Much of the country remains in extreme or exceptional drought and no one is sure how yields will turn out in 2013. Next year’s yield will depend on weather next spring and summer but this week we take a look back at previous year’s when yields fell well below trend to see what happened to yields the following year. The pattern may not be repeated this year, but the data shows that big rebounds in yields are clearly possible.
For this analysis we used corn yield data from 1960 through 2010. The first step was to estimate the trend yields over time. We broke the history down into decades and estimated the trends for the individual periods. Then we went back through and excluded the years when the actual yield deviated from the trend by more than the standard error of the calculation and re-estimated the trend.
With this information we could locate years when the actual yield was more than 5 percent below the trend for that year, and then determine how the actual yield the following year compared to trend.
Using this criteria there were 13 years – out of the total 51 – when the yield was at least 5 percent below the trend. Five percent below trend would probably not be considered a year of extreme, but it is a place to start the analysis.
The actual yield was above trend the year after the low yield 4 times, essentially on the trend 3 times and below trend 6 times. It is worth noting that there was an extended period of low yields in the 1970s with yields below trend in 1974, 1975, 1976, and 1977. Yields did increase in each year after 1974, but didn’t get back to the trend until 1978.
There have been 6 years since 1960 that the actual yield has been more than 10 percent below trend. The most recent year for this, prior to 2012 was in 1993 and that deviation was due to flooding and not due to drought.
So the most recent, and probably the most similar case we have is the drought of 1988. In 1988, the U.S. corn yield fell to 84.6 bushels per acre, while the trend was 115. This is a 26 percent deviation from trend, very similar to the deviation this year. In 1989, the national average corn yield bounced back to 116 bushels per acre, slightly above the calculated trend. However, yields came in below trend in earlier low yield years, such as 1984 following the 1983 drought and 1981 after the 1980 drought. However, the deviation below trend was only 2 percent in 1981 after the 1980 drought.
The only conclusion that seems clear from this data and analysis is that the low yields in 2012 do not necessarily indicate another low yield in 2013. However, below trend yields following low yield years are more frequent than above trend yields. It is useful to note that in 1989, the year following the drought most similar to what happened in 2012, the national average yield actually came in a little above trend.
The purpose of the analysis was to see how yields bounced back following years with poor crops. But the analysis also showed some interesting changes in trend yields. It is true that the calculated trend yield can change significantly depending on what time frame is used. For this analysis, we stuck close to the decade delineation for the trend calculation.
Using this approach, the data show a slowdown in trend yield growth from the 1960s to the 1970s to the 1980s. Then the pace of growth in trend yields turns up again in the 1990s, rising further in the 2000s. Based on the data from 2000 through 2010, the national average U.S. corn yield is increasing at a rate of about 2.2 bushels per acre per year, and the trend would put the yield for 2013 at about 165 bushels per acre. Adding in the data for 2011 would cause the trend yield for 2013 to be lower.