Field Checkup

March 28, 2009 06:20 AM

A few years ago, you carried your management map in your head. Past experience, the sound of the combine as you harvested and a soil map told you everything you knew about which spots yielded well, which ones yielded poorly and why.

Modern technology, such as yield monitors and Normalized Difference Vegetation Index (NDVI) mapping, lets you give your field a checkup at any time during the season. NDVI mapping uses common color photography and infrared photography to show differences in vegetation across a field. It lets you spot problems almost as they occur, sometimes in time to correct them before harvest.

NDVI technology is so new that even consultants, such as Ken Ferrie and his son, Isaac, of Crop-Tech Consulting, are just now learning how effective it can be. "NDVI mapping is about where yield monitors were 10 years ago,” Ken says.

This past season proved that the Ferries' optimism was not misplaced.

To test the NDVI concept, Jack Pitts of McLean, Ill., had NDVI photos shot of two of his corn fields in mid-August. "The concept of a timely snapshot of the plants' status during the growing season, rather than after the fact with yield maps, seemed a wise use of our money,” he says. 

Sure enough, when Pitts and the Ferries reviewed the NDVI maps they saw "two rows of corn that were absolutely getting hammered,” Ken says. To find out the cause, Isaac downloaded the photo onto a handheld GPS unit and set out into the field.

"The ears in those two rows of corn were stunted and deformed,” Isaac says. "Soil nitrate tests showed those rows had run out of nitrogen. Eventually, we realized that one knife on the farmer's sidedress bar either got plugged or did not seal properly. He was applying anhydrous ammonia on 60" centers, so it affected two rows of corn.”

Yield loss confirmed. A hand yield check showed the two nitrogen-starved rows yielded 100 fewer bushels per acre than the other six rows in each eight-row planter pass. That averaged out to a 25-bu.-per-acre yield loss across the entire field. On top of that, the applicator continued to pump out just as much ammonia, so the nitrogen that should have gone on those two rows either was lost to the atmosphere or applied to the other six rows, which didn't need it.

The problem did not show up on Pitts' yield maps. "It was an eight-row applicator and an eight-row corn head,” Isaac says. "So every combine pass took six good rows and two bad ones, all the way across the field. The resolution on the yield map was not fine enough to reveal the problem.

"On a yield map, a pixel is 15', 20' or 30' wide (depending on header width) by one to three seconds of travel. In our NDVI aerial photo, a pixel was 3½'x3½', so the problem affecting only two rows showed up clearly.”

"Without the NDVI maps, we never would have known we had a problem,” Pitts says. "We had calibrated the applicator using the pressure gauge and were confident we were doing things correctly. Looking from the road at that tall corn, we would not have spotted a problem affecting only two rows. And, with the field averaging 229 bu. per acre, we wouldn't have dreamed we were losing any yield.”

Timely tiling. If a problem results from poor drainage, NDVI mapping can give a farmer a leg up on getting it corrected. "Many farmers use yield maps to see where they need to put more tile,” Isaac says. "But you can't study a yield map until after harvest season. That makes it difficult to get tiling done before the next growing season.

"But if you shoot NDVI photos in July, you can map the wet spots and get a tile contractor lined up and ready to go as soon as the crop is harvested. You can run the tiling machine literally right behind the combine.”

Drainage problems show up in soybeans, as well as corn. "In 2008, because the season was so late, some of our clients decided not to wait for harvest to begin tiling,” Isaac says. "They tiled through the standing soybeans, figuring the increased corn yield in 2009 would be worth the cost of some soybean yield in 2008.”

The benefits of in-season NDVI mapping also showed up in a nitrogen timing study the Ferries conducted. "We wanted to see how various hybrids reacted to three nitrogen application methods—applying all the nitrogen before planting, sidedressing it at the V6 stage and sidedressing at tasselling,” Isaac explains. "We shot NDVI images three times during the growing season. From the NDVI maps, we could see how every hybrid responded to the different nitrogen treatments.”

Here's another way NDVI images can pay off for growers: "In 2008, we had a lot of drowned-out corn in some parts of central Illinois,” Ken says. "In some other areas, we had green snap from windstorms. Some of our clients used NDVI maps to identify the areas and extent of damage.

"Then, they went to the field, took yield checks and determined how much yield they had lost. Without that information, they might have sold more corn than they produced, or they might have been afraid to sell and missed out on some good prices.”

Although yield monitors are still essential tools, NDVI maps can provide a substitute in situations where monitors are not practical. That includes crops, such as hay, for which there are no yield monitors and situations involving small fields and/or multiple combines running in the same field.

"Small patches usually generate poor yield maps because headlands and point rows make up a large percentage of the field,” Isaac says. "These areas create ‘noise' in the yield map. An NDVI map filters out that noise.”

When you run multiple combines in the same field, it's very difficult to get all the yield monitors calibrated the same. "One yield monitor almost always reads 2% or 3% higher than the other,” Isaac says.

"So your yield map shows streaks all the way across the field. In that situation, it's almost impossible to determine whether there's a problem or just a calibration issue. Unloading on the go can cause up to 10% difference in yield monitor readings because of the power requirement of the unloading auger. This causes streaking on your yield map. If the streaks appear in an NDVI map, you know they did not result from yield monitor errors.”

Another source of yield monitor error is tough-cutting soybeans. "As you harvest at night and beans get tougher to cut, the accuracy of the yield map can be impacted two ways,” Isaac says. "First, the tough cutting puts a power demand on the combine, and you can start blowing beans out the back of the machine. An NDVI map will filter out those effects.”

Considering that NDVI mapping costs as little as $2 per acre to $4 per acre, the potential returns are pretty good. The ideal situation is to combine a yield map, an NDVI map, a tile map and a soil map with fertility levels. After you have accumulated a few years of repeatable data—the more years, the better, including dry and wet seasons—you'll be able to designate management zones within fields, where yields tend to be higher or lower. From those, you can move into variable-rate application of nitrogen, phosphorus, potash, lime and seed.

"Don't designate a management zone based on one year's data,” Ken cautions. "If a problem occurs only one time, it may be nothing more than the wrong hybrid or too little fertilizer.”

You can e-mail Darrell Smith at

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