This week's crop condition report for corn showed 69% of the U.S. crop rated as either good or excellent, the same as last week though this week 17% was in excellent shape vs. 15% last week; 52% in the good category vs. 54% last week with the very poor, poor and fair readings the same as the prior week at 2%, 7% and 22% respectively.
Using our usual ratings system (we weight the crop based on the percent in each category and assign that category a factor of 2 for very poor, 4 for poor, 6 for fair, 8 for good, and 10 for excellent and then sum the results), this week's rating is 750 vs. 744 last week.
That is the number we plug into the trend-line in the scatterplot on the accompanying graphic that plots U.S. crop conditions as of week 29 (this week) vs. the percent that final national corn yields deviated from the 30 year trend.
Using this 750 number times 0.001083 -0.079954 results in a prospective 2015 corn yield 1.3% above trend and if trend this year is 166 bushels per acre (bpa) this calculates to 168.1 bpa which is well above the 162-165 bpa range that has been bandied about.
The standard error is 0.044% to there is a 67% chance yields are 3.2% below trend (160.1 bpa) to 5.8% above trend (175.6 bpa).
The problem with a national rating is that crop conditions this year are so varied with the Eastern Corn Belt states of Illinois, Indiana and Ohio seeing some of their worst corn crop ratings as of week 29 in many years save for the disastrous 2012 drought.
On the other hand many northern and western producing states have very high crop ratings suggesting record yield potential.
Note that the USDA has been calculating national crop condition ratings since 1986 with the 1986-2014 week 29 average 732 with the 20 year average 726 and the 10 year average 714 so using this criteria on the whole 2015 crop conditions at least so far this year are above average.