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Issue No. 21, Article 10/August 12, 2005

Crops in August: Moving Toward the Finish Line

Crop condition ratings continue to suffer, topsoil moisture is short over most of Illinois, and organized, general rainfall is nowhere in the forecast. Does this sound familiar? It's with some sense of relief we approach mid-August this year, with physiological maturity set to arrive in the next month for much of the corn crop and not far behind for soybean. We already know that crops in many fields will yield less than average, and much less than hoped, especially after a season that started out well. We'll know the toll of such weather soon enough.

If there's a slight positive in the corn crop development estimates, it's that kernel stages are not greatly different from what accumulated growing degree-days would predict. Under very serious drought, kernel development tends to accelerate as the crop runs out of water. The fact that this is not happening widely across the state indicates that the crop is still alive and still filling kernels, even if at slower than normal rates due to lack of water. This is not the case in the driest areas, where leaf loss is starting to increase and where the crop is not going to fill much more. On average, though, the percentage of kernels that have reached the dough and dent stages is about average. This partly reflects some growth delay because of dryness in June and July and so is not altogether positive. But these days we'll take even the smallest signs of hope.

There is great anticipation of the first official corn and soybean yield estimates, to be released later this week. These estimates are made using extensive data gathered from ear and kernel counts, with kernel number per acre estimated, and this number then divided by expected number of kernels per bushel. While kernel counts are never perfect, they're usually more accurate than the guess of final kernel size can be at this stage, under these conditions. The default kernel count is 90,000 per bushel. Under the decreased prospects for kernel filling rates this year, that will probably be optimistic. While it's an oversimplification, many fields with the lowest kernel counts are also those with the poorest prospects to fill the kernels that exist. If so, it might be appropriate to divide low kernel counts (less than 300 per ear, and less than 80 to 100 million per acre) by 100,000 or more kernels per bushel to produce yield estimates. In fields with intact canopies and high kernel counts (150 to 180 million per acre), the factor to divide by might be lowered to reflect better prospects for filling kernels. Dividing by less than 80,000 kernels per bushel is probably not realistic for even the best fields this year.

Soybean yield estimates are made the same way as corn yield estimates, but with even more uncertainty in early August. While final seed size is likely to be at least as variable as that in corn, seed number per acre is also not yet fixedÑpod loss and failure of seeds to fill continuing to threaten as conditions stay dry. About all we can do is scout, protect the canopy if it needs it, and hope for rain to bring back the photosynthetic activity that the soybean crop needs to fill its seeds. Unlike in 2003, we do see the yield challenges coming this year. That doesn't make it much easier, just less full of surprises.--Emerson Nafziger

Emerson Nafziger

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