Route 2: CSI: Corn Field

12:00AM Aug 27, 2008
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Darrell Smith, Farm Journal Conservation & Machinery Editor
You've calibrated your yield monitor, and you understand how to produce a good yield map. Now your accurate yield map will help you do two things.
First, it will help you notice and start correcting yield-sapping problems. Then, as you gather more sound data in the coming years, it will enable you to do variable-rate application—a goal that becomes more and more appealing as input prices rise.
The problem-spotting analysis, which you will perform after every harvest season, is sort of like a detective solving a crime—"CSI: Corn Field,” if you will. "A single season's yield map is a report card,” says Farm Journal Field Agronomist Ken Ferrie. While you need multiple years of data to make major management changes (more on that later), there are some problems you can quickly correct.
The key to finding those problems is gathering all your operators—fertilizer applicator, planter, sprayer, combine—when you analyze your yield map.
"The first thing to do is find problem area zones caused by man-made activities,” Ferrie says. "Man-made zones usually show up on the map as straight lines, and they do not repeat themselves from year to year.”
If a line on the map runs at an angle to the rows, the tillage operator may remember, "Oh, yes, that's where I stopped the day we got rained out.” If the line runs with the rows, the planter operator may remember that he ran out of starter. If he does, the mystery is solved.
Keep in mind that there can be many reasons for a change in yield to appear on your map. Understanding your equipment will help you explain some of those changes.
"A change in yield on the map may result from unloading on the go,” Ferrie says. "Even with a calibrated monitor, the yield reading can go down when you start the unloading auger. This kind of error may produce a repeatable pattern on the map. It tends to show up most when the combine is fully loaded and pulling hard.”
Off the straightaway. After you have identified man-made causes, look for areas with yield changes that don't correlate with straight lines.
"When you find an area, compare it with maps from other years and see if it showed up then,” Ferrie says. "Collect information from the operators. Did the combine operator recognize the area as lower yielding? What is his interpretation? Were there weeds? Were there no corn plants? If not, maybe the problem was with stand establishment. If so, what caused it? Was the soil compacted from working it wet?
"If the combine operator didn't notice anything, it suggests a less-visible cause—a gradual yield robber, such as disease or low fertility.”
If you discover the reduced yield is in a poorly drained area, the map, with its documentation of yield loss gives you ammunition to propose tiling to your lender or the landowner.
If you have multiple years of maps, look for areas that are repeatable from year to year. If you can't explain why yield dropped off, overlay a soil type map and a soil test map on top of the yield map, and see if the yield issue is related to either one. "If you can attribute the reduced yield to fertility or pH, you can start to fix it,” Ferrie says.
High soil test readings and low yield may suggest a drainage problem. If you don't harvest a normal yield, nutrients applied as fertilizer can build up.
"You may find poor drainage affects a portion of one soil type—maybe five acres out of 10,” Ferrie says. "If so, the five acres are a management zone.”
If you can't find the cause of low yield after overlaying a soil map, break the low-yielding portions into a separate yield zone. If the problem shows up only in soybeans, test for soybean cyst nematode. If it shows up only in corn, test for corn nematodes.
Created during the growing season, a Normalized Difference Vegetation Index (NDVI) map can help your detective work with yield maps. A NDVI map can be a tool for determining when a problem occurred.           Source: Crop-Tech Consulting
Make big changes slowly. If this is your first year of making accurate yield maps, be cautious about making major management changes based on your data. "Making major decisions from one year's yield data can come back to bite you,” Ferrie says.
"There are so many variables in yield—weather, planting date, soil type, pests, fertility and on and on—that you need years of good yield history to create management zones,” Ferrie explains. "It's the old story: junk in, junk out. If you have 15 years of junk yield history, you may make management decisions based on something that isn't really happening. I like to see at least two years of crop response, and I begin feeling comfortable with about six years of data.”
When you have enough years of yield maps, you can make management-zone maps, which let you make variable-rate application of fertilizer and lime to correct problems and avoid wasting inputs.
"With a management-zone map, it really helps to separate corn from soybeans,” Ferrie says. "Some software combines results from corn years and soybean years to make one map. But some problems that affect corn don't affect soybeans, and vice versa. This is especially true with soybean cyst nematodes and fertility, as well as insects and acid soil.
"If a client brings in one composite map generated from five years of yield maps, I ask him to go back and get the original yield maps,” Ferrie adds.
To create management zones, the two most essential maps are the ones you may be tempted not to collect—maps reflecting yield in dry years and wet years. "In those years, it's natural to feel frustrated,” Ferrie says. "You don't feel like spending time to calibrate the yield monitor. But you'll learn more from those two seasons than you will from 10 normal seasons.”
Although the soil type may be the same in two parts of the field, one area may yield better than the other in a dry year because of different topsoil depth, Ferrie points out. In a wet year, a yield map will reveal where water damage is most prevalent. That will tell you how to manage drainage and nitrogen applications (because of the risk of leaching and denitrification) in the future.
"It's valuable to record how hybrids perform in different situations, such as neutral and acid soils, or in wet soil,” Ferrie notes. "Often, seed companies no longer have this information. Or, by the time they get it, a hybrid is already being phased out.”
If you can't find the reason one area yields differently from another, keep good records of everything you know about it, including the hybrid that was planted there. Check the area several times a season to determine when yield is being lost. At the same time, take stand and ear counts and watch for uniformity. Eventually, you'll identify the cause of the problem.
"Sometimes the answer is something simple, such as a colony of 13-lined ground squirrels in a no-till field,” Ferrie notes. "You just have to find it by asking the right questions: Does the problem show up in wet or dry years? Does it affect corn, soybeans, wheat or all three?” (If it affects all three crops, it could be a sign of acid soil; if it only affects soybeans, it might indicate a high-pH soil.)

To create management zones, the most essential yield maps are the ones you may be tempted not to collect—ones showing yields from wet and dry years. Above, the maps from the same field show how variability can differ based on the growing season. Knowing that can help you determine the culprit so you can pinpoint the solution. 
Source: Crop-Tech Consulting

Yield spurts. Look not just for problem areas but also for areas that yield higher than the rest of the field. "If you have 40 acres of a certain soil type scattered through a 120-acre field, and five or 10 acres yield better than the rest, make the high-yielding area a management zone,” Ferrie says. "Try to figure out what's going on there, and if it's something that could be repeated on the rest of the soil type.
"With corn, a common reason for higher yielding areas is that the soil there has a greater ability to supply nitrogen, phosphorus, sulfur or all three. That means we might be able to reduce inputs in that area.”
To make your yield maps even more helpful, overlay them with a Normalized Difference Vegetation Index (NDVI) map. "A NDVI map, created during the growing season, can help you determine when a problem occurred,” Ferrie says.
"If symptoms show up on a NDVI map shot prior to pollination, you would suspect the yield loss may have resulted from early season stress, such as nitrogen deficiency or seedling blight. If something happened at an angle, such as an anhydrous ammonia knife plugging, it will show up on a NDVI map much more clearly than on a yield map,” Ferrie adds.
Once you identify a management zone, you can develop a plan of attack. If the issue is low fertility, you can correct it by using variable-rate application. If lower yield simply results from light soils with poor water-holding capacity, you can treat the area accordingly by changing the method, timing and placement of nutrients. You might want to split the nitrogen application and lower the plant population in that area.
The essential first step: Sit down with your staff at the end of the season and study your yield map/report card. Good things will begin to happen.
"It will make you feel like you're more in charge of events,” Ferrie says. "When your partners or employees participate in the evaluation, you'll see a change in their perspective because they'll understand that what they do makes a difference. They'll begin to make notes and report problems they notice in the field and spend more time calibrating yield monitors. It's a fun process to watch.”
Having more detectives on the scene solves the mystery quicker!

You can email Darrell Smith at [email protected].