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Northern Illinois Agriculture

University of Illinois Extension
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Yield checks

Posted by Russel Higgins - Corn

Here we are, in the third full week of August in the 2012 growing season. Crops are beginning to show signs of maturity. In those areas of northern Illinois that missed recent rains the maturity is being hastened. Hit and miss thunderstorms have been rule rather than exception around the Northern Illinois Agronomy Research Center for the past month. We have accumulated enough rain to resuscitate our dormant grass in the plot alleyways; recent visitors heard the sound of mowers for the first time in weeks.

This past week I have spent considerable time in fields trying to get a better estimate on expected corn yields in our general area. Projected yields have ranged from 87 t0 172 bushels per acre. While in corn fields I have made several discoveries,

  • Giant ragweed is starting to shed its pollen, I exited one field covered in a yellowish hue
  • Dry weather is favorable for Garden/Banana spider survivability (They are everywhere in the corn fields)
  • There is incredible variability in the ears, not only between fields and varieties, but within fields
  • It is apparent that those fields with the highest water holding capacity (deep dark soils) have had a considerable yield advantage in the fields I visited

Most of the fields visited were at or near the R5 of dent stage. Any additional stress the corn plant experiences at this stage can reduce kernel weight, but will not reduce kernel numbers.

Are yield checks accurate predictors of final yield? The closer to harvest and the greater number of samples included will increase the confidence of the yield estimate. If for no other reason, yield checks get you into your corn fields where you can also check fields and varieties for standability and the possible presence of ear or stalk diseases. This could be instrumental as you schedule you plan your harvest schedule. How is a yield check performed? There are several variations used, but I'll share the procedure I use.

The first step is to determine your harvest population. This is done by measuring 1/1000 of an acre (17 ft. 4 in. if you planted 30 inch rows) and counting the ears harvestable by the combine in that distance. The next step is determining the number of kernels per ear, this accomplished by selecting random ears, one suggestion is every fifth ear (the greater the number of sample ears the greater likelihood of an accurate kernel count). Multiply the number of rows of kernels on an ear by the number of kernels in a row. I generally ignore the kernels at the butt and tip end of the ear (I would rather under than over estimate yield). Average the number of kernels on your collected ears. Now were ready for math

(Average number of kernels per ear X number of harvestable ears in 1/1000 acre) / 90

The number 90 (thousand) represents the approximate number of kernels in a bushel of corn. This number can be changed to compensate for poor or exceptional conditions during grain fill which will result in smaller or larger kernels. For most in northern Illinois in 2012 I would advise using a larger number.

Kernel numbers per bu. relative to growing conditions during the grain fill period.

Growing conditions - Range in kernel number per bu. (thousands)

Excellent           75 to 80

Average            85 to 90

Poor                 95 to 105

Let's do a quick example using data collected from this morning's field visit. The corn was planted in 30 inch rows.

Harvest population (ears counted in 1/1000 of an acre, 17' 4") –  29 (thousand)

Average kernels per ear - 418

(Average number of kernels per ear X number of harvestable ears in 1/1000 acre) / 90

(418 x29)/90 = 134 bushels per acre

This number actually exceeded my expectations for the field and upon closer inspection I determined the kernel size was smaller than an average year. I am going to retry the formula estimating 100,000 seeds per bushel.

(418 x29)/100 = 121 bushels per acre

You are now armed with the skills, go forth and estimate your corn yields!

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