If you aren't using NDVI (Normalized Difference Vegetation Index) maps yet, you might want to take a look at this exciting new technology. Based on remotely sensed images, NDVI mapping is affordable for any size farm and can provide valuable information from an eye in the sky.
This summer, FARM JOURNAL welcomes back graduates of the 2008 event—to move to the next level—and invites other producers to experience our Corn College. Led by Field Agronomist Ken Ferrie, the 2009 event will focus on capturing every bushel possible from every acre for maximum profitability. Early bird registration (prior to June 1) is $350; regular registration is $425; and registration for multiple attendees from one farm is $300. To register, call (800) 909-3681. E-mail questions to CornCollege@farmjournal.com.
NDVI maps are based on near-infrared and red light reflectance. "They are an effective way to show changes in crop health," explains Isaac Ferrie, who worked with the technology in the Farm Journal Test Plots. "You don't have to understand everything about light reflectance to use these maps, any more than you have to understand everything about the nitrogen cycle to benefit from applying nitrogen. All you need to understand is that these maps are useful."
This past season, Isaac and his father, Farm Journal Field Agronomist Ken Ferrie, used NDVI maps for more than 9,000 acres of crops—in our plots and for Crop-Tech Consulting clients. The maps were based on photos shot from aircraft. They also experimented with NDVI maps made from other types of remote sensors. In either case, the maps were used to create management zones. The results were shared with the 2008 Corn College attendees.
"We use the maps not to tell what the field yielded, but to show us the variability within the field," Isaac says. "When we find a low-yielding zone, we try to determine what caused the low yield and see if there's anything we can do to raise it. NDVI maps tell us whether our soil-testing zones and yield zones are in the right spots or if they need to be modified.
You might call it minimizing the downside. "When pushing yield to the next level, it's not just about raising yield on good areas but also about minimizing yield loss on poorer areas," Isaac explains. "After you accumulate data from a number of seasons (including wet and dry years, which provide the most important information), you can figure out how to manage yield zones in wet or dry seasons."
NDVI maps are also useful for analyzing drainage. "Using the maps, you can figure out how well your tile is working and where you need to add more or move lines closer together," Isaac says.
A popular breakout session at the 2008 Farm Journal Corn College focused on using Normalized Difference Vegetation Index maps as an agronomic tool to help target scouting and direct in-field decisions.
In-season comparisons. NDVI maps can tell you when a problem occurred if you compare a map made early in the season with one made later, or if you compare an early map to a yield map made at harvesttime.
"A yield map shows spatial variability," Isaac explains. "An NDVI map shows variability over time."
An NDVI map won't reduce the time you spend scouting fields, Isaac points out, but it will make your time more productive.
"There's no substitute for scouting," he says. "But instead of wandering around, trying to cover the whole field, an NDVI map tells you where to look, where the trouble spots are and how big they are, so you can focus your energy and attention. And it makes sure you don't miss spots.
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"If you find a disease problem in a field, a map can save you from having to walk the entire field to find out how far the disease extends. If you look at multiple pictures, you can likely tell when a problem arose—and use that knowledge to try to avoid similar problems in the next crop year."
As Isaac sees it, NDVI mapping is the next step beyond yield maps. In terms of farmer acceptance, the tool is about where yield monitors were 10 or 15 years ago, he says—beginning to catch on.
"NDVI maps show much more detail than a yield map," Isaac says. "The resolution on a yield map is equal to the width of the combine header; but with aerial imagery, one pixel is about 3½'. The difference between a yield map and an NDVI map is like the difference between a very cheap digital camera and a very expensive one."
NDVI maps can be used with, or even in place of, yield monitors. "With multiple combines in the same field, calibrating the monitors within a few bushels of each other is pretty hard and you wind up with streaks through the field," Isaac says. "That makes it hard to see management zones. If you use custom harvesters, you may have a difficult time obtaining yield maps. With crops, such as hay, that have no yield monitors, an NDVI map can take the place of a yield map."
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Obtaining images. There are several ways to obtain NDVI images, and more are likely to become available soon. Each method has its own advantages and disadvantages.
The most common method is aircraft. "An airplane can fly 1,000 to 2,000 acres in a day, weather permitting," Isaac says. "Problems include weather—you need a sunny day to measure light reflectance. On a given day, you might have only two hours that are free from clouds.
"You can take pictures at any time; but the later you wait in the season, the more the NDVI map will correlate to a yield map. We try to take our photos a week before tasselling because we are interested in seeing wet spots and nitrogen deficiency."
However, tassels popping out will change the reflectance values—so a picture taken at tasselling time can show you how even the field is in maturity. "That tells us how evenly it will pollinate," Isaac says.
Full-sized aircraft usually require a minimum order of 1,000 acres, Isaac notes. So you might want to combine your order with a neighbor's.
Remote-controlled aircraft. A possible option in the future is remote-controlled aircraft. The plot crew tested a 7' aircraft carrying a camera controlled from the ground, which is not yet available to the public.
"On the plus side, there is no minimum acreage requirement," Isaac says. "On the negative side, you can't cover as many acres as a full-sized aircraft before you need to refuel or recharge batteries. Windy conditions can make the small aircraft hard to control. But this is something a soil-testing company could carry on their truck and use as needed. As the technology becomes smaller and cheaper, you may even see remote-controlled aircraft on farms."
Satellite imagery is available at various Internet sites. "You can buy photos by the square mile or the kilometer for only 25¢ to 75¢ per acre," Isaac says.
"A company called SatShot (www.satshot.com) lets you purchase photos by the field," Isaac continues. "They will give you the photos from the day they shoot the field and any other pictures of the field that they have from the past 20 years to compare crops from various years.
"A problem with satellite imagery is resolution—the pixel size is 30' to 60'. Also, the satellite is only over a given field every 10 to 21 days. If it's cloudy that day, you have to wait for the next flight to take more." Ask your provider if you will have to pay for photos taken on a cloudy day (which will only show the clouds).
Other remote sensors. Also in the future are sensors mounted on ground application equipment. The plot crew has tested sensors, such as GreenSeeker, made by NTech Industries (www.ntechindustries.com).
Mounted on applicators, the sensors ride over a crop row, detecting changes in leaf color as they go. "If a dealer mounted these sensors on a sprayer, he could map every acre as he
applies herbicides and then send his customer an NDVI map," Isaac says.
An advantage of such sensors is that you don't need as much crop canopy to get a reading because they are mounted directly over the row. So they can be used earlier in the season, allowing time to correct problems.
Farmers often fear that NDVI mapping is too expensive, especially for modest-sized farms. "But with NDVI mapping, there's no startup cost," Isaac says. "You don't have to buy any equipment—only the service, and the actual cost is around $2.50 to $4 per acre. You can choose fields to try it on and see if you like the results."
Compared with setting up for variable-rate farming, which requires new equipment, the cost of NDVI mapping is very small, Isaac adds. Yet, the NDVI maps lay the groundwork for variable-rate application.
You can e-mail Darrell Smith at firstname.lastname@example.org.