The historic pattern of U.S. winter wheat yields
4. The correlation discussed above is based on comparing deviations in pairs of adjacent years. It is also interesting to examine whether longer “runs” in the deviations occur. The distribution of runs of consecutive positive or negative yield deviations is summarized in Table 1. There were 6 single year runs with positive deviations and 5 single year runs of negative deviations. This simply means that a positive deviation was followed by a negative deviation and vice versa in these 11 years. The longest run of negative yield deviations was 8 years (1961-1968), but there were also runs of 5 years (1974-1978) and 4 years (2004-2007). Positive deviations had runs of 7 years (1979-1985) and 5 years (1969-1973 and 1997-2001). Since negative deviations and positive deviations occur with almost the same frequency, it is logical that the number and length of runs of negative and positive yield deviations would be similar. click image to zoom5. Given that the 2011 yield was below trend, of particular interest is the length of previous runs of negative deviations. There have been five previous instances of single year negative deviations from trend yield and five instances of 2 or more consecutive years of negative deviations from trend yield. Given that multiple-year runs of negative yield deviations have occurred relatively frequently in the past, we should expect them to happen again at some point in the future
We estimate the trend yield for U.S. winter wheat in 2012 to be 47.5 bushels per acre based on data from 1960-2011. For any particular year, including 2012, history suggests a 48 percent chance of an average U.S. winter wheat yield above trend and a 52 percent chance of an average yield below trend value. The odds slightly favor a winter wheat yield below trend in 2012. More specific expectations about the 2012 average yield will unfold as the crop comes out of dormancy and spring weather prospects emerge. The USDA will release the first yield forecast on May 10th.
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