Editor’s note: The following was written by Todd Hubbs and Scott Irwin, with the University of Illinois Department of Agricultural and Consumer Economics, for farmdoc Daily on May 13.


A perennial question in the corn market is how much does late planting hurt yield. This is just the flip-side of the question of how much does early planting help yield.

Based on recent weekly Crop Progress reports from the USDA, the 2020 U.S. corn crop is on track to be planted at an average pace. Sixty-seven percent of corn acreage in 18 major producing states was planted as of May 11, one percentage point higher that the 5-year average for this week.

By comparison, only 30% of the crop in 2019 was planted as of this date due to exceptionally wet conditions. The extreme lateness of planting the 2019 corn crop represents a valuable data point to help estimate the impact of late planting on corn yields.

The starting point for the analysis is defining the beginning date for a significant late-planting penalty for corn yields. There is not complete agreement on the optimum planting window for maximizing corn yields or the date when late planting begins to impose a substantial yield penalty. Both the optimum window and cutoff date for a significant late planting penalty also varies by geographic location.

For market analysis purposes, however, it is useful to identify one date for the end of the optimum window that can be applied to the entire Corn Belt. Acreage planted after that date would be considered to be planted late, and yield potential would be expected to be reduced as the percentage of the acreage planted late increases.

In order to set the cutoff date for late corn planting, we reviewed information generated from agronomic research relating planting date to corn yields at the farm-level. For example, field trial data provided by Professor Emerson Nafziger indicates yield loss in Illinois is minimized by planting in mid- to late-April. The yield penalties become increasingly large as planting is delayed after mid-May.

Based on the agronomic field trial results, we have previously defined the beginning date for substantial late planting penalties on corn yield in the U.S. to be May 30 from 1980 through 1985 and May 20 from 1986 onwards. The change in cutoff dates in the mid-1980s reflects recommendations for earlier planting that appeared around that time. We continue to opt for these cut-off dates in the present analysis. On average, late planting was 17.7% with the bulk of the observations between about 5% and 25%.

At 50%, 2019 was the highest level of late planting over the 40-year sample period. There were only two other years (1993 and 1995) when late planting exceeded 40%. There is no evidence of a trend up or down over time in the late planting percentage, which suggests that producer behavior in aggregate with respect to late corn planting has been quite stable.

The next step of the analysis is to estimate the impact of late planting on the U.S. average corn yield. The first method we use is to directly estimate the relationship between the trend deviation for U.S. corn yield and U.S. corn acreage planted late over 1980 through 2019. As expected, there is an overall negative relationship between late planting and corn yield deviations from trend. Specifically, for a 10% increase in late planting the U.S. average corn yield decreases by 2 bushels per acre.

The model implies that when late planting is zero the increase in corn yield above trend is 3.5 bushels. This is the maximum benefit of early planting on the U.S. average yield of corn according to this regression model. In comparison, the maximum loss in yield from high levels of late planting can be much larger. For example, the model predicts that the level of late planting in 2019 (50%) leads to a 6.3 bushel decline below trend for the U.S. average corn yield.

Because of this low explanatory power, the regression model in Figure 3 should be treated with a good deal of caution. This does not mean that late planting should be ignored when projecting corn yield, but rather, other factors, in particular summer weather, are typically more important in explaining deviations from trend yield.

Late planting was well above average in 2009 but the corn yield was a record at the time due to a cool, wet summer. Conversely, late planting in 2012 was at the low end of the sample range but corn yield was extremely low relative to trend due to the severe drought that summer.

In sum, despite the differences in the methods and data, the estimates of the impact of late planting on the U.S. average corn yield are quite similar, ranging between 1.7 to 2.2 bushels per acre for a 10 percent increase in late planting. A median estimate is therefore about 2 bushels per acre for a 10 percent increase in late planting.

The impact of late planting on projections of the U.S. average corn yield is always an important issue at this time of year. At the present time, it appears that U.S. corn acreage planted late (after May 20) will be average to below average. This could boost projections of the 2020 U.S. average corn yield by as much as 1 to 2 bushels per acre. Of course, the main show — summer weather — for determining corn yields is still on the horizon.