The 5 state numbers for soybeans show a similar pattern to corn below. Again Iowa sows the biggest changes so far. Again, I'm not ready to attribute this to greater fluctuations in soybean conditions just yet, it could also be the sensitivity of the parameters. Many analysts who use this data simply create a fixed weight index where Index=1*very poor + 2* poor + 3*fair + 4*good + 5*excellent and do this at a national level. I do each state individually and estimate the weights individually with the only constraint in estimation that they have to be non-decreasing with condition.
I also include a variable on trend and may include a variable on planting progress. It is the parameter on planting progress which got me into so much trouble last year as Illinois had the biggest planting delay in 4 decades but had reasonably cool and moist temperatures the rest of the year which lead to better yields than my simple model would have anticipated. The planting progress is basically an intercept shift. If you want to read more on Illinois crop yields from last year I'd suggest the folks at U of Illinois who do far more detailed models than I do here.
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