Here's a primer on how the Pro Farmer Midwest Crop Tour arrives at its corn yield estimates and soybean pod counts.
It's the first day of the Pro Farmer Midwest Crop Tour. Yield estimates and pod counts are starting to pop up on Twitter. There's a distinct buzz about it, and I'm happy to be out walking through corn that's much taller than I am.
I'm notorious for tweeting field-by-field on crop tours. Some people think that's distorting -- after all, one field doesn't represent what a whole state looks like, or for that matter, what a whole county looks like. I fully agree. However, I like to tweet frequently because it allows me to convey details about each field that might explain why our yield calculation came in where it did or if our estimate doesn't take into account the pest, disease, development or other issues we can see.
So, please remember that each tweet comes from one field and represents that one spot. (You can follow me on the tour's western leg at https://twitter.com/… and crops editor Pam Smith on the east at https://twitter.com/…. For tweets from the whole tour, follow the hashtag #pftour13)
One of the other key things to understand about the crop tour is the methods we use to arrive at yield estimates on corn and our pod counts on soybeans. The tour doesn't issue yield estimates on soybeans because even in a typical growing season (if there is such a thing) late August and early September weather determine how pods fill and how big the beans become. Those variables make it nearly impossible for early tallies like the crop tour's to be accurate. Instead, it computes the number of pods in 3' by 3' square.
For corn, scouts walk into the field, past the end rows. The Pro Farmer editors recommend walking 35 paces past the end rows. They measure the row width and hook a 30' length of rope on a corn stalk. Next, scouts count the number of ears in two rows along the rope, and pull the 5th, 8th and 11th ears from one of the rows.
Then scouts find their way back to the car and husk the ears. They measure the length of the grain on each ear and count the number of kernel rows around. Often, scouts break an ear in half to look at kernel depth and the milk like, but this isn't an official part of the formula.
Now, to the math. Scouts average their ear counts. For instance, one 30' row has 51 ears and the other has 54. They use 52.5 ears in the formula.
Next, scouts average the grain length. If the 5th ear is 7 inches, the 8th ear is 6.5 inches and 11th is 6 inches, scouts will use 6.5 inches. Then they figure the average number of kernel rows, and for the purpose of this example, we'll say all ears have 16 rows of kernels.
They plug it into the formula:
Ear count (52.5) x grain length (6.5) x kernel rows (16) = 5460/row width (30') = 182 bushels per acre.
The soybean calculations, on their face, are simpler than corn. However, two people must work together to take corn yield estimates and on soybeans they take two separate counts and average them together.
Scouts get past the field's end rows and use a measuring tape to determine row width and define a 3' section of a row. They count all of the plants in the row and then pull three random plants from it.
Once they're back at the car, they count pods on each plant and figure an average pod-per-plant count. Then scouts plug their numbers into this formula (I inserted numbers for this example):
Number of plants in 3' (14) x Average number of pods per plant (35) = Pods in 3' (490) x 36 (makes it a square) = 17,640/row spacing (15) = 1176 pods in a 3' by 3' square.
I know it's hard to visualize what those pod counts really mean, especially when you're used to thinking about crops in terms of bushels per acre. One mathematical wizard I know once told me that if you divide the pod count by 27, you get something that approximates yield. In this case, 1176 pods work out to be about 43.5 bpa. I'm neither an agronomist nor mathematician, but if it helps you visualize the tour's findings, feel free to use this trick. I just wouldn't put too much stock in that yield estimate's accuracy.
Katie Micik can be reached at firstname.lastname@example.org