Fall Yield Estimates and Errors

In August, the USDA estimated the national average yields at 174.6 for corn and 50.0 for soybeans. While current yield estimates are low enough to keep ending stocks sufficiently tight, it's worth stepping back to consider how much variability - or error- there is from various yield estimates.

Figure 1 shows the error of various corn yield estimates throughout the growing season. As one might expect, yield estimates significantly improve throughout the summer and fall. By November, the estimates are even better and the errors are very small.

Perhaps overlooked is how much yield variability still lingers in the early Fall. For corn, historic errors have been nearly 10 bushels per acre (plus or minus) four out of the past 21 years. Most recently, the August 2020 yield estimate was 9.8 bushels above the final.

Figure 2 shows the data for soybeans. Overall, the story is similar in that 1) errors reduce throughout the autumn and 2) more error exists than many might expect. For soybeans, the data suggest the August estimates aren’t much more accurate than the trend estimate or the May WASDE estimate. In short, considerable soybean yield uncertainty remains.

Historically, August yields estimates have been off by 3 or more bushels seven out of the past 21 years or 1/3 of the time.

While not directly shown, soybean yield forecasts have more error at every forecasted point (when measured as a %). Specific for the August estimates, soybeans have an average error of 5.3%, or 2 bushels per acre, compared to 3.1% for corn.

Wrapping it Up

While the August data and estimates are insightful, helpful, and valuable, we have to remember these estimates have less certainty than one might initially think. Broadly speaking, there often appears to be more confidence or certainty in the yield estimates than the data would suggest we should have. This isn’t a criticism of the data or estimates, but a reminder for those of us using the data. In short, the August yield estimates are a long way from final, and in some cases, things could still change significantly. This is especially true for soybeans.

In short, don't be caught by surprise if corn and soybean yield estimates change significantly throughout the fall. While the growing season is largely over, much is still unknown about the final size of the crop.