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Yield maps made simple

Gregg Carlson has been applying precision technologies to agriculture longer than anyone, perhaps.

“It started for me when I was in the Army Rangers in 1972,” says the South Dakota State University precision farming agronomist. “I first saw how GPS could locate our position, and I remember thinking, ‘How cool is this?’ But I remember also thinking, ‘It will be even cooler for agriculture, to be able to farm by precise location.’ ”

Carlson went on to become an agronomist, and his chief area of interest since has been application of precision techniques to improve farm performance and economics. It's been a long journey, and finally there are some payoffs.

“I call myself a ‘why's guy’, I always ask, ‘Why?’ Why does one part of a field consistently yield more? We have all this data, how can we turn it into usable information? There are lots of unknown returns to yield maps, and the known returns are small. But I think we can improve them,” Carlson says.

50×50 Grids

For the serious yield mapper, it's worth your time to manage the data you have accumulated, he continues. And it doesn't have to be complicated.

“I start with the two things I most want to know: What is the average yield in a field or zone in a field? What is the variation of that field over time?”

Carlson prefers to look at data in 50×50-foot grids, giving about 3,000 data points per 160-acre field per year.

“You can't really manage plots any smaller than 50×50, and that many data points is a workable number for even a slow computer,” Carlson says.

With the yield data in hand, Carlson puts fields into three categories:

1. The consistently high yield areas.

2. The consistently low yield areas.

3. The consistently inconsistent areas.

“Then I treat them differently,” he says. “And I always associate a specific year data with that year's precipitation data. The precipitation information often explains a lot of yield patterns year to year.”

1. Consistently high

These are your best fields and areas of fields, always at the high end of yields, and where you want to ensure that inputs do not limit yield potential. Give them fertilizer, seed population rates, and racehorse hybrids that will maximize yields.

“You don't want to limit the yield potential in these areas of the field,” Carlson says.

These areas are also the fields where you are mining plant nutrients the hardest, he says. “You're taking lots of nutrients off of them at harvest, and as a result, they are often your lowest soil test fields for phosphorus and potassium. Give them all they need; you want to push these areas. They're the areas where someday we're going to grow 300-bushel corn, 100-bushel soybeans, and 150-bushel wheat.”

2. Consistently low

When your yield maps show you these zones, it is a good time to ask why. “Is it drainage? Soil pH? Hard pan? Weeds? As an agronomist, you need to know what's going on in these areas, why they are consistently low yielding, and develop a plan to improve them,” says Carlson.

While the temptation might be to pour more inputs onto these consistently low fields, that would probably be a mistake. They are not being mined as hard for nutrients, and fertilizer applications at the same levels as your high-yield fields could be more than needed.

“Plant defensive hybrids that excel in tough environments. The racehorse would be an underperformer here, too,” Carlson says.

3. Consistently inconsistent

These are those really perplexing areas where you see a high deviation of yields from year to year. In the case of corn, Carlson uses 15 bushels an acre standard deviation as a guide point, looking for zones that have been, in some years, as much as 15 bushels both over and under the average.

“If it deviates less than 15 bushels, that's a pretty consistent part of the field, and you can manage it consistently,” he says.

“Asking why is really a tough one here,” he says. “It's often a drainage issue. In a drought year, these fields may give the best yields; in a wet year, they're too wet. But inconsistent yields could be the result of something else, like weeds, diseases, insects, or other factors that are there one year but not another. I like to look at a five-year composite map on these fields and see what it tells me about the inconsistencies.”

Once you've identified these three areas – high, low, and inconsistent – you can start to apply some precision farming techniques and vary planting population, hybrids, and fertilizer at least field by field, if not within fields.

“I believe it will pay you to apply phosphorus and potassium at variable rates as you move from high-yield to low-yield areas,” Carlson says.

The same goes for planting population. The ability to vary seeding rates over a field is here now with some planter systems and soon will be with others.

“For instance, at current corn economics and with seed at $240 a bag or more, we know the optimal population to get 200-bushel corn is 35,000 seeds per acre,” he says. “But if a realistic yield goal for another zone is 120 bushels, that optimal seeding rate is 25,000 per acre.”

Planting too much seed for the yield potential in a field or zone can give you a double whammy – you pay too much for the extra seed and you get lower yields, too, says Carlson. “We've seen where changing hybrids based on field conditions and yield potential can add 10 to 20 bushels an acre to yields, just getting the right hybrid on the right soils. Those are big gains at today's prices!”

Still much to learn

The irony of this is that precision farming probably does the least good for your best fields.

“If you've got consistently high-yield fields with little variability, you just give them everything they need, everywhere, and that's all you have to worry about.

“But those highly variable fields like we tend to have in our area, that's where the questions are the most difficult to answer, but where there is much to gain,” says Carlson. “We need more research; we're just beginning to understand yield variability and what to do about it. You can do some of it on your own farm to learn which fields need help the most and then manage to each field's or zone's yield potential.”

Carlson says this technology comes at an exciting time in agriculture. “For many years, we did all these things to improve yields, knowing that the more we grew, the lower the price was going to be. That's not true today, with this great market we have from ethanol production. If we grow more corn, we can just build more ethanol plants.”

Control Where You Need it Most

There are some areas of your farm expenses over which you have limited ability to vary inputs. Machinery is generally going to cost you the same amount per acre, whether that acre is high-yield potential or low-yield, corn or soybeans. You can't variably apply machinery.

Seed and fertilizer, on the other hand, are different stories. The technology is here to vary them between fields, even within fields, with variable-rate technology.

The good news in that is seed and fertilizer are your two biggest crop input items (excluding land and labor).

For a corn crop, here's an average breakdown of costs from South Dakota State University precision farming agronomist Gregg Carlson:

• Seed 32%

• Fertilizer 29%

• Machinery 16%

• Pesticides 9%

You can have variable control in the areas that cost you the most.

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