This window runs from roughly early April though early May. Second, the yield penalty is non-linear and increases sharply for planting past mid-May, with reductions as large as 10 to 20 percent.

Next, we review the relationship between late planting and corn production for the state of Illinois (farmdoc daily, May 9, 2022). Figure 2 shows the estimated deviation relationship between the state and corn acreage planted late 1980 through 2021. The deviation trend is computed based on a linear trend for Illinois average corn yield over this period. The figure shows that, as expected, there is an overall negative relationship between late planting and corn yield deviations from trend. Specifically, the Illinois average corn yield decreases by about 1.3 bushels per acre. However, the overall effect of late planting is linear and rather modest, causing the Illinois corn yield to deviate above or below trend by no more than two bushels in most years.

The previous results indicate that the effects of regression estimation using state-level data are linear and fairly small when compared to the non-linear and large late planting effects based on agronomic field trials. As discussed earlier, a possible explanation is a negative correlation between spring and summer weather. If this is the case, the negative impact of late planting on corn yields would tend to offset the summer growing season. We examine the historical correlation of spring and summer weather in Illinois over 1980 through 2021. Figure 3 presents the correlation between total April-May precipitation and total June-August precipitation. Figure 4 presents the correlation between average April-May temperature and average June-August temperature. Figure 5 presents the correlation between the April-May precipitation and temperature interaction and the June-August precipitation and temperature interaction. The interaction is most computed as the product of precipitation and temperature for a given period. It is possible that the interaction variable like this represents a better growing season.

This is one case where the results are unanimous. There is virtually no evidence of any correlation between the variables shown in Figures 3-5. The R2 in each case is only about one percent. We also checked for all monthly observations for precipitation and temperature in Illinois for April through August. In other words, April-June precipitation correlation, April-July precipitation correlation, and so on. Out of 24 such correlations, only one is statistically significant, no more than one would expect based on random chance. It is safe to say that springtime weather in Illinois is completely uncorrelated with summer weather. This is consistent with the monthly correlation of weather data in Illinois and surrounding states for different periods in the year (farmdoc daily, March 8, 2012; April 2, 2012).

The complete lack of relationship between springtime and summer weather means that this type of correlation cannot be explained. This leaves us with the potential explanations offered in our previous farmdoc daily article of May 9th:

  1. The planting trial results are not the same as deviation from trend yield. The maximum trial yield in any year can be above the trend yield for the state or the entire US, and the maximum varies across trial locations. This creates an inherent apples and oranges problem when comparing results from both types of studies.
  2. The planting trial results are site-specific within Illinois, while the data used in regression studies are state averages. Average State Observations
  3. The explanatory power of planting date in field trials studies are not as high as commonly perceived. For example, the R2 of the regression equation shown in Figure 1 in only 12.6 percent. This means that there is a wide variation in the yield due to field trials, especially the later planting date. As an example, the lowest yield observation for corn in Figure 3 occurs on June 2nd at 62.7 percent of maximum yield. Other trial results for the same date result in the maximum of 100 percent. The lesson is that there is a wide range of potential outcomes on a given planting date even in field trials.
  4. There may be a small sample problem with state aggregate data used in regression studies. As shown in Figure 2, there is only a handful of years with late planting greater than 40 percent. These are the years where we would expect to have large scale planting effects, which are not significant. It may be the case that summer weather was better than average simply by chance in this small handful of years. This was certainly the case in 2019, which was a record year for late planting. If we assume that the field trial data is the best available, this means we can expect to see a cluster of years with high levels of late planting and large negative deviations below trend yield. At that point, the aggregate data used in regression studies would show a similar non-linear relationship as found in field trials.

Implications

The estimated size of late planting effects is small and small when compared to the non-linear and large-scale planting effects based on agronomic field trials. A possible correlation between spring and summer weather. If this is the case, the negative impact of late planting on corn yields would tend to offset the summer growing season. We examine the historical correlation of spring and summer weather in Illinois over 1980 through 2021 and find virtually no correlation. Due, correlation in spring and summer weather cannot be an explanation for the puzzling results across different types of planting date studies. Other possible explanations exist but they will take some time and additional data.

References

Irwin, S. “The Impact of Late Planting on the State Average Yield of Corn in Illinois.” farmdoc daily (12): 66, Department of Agricultural and Consumer Economics, University of Illinois at Urbana-Champaign, May 9, 2022.

Irwin, S. “What Do We Know About Planting Dates and Corn and Soybean Yield from Agronomic Field Trials?” farmdoc daily (12): 51, Department of Agricultural and Consumer Economics, University of Illinois at Urbana-Champaign, April 14, 2022.

Irwin, S. and D. Good. “Do Warm Winters Tell Us Anything about Summer Temperatures and Corn Yields?” farmdoc daily (2): 46, Department of Agricultural and Consumer Economics, University of Illinois at Urbana-Champaign, March 8, 2012.

Irwin, S. and D. Good. “Winter Precipitation and Corn Yield.” farmdoc daily (2): 61, Department of Agricultural and Consumer Economics, University of Illinois at Urbana-Champaign, April 2, 2012.

Nafziger, E. “Personal Communication.” April 11, 2022.

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