Estimating Corn Yields

If you read our last blog (What Makes a Bushel?), then you already know that we are concerned about bushels and ultimately yield. We spend a good amount of time looking for economical and profitable ways to drive yield higher. The fun thing about this time of year is that we begin to figure out how we performed. The yield monitor in the combine is the true measure for us, but we do spend some time trying to estimate what we should expect in the coming months.

Four Variables of Corn Yield

Number of Plants - Population is the number of seeds planted per acre. We select and plant certain populations with incredible accuracy. Most of our corn is currently planted around 35,000 seeds per acre. Once in the ground, factors such as temperature, moisture, disease, insects, soil compaction, and seed quality impact the actual number of seeds that emerge and develop in to viable plants.

Girth of Ears - The number of kernels around an ear can range from about 14-20. This number is determined by hybrid and plant health early in the season. When the plant thinks it is going to have sufficient water and nutrients to support a bigger ear, then it produces one. If you didn't read our earlier blog, then you can learn more about pollination in It's Pollination Day!

Length of Ears - The number of kernels along the ear can range anywhere from 0-50. This number is also determined by hybrid and pollination, but the biggest factor here is weather after pollination. You can have a perfectly pollinated ear with the potential for 40 kernels long, but hot dry weather or lack of nutrition can cause the plant to begin aborting kernels from the tip back.

Seed Weight - The factor for kernels per bushel is difficult to calculate when the grain is not yet ripe or dry. In a normal year, 90,000 is a good number. But, that value could range from 60,000 to 120,000 depending on growing season conditions. Good moisture and cool conditions at the end of the plant's life allow it to add size, density, and weight to the kernels. Obviously heavier kernels means that it takes less to make a bushel.

As you can see, we are dealing with a lot of moving targets as we try to estimate yield. Emergence is important as this is when your stand or plant population is essentially set. Then pollination is crucial as this is when the potential number of kernels are established. And finally, late season weather is significant as it will determine the size and density of the seeds that will ultimately drive the weight of the sample.

If you would enjoy the technical nuts and bolts of these calculations, then please continue reading. If not, then you can check out our counterpart to this blog titled Estimating Soybean Yields.

Yield Check

To perform a yield check for corn we measure off 17 feet 5 inches, which is equal to 1/1000th of an acre in 30" row spacing. Then we count the total ears along this length while picking a few ears at random. After husking the ears, kernel counts are made by counting around and along the ear. Typically you will find somewhere between 14-20 kernels around and 30-45 kernels long. Then comes the math... multiply the girth, by the length, by the ear count, and divide by the number of kernels per bushel, and multiply by 1000. This math gives you a yield per acre.

Number of ears, girth, and length are pretty objective, but the number of kernels per bushel is rather subjective prior to harvest. This is the big question and probably the most challenging factor in a yield estimate. The size or depth of the kernel along with the density of the grain vary widely depending on nutrition, weather, and variety.

Below, you can see a couple of yield calculations. Pay close attention to the kernels per bushel as that is the only variable being changed. This simple example makes it quite obvious why yield estimates are far from an exact science!

EXAMPLE 1: