Hi, I’m Jim Johnson, Soils and Crops Consultant with the Noble Foundation. Today, we’re going to calibrate this grain drill to plant some cover crop seed.Many times you can use the chart under the lid of a grain drill to get the right idea of where to set the drill to plant the right amount of seed.
A lot of times you need to use that as a starting point and calibrate from there to be sure.This cover crop seed, that’s what we’ll be doing,is using the chart as a starting point and adjust from there.
What we’ll do is we’ll jack this drill up.There’s a wheel – a drive wheel –every drill, no matter what make or model it is, is going to have a drive wheel on it somewhere: either on front or behind, maybe on the ends. And we’ll measure the distance around that drive wheel.
Then we can do some mathand determine how much of an acre that one row unit plants in 20 turns of that drive wheel. And do a little more math and calculate how much seed we need to catch to put out the 57 pounds per acre of seed that we have targeted for this field.
So on this particular drill, we’ve got a drive wheel at each end. This drive wheel runs the boxes on this side of the drill. The other drive wheel runs the boxes on the other side. And so, attached to this drive wheel is a sprocket and chain. The chain runs the shaft that turns the seed meter.
What we’re about to do is jack up this drive wheel, so that it turns freely. Once we have it where it turns freely, we can measure the distance around it so we know the circumference so we know how many feet in one revolution.
What we need is a tape measure and a jack – either a handyman jack or a bottle jack. So we’re just going to jack this up until this wheel turns freely.
[Sound of jack clicking]
Now that our wheel is free, we can measure the circumference.
[Rattling of tape measure]
So, I’m starting at one foot. I’m at eight feet and eight inches, but I need to subtract off a foot for my tail. So the circumference of this wheel is seven feet and eight inches. And that’s the number we’ll use when we do our math.
We’ll mark the wheel with a piece of tape so it’s easy to count revolutions.
[Sound of duct tape]
Next we need to measure our row spacing. Our width across two row units is one foot and three inches: 15 inches for two units. So we have a seven and a half inch row spacing. This is a cover crop mix, multiple species. Has small grains, legumes, some other plants: broadleafs, brassicas. And we’re going to plant this mix at 57 pounds per acre.
What we need is enough seed to cover the bottom of the drill. This is plenty of seed just to do our calibration. Since there’s no crop on the chart for “cover crop mixture,” we’ll just use the wheat line on the chart.
So here’s our wheat line and we’ll follow it across and we want about 60 pounds of seed. So we probably need to start at 14 on the index. So since we’re going to use our wheat setting to start our calibration, we will set this meter-lever handle at 14 on the index.
What we need to do is pull our seed tubes loose so that we can catch the seed that falls out in a cup so that we can weigh how much comes out in a cup. I usually do four random cups, somewhere that it’s easy to reach.
So I’m going to turn this wheel 20 times so that we can catch our seed in our cups for our calibration.
[Sound of machinery turning]
So we’ll do some quick math to determine how many grams of seed we need to catch to equal 57 pounds of seed per acre. For step one, we’ll take the circumference of the tire in feet multiplied by the row spacing in feet multiplied by the number of times we turned the wheel to get the square feet of one row unit of test area.
For step two, we’ll take the answer from step one – the square feet per one row unit of test area – multiplied by the pounds of seed desired per acre, divided by 43,560 square feet per acre and that equals the pounds of seed to catch per row of test area.
But that isn’t very convenient to use since it’s a decimal, so for step three we’ll take the answer from step two multiplied by 454 grams per pound to get grams of seed to catch per row of test area.
So let’s plug our numbers into these formulas. If you recall, the circumference of the wheel was seven feet and eight inches. So we know that seven feet eight inches is 7.666 repeating feet. And our distance between rows was seven and a half inches. And we know that seven and a half inches is .625 feet. So the area in our field when we turn the wheel 20 turns is 7.6 repeating feet times .625 feet, times twenty turns. This gives us an area of 95.8 square feet. This is the answer to step one.
For step two, take the answer to step one, 95.8 square feet,
and multiply by the pounds of seed desired per acre, which is 57 pounds per acre.
Then divide by 43,560, which is the number of square feet per acre.
This equals .125358 pounds of seed to catch per row in
our test area, and is the answer to step two.
For our final step, we’ll convert pounds to grams by multiplying the answer from step two by 454 grams per pound. Our final answer is that we need to catch 56.9 grams in our 95.8-foot test area to equal 57 pounds per acre. So we’ll zero our scale with an empty coffee cup. And now we can weigh the seed in each of our cups.
Knowing that we need 57 grams, this first one comes out 55 and a half. That’s certainly within reason. The next cup, 59. Looks very good so far. Our third cup is 50 — that’s just a little light. And our fourth cup is 47. So we may want to increase the rate by one notch on the seeder and catch one more time just to be sure.
Repeat the process of catching and weighing the seed until you are within two or three percent of your target weight. You’ll need to do this every time you switch to a new seed and that way you’ll feel confident that you’re putting out the correct amount of seed that you want to put out. And you’ll know that when you finish the field, you’ll also finish your seed rather than running out of seed before the field is done or running out of field before the seed is all gone.