If you estimate yield before harvest, how close can you get? To a large extent, Dave Nanda says, it depends on what factor you choose to represent kernel size in the yield formula.
“You can estimate ears per acre with some accuracy, and if you do checks in enough random spots, you should be close on number of rows of kernels per ear and number of kernels per row,” says Nanda, director of genetics for Seed Genetics Direct. “What is harder to determine is how kernel weight will impact final yield.”
The 2020 edition of the Purdue University Corn and Soybean Field Guide outlines the steps in the yield formula. It was originally developed by the Department of Agricultural Engineering at the University of Illinois decades ago. You can learn more about the formula by visiting kingcorn.org. Bob Nielsen, the Purdue Extension corn specialist, maintains the site and has experimented with the yield formula over the years.
To use the formula, first count the number of harvestable ears in 1/1,000 of an acre. If you’re in 30-inch rows, that’s in 17 feet, 4 inches of row length. Next, count the number of rows of kernels and kernels per row on every fifth ear in that 1/1,000 of an acre. Multiply ear number by rows of kernels by kernels per row and divide by a factor representing kernel weight.
Repeat the procedure at multiple locations, picked at random, to arrive at an average yield estimate.
Nielsen notes that when the formula was originally developed, the kernel factor was typically 90, representing 90,000 kernels per 56-pound bushel. Some of today’s hybrids produce bigger, deeper kernels, meaning that 85, 80 or even 75 might be a more representative factor.
However, Nielsen notes in the Purdue crop guide that depending on the year and conditions during grain fill, number of kernels per bushel can vary by as much as 20,000 for the same hybrid. You can find actual counts all the way from 65,000 to 100,000 kernels per bushel.
Nanda recommends staying conservative to represent kernel weight in the formula. That way you’re less likely to overestimate final yield.
Here are three examples using 30,000 ears per acre, 18 rows per ear and 38 kernels per row. The only difference is in the factor used for kernel weight.
1. Factor is 90. If you do the math, you wind up with 20,520 divided by 90, or 228 bushels per acre. That estimate is for corn at 15% moisture.
2. Factor is 100. Perhaps it turned dry late and kernels are smaller. Now the math becomes 20,520 divided by 100, or 205.2 bushels per acre.
3. Factor is 80. You might choose this number only if the hybrid is known to produce large kernels, plenty of moisture is present, and other conditions for grain fill are favorable. The math is 20,520 divided by 80, or 256.5 bushels per acre.
That’s a difference of 50 bushels per acre, just depending on which factor you use for kernel size and weight. Even if yields are at a lower level, the relationship will be similar.