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Taking a stab at pre-harvest yield estimates

Agriculture.com Staff 07/06/2010 @ 5:22pm

Actual corn and soybean yield determinations can only be made at harvest using accurate area and grain weight measurements, but there are ways to make ballpark pre-harvest estimates.

We've entered the window when rain is busy making grain in the Corn Belt. Many factors in addition to precipitation go into making a corn or soybean crop, though. In areas with low rainfall, for example, stored soil moisture comes into play. In such areas good weed control and residue management can play an important role in keeping yield loss at bay.

Farmpro10, a member of Agriculture Online's Crop Scouting Talk group, recently asked if anyone knew the formula for estimating yields for soybeans and corn ahead of harvest. Several farmers offered their thoughts and methods in response. We tracked down University sources on the issue and found their favored methods for calculating yields for each crop.

With that said, it's probably a bit too soon to be making yield calculations for this year's corn and soybean crops. Still, following are some tried and true methods. You may want them in August.

Bob Nielsen, Purdue University Agronomist, says the Yield Component Method is the most commonly used method in the field. Sometimes referred to as the Slide Rule Yield Calculator, this method was developed by the University of Illinois.

The slide rule method can be used when kernel development has reached the late milk to early dough stages (R3 to R4). Under "normal" conditions, this point in kernel development occurs about 25 days after pollination is complete, Nielsen says. "Estimates made earlier in the kernel development period risk being overly optimistic if subsequent severe stresses cause unforeseen kernel abortion prior to about the roasting ear stage (R3 or milk)." For help determining grain fill stage, including color photos, read a Purdue publication he wrote: Grain Fill Stage in Corn.

Nielsen points out that the Yield Component Method offers a ballpark figure that should be used for planning purposes only, since kernel size and weight will vary depending on hybrid and environment. Yield will be overestimated in a year with poor grain fill conditions and underestimated in a year with excellent grain fill conditions.

You can tinker with the equation a bit, though. If you believe that kernel weight will be less due to stress during grain fill, you may elect to replace the value of "90" in the equation with "100" to reflect the potential for smaller and lighter kernels. In a good crop year, you may elect to replace the value of "90" in the equation with "80" to reflect the potential for larger and heavier kernels, Nielsen says.

Keep in mind that with any method of estimating yield, if you have a field that isn't uniform you should take more samples than you would for a uniform field, because crop uniformity greatly influences the accuracy, says Nielsen.

Actual corn and soybean yield determinations can only be made at harvest using accurate area and grain weight measurements, but there are ways to make ballpark pre-harvest estimates.

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