In a recent daily farmdoc article (May 9, 2022), corn planting date results from agronomic field trials in Illinois were compared to state-level estimates of late planting impacts. Field trials indicated that the yield penalty for planting after mid-May is non-linear and substantial, with reductions of up to 10-20%. In contrast, state-level regression estimates indicated that the impact of late planting is linear and rather modest, causing Illinois corn yield to deviate above or below trend. no more than two bushels in most years. Several explanations for the different results between the two types of studies were discussed in the May 9 article. One possibility not explored in this article was a negative correlation between spring and summer weather. If so, cool, wet springs would tend to be followed by a better than average summer. Therefore, the negative impact of late planting on corn yield would tend to be offset by good weather conditions during the 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 confusing results of different types of planting date studies.
We begin by briefly reviewing agronomic field trial results for corn planting dates in Illinois. These data have been presented previously daily farmdoc articles (April 14, 2022; May 9, 2022). Figure 1 shows results from 42 field test sites in central and northern Illinois over the period 2007-2021. Data from these experimental field trials reveal two key patterns. First, there is a relatively wide time window to plant corn in Illinois and expect “normal” yields. This window runs approximately from early April to early May. Second, the yield penalty is non-linear and increases sharply for plantings after mid-May, with reductions of up to 10-20%.
Next, we examine the relationship between a late planting measure and corn yield for the state of Illinois (daily farmdoc, May 9, 2022). Figure 2 shows the estimated relationship between the statewide Illinois corn yield trend deviation and the area of corn planted from the end of 1980 through 2021. The trend deviation is calculated based on a linear trend in the average yield of Illinois corn over this period. The figure shows that, as expected, there is an overall negative relationship between late planting and maize yield deviations from trend. Specifically, for a 10% increase in late plantings, the average corn yield in Illinois decreases by about 1.3 bushels per acre. However, the overall impact of late planting is linear and rather modest, causing Illinois corn yield to deviate above or below trend by no more than two bushels most years. years.
The previous results indicate that the estimated size of the impacts of late planting from the regression estimation using state-level data is linear and quite small compared to the nonlinear and large impacts of late planting based in agronomic field trials. As stated earlier, one possible explanation is a negative correlation between spring and summer weather conditions. If so, the negative impact of late planting on corn yield would tend to be offset by the good weather of the summer growing season. We examine the historical correlation of spring and summer weather in Illinois from 1980 to 2021. Figure 3 shows the correlation between total precipitation from April to May and total precipitation from June to August. Figure 4 shows the correlation between the average temperature from April to May and the average temperature from June to August. Figure 5 shows the correlation between the interaction of precipitation and temperature from April to May and the interaction of precipitation and temperature from June to August. The interaction is calculated as the product of precipitation and temperature for a given period. It is possible that an interaction variable like this better represents the impact of growing season weather on corn yield.
This is a 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 the correlation between all individual monthly observations of precipitation and temperature in Illinois from April through August. In other words, the rainfall correlation from April to June, the rainfall correlation from April to July, etc. Of 24 of these correlations, only one is statistically significant, no more than one would expect based on chance. It’s safe to say that spring weather in Illinois is completely unrelated to summer weather. This is consistent with previous work on correlating monthly weather data in Illinois and surrounding states for different times of the year (farmdoc daily newspaper, March 8, 2012; April 2, 2012).
The complete lack of a relationship between spring and summer means that this type of correlation cannot explain the confusing results of different types of planting date studies. This leaves us with the potential explanations offered in our previous daily farmdoc May 9 articleand:
- Planting trial results are reported as a percentage of maximum yield in a trial site for a given year, which is not the same as a deviation from trend yield. The peak test yield in a year can be well above or below the trend yield for the state or the entire United States, and the peak varies between test sites. This creates an inherent apples-and-oranges problem when comparing the results of the two types of studies.
- Planting trial results are site-specific in Illinois, while data used in regression studies are state averages. Average state observations may still be at too high an aggregation level to accurately reflect local level impacts.
- The explanatory power of planting date in field trial studies is not as high as commonly thought. For example, the R2 of the regression equation shown in Figure 1 is only 12.6%. This means that there is wide variation in yield results in field trials for a particular planting date, particularly the later the planting date. As an example, the lowest yield observation for maize in Figure 3 occurs on June 2n/a at 62.7 percent of maximum efficiency. Other test results for the same date give a maximum yield of 100%. The lesson is that there is a wide potential range of yield results at any given planting date, even in field trials.
- There may be a small sample problem with aggregate state data used in regression studies. As shown in Figure 2, there are only a handful of years with over 40 percent late planting. These are the years when the impacts of late planting would be expected to be large, mirroring the non-linear impacts seen for very late planting dates in field trial studies. Summer weather may have been better than average just by chance during this small handful of years. This was certainly the case in 2019, which was a record year for late plantings. If we assume that the field trial data are the best available, this means that in the future we should expect to see a group of years with high levels of late sowing and large negative deviations from to trend performance. At this point, aggregate data used in regression studies would show a nonlinear relationship similar to that found in field test studies.
The estimated size of late planting impacts from regression estimation using state-level data is linear and quite small compared to the nonlinear and large impacts of late planting based on agronomic trials on field. A possible explanation for this difference is a negative correlation between spring and summer weather conditions. If so, the negative impact of late planting on corn yield would tend to be offset by the good weather of the summer growing season. We examine the historical correlation of spring and summer weather in Illinois from 1980 to 2021 and find virtually no correlation. Therefore, the correlation between spring and summer weather conditions cannot explain the confusing results of different types of planting date studies. Other possible explanations exist but sorting them out will take time and additional data.
Irwin, S. “The Impact of Late Planting on Average State Corn Yield in Illinois.” daily farmdoc (12):66, Department of Agricultural and Consumer Economics, University of Illinois at Urbana-Champaign, 9 May 2022.
Irwin, S. “What do we know about planting dates and yield of corn and soybeans from agronomic field trials?” daily farmdoc (12):51, Department of Agricultural and Consumer Economics, University of Illinois at Urbana-Champaign, 2022 Apr 14.
Irwin, S. and D. Good. “Do warm winters tell us anything about summer temperatures and corn yields?” daily farmdoc (2):46, Department of Agricultural and Consumer Economics, University of Illinois at Urbana-Champaign, March 8, 2012.
Irwin, S. and D. Good. “Winter Rainfall and Maize Yield.” daily farmdoc (2):61, Department of Agricultural and Consumer Economics, University of Illinois at Urbana-Champaign, April 2, 2012.
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