Why Corn Yield Check Formula May Be Off This Year

CROP WATCH: Stress in grain fill skews the corn yield formula.

Published on: Sep 16, 2013

We reported a couple weeks ago that a yield check on the Crop Watch '13 field came up with 189 bushels per acre. Actually, if you factor in the ends, it's closer to 185 bushels per acre. Our checks were confined to the northern third of the field, so don't base your contest entry guess for the Crop Watch'13 field on our information by itself.

Those entries are due October 15. More than $3,000 in free seed from Seed Consultants, Inc., is on the line. You can email your entry to tbechman@farmprogress.com. Just include your name, address, phone and number of acres farmed, plus your average guess for the field.

The problem with the yield estimate formula that produced the 189 bushel per acre guess is that it is just that – an educated guess. Purdue University Extension corn specialist Bob Nielsen, helping author the Purdue Corn & Soybean Field Guide, makes it clear. The formula underestimates yield in a year with a good grain fill period late. But it over estimates yield in years with stress during grain fill.

Wait and see: You wont know how much damage the late stress did to yield until the combine enters the field.
Wait and see: You won't know how much damage the late stress did to yield until the combine enters the field.

There has been stress during grain fill – dry weather, heat and early symptoms of nitrogen deficiency. The N deficiency may be more tied to the roots not getting it than an actual shortage, but the effect is the same. The plant thinks it's short on N and adjusts accordingly.

One thing it does is cut back on the number of kernels by aborting kernels in the tips. It had already done that when we checked yield. Ears that would have had 40 or more kernels per row had 30 to 32. That's a big heat.

What's still out there are depth of kernel and how much starch is packed into each kernel. Test weight could suffer.

The formula assumes it takes 90,000 kernels for a bushel volume measurement of corn. If it takes out 9,900 kernels, you're off 10% already. That knocks 189 bpa to about 170 bpa.

If you tack on another 10% error, know you're down to barely over 150 bushels per acre. It's a far cry from last year's crop watch'13 field yield of 55 bushels per acre, but it's not 200 bushels per acre either.