Crop-Sensing System Measures Crop Health

Like most farmers, Matt Helmke, who farms about 900 acres in partnership with his dad, Myron Helmke, near Clatonia, Nebraska, applies a portion of the nitrogen destined for corn production prior to planting. In his case, it amounts to about 60 pounds applied with the planter.

However, Helmke also believes that sidedress nitrogen provides an advantage in and of itself for improving yield, primarily because he is able to apply nutrients when the crop is actually determining its yield potential. Plus, it puts the nitrogen where it’s needed, when it’s needed, so plants aren’t as likely to run out of nitrogen before the end of the growing season.

On the other hand, he doesn’t believe in putting more nitrogen where it’s not needed or starving plants that do need it. That’s why he began using an OptRx crop-sensing system approximately five years ago to variable-rate the sidedress operation. Like Trimble’s GreenSeeker system and Topcon’s CropSpec system, OptRx uses optical sensors to measure and quantify crop health or vigor and to provide instant rate adjustment when coupled to variable-rate controllers.

“This is our fourth year with the system,” says Helmke, who is also a sales and service representative for Miller Sales near Clatonia. “We have three sensors on a Hagie sprayer with a 60-foot boom and auto guidance. Readings are then averaged for one rate across the full width of the boom, so it really doesn’t get down to the individual rows. From what I’ve seen, that is adequate in most situations and much more cost-efficient.”

Even though OptRx often picks up many of the same spots as the yield mapping system, Helmke believes in-season crop monitoring still provides some advantages.

“We still find some inconsistencies,” he says. “We could variable-rate a sidedress application based on yield maps from the past few years, but we still find some spots in the bottom that benefited from the OptRx application. It may be that it was too wet in that spot last year, and this year the moisture is just right, but some of the nitrogen has leached out. We just never know.

“Other times, the application is just the opposite of what we would expect,” he continues. “The system often puts more nitrogen on the poor soil and less on the better areas with the higher potential. It may be because those areas don’t hold nitrogen as well. Or it could be because the soybean crop was also poorer on those areas and didn’t produce as much nitrogen in the root zone.”

The system is also put to the test on terraced fields. The problem, he says, is that even though his granddad was progressive for his time in putting in terraces, the dirt movers generally scraped up topsoil from one part of the field and shaped it into terraces in another. Hence, crop health can vary from one pass to the next.

“Obviously, it’s not perfect, because we’re taking a 60-foot swath. But this just seems to be a little more resolute,” he notes.

Sensing success
About 100 miles to the northwest, Kurt Kroeger, who grows around 1,300 acres of corn near the Danish settlement of Dannebrog, Nebraska, has seen similar success with OptRx.

The difference is that most of Kroeger’s corn and soybean crops are grown under irrigation on ground that varies from Sandhill knobs to fertile bottom ground. He also applies sidedress nitrogen on corn with a tractor and cultivator at the V8 to V10 stage, in contrast to Helmke’s sprayer application.

“I put on about 34 pounds of nitrogen per acre with the planter. Then, I’ll generally make two passes with the irrigation pivot, applying another 30 pounds in each pass,” says Kroeger, noting that there are 17 pivots on the farm. “Finally, I’ll use the OptRx system connected to the Capstan liquid fertilizer controller when I run the sweeps down the rows, since I ridge-till all the corn. That application will range from 10 or 15 gallons to 65 gallons per acre.”

Like Helmke, he says OptRx typically assigns more nitrogen to the poorer areas, like the sandy hills, and less to the bottom ground. Kroeger says the system also compensates with more nitrogen on the fields that are still gravity irrigated, since they don’t get the two applications from a pivot.

“I’ve found that corn hybrids vary in their vegetative index or color,” Kroeger says, noting that he has done test plots with Pioneer to see how different hybrids respond to variable-rate nitrogen applications. “Vegetative index can mean different things with each hybrid, so I have to know my hybrid.”

That’s one reason sensor manufacturers recommend putting down a nitrogen-rich strip in each field or for each hybrid as a gauge in which nitrogen is not the limiting factor. That strip is then used to calibrate the vegetative index (known as NDVI) and to determine the benefit of additional N in each area of the field.

Other tips include not trying to use the system when there is dew or rain on the leaves, since the leaves reflect more light when they’re wet.

“It is also recommended to mount the sensors so they run about 3 feet above the crop,” he notes. “It’s hard to get a good reading at night, too, since the plants don’t produce chlorophyll without sunlight. Also, you need to keep the lenses clean.

“I’m totally sold on the system, though,” Kroeger adds. “I’ve been using it since 2010. That first year, I did about everything I could do wrong to make it fail, and it hasn’t done so yet. If you need proof, you can just look at one of my fields I’ve sidedressed using the OptRx. It makes the entire crop a lot more uniform, no matter what the soil type.”

According to Peter Scharf, professor of agronomy at the University of Missouri and MU Extension specialist, even more farmers could benefit from sidedress nitrogen applied with the aid of color sensor technology. In fact, he has been conducting research on the practice since 2004, when the industry was still in its infancy.

“We did 55 replicated field-scale, on-farm demonstrations where we compared sidedressed nitrogen applied with the aid of crop sensors with the rate that the producer chose,” explains Scharf. “On average, over the 55 trials, we made about 2 extra bushels per acre and saved 14 pounds of nitrogen per acre.”

Calculating ROI
The system doesn’t come without cost, however. Scharf says returns on investment depend on corn and nitrogen prices.

Equipment costs are $15,000 to $20,000 at current prices, and returns were $15 per acre in his study. Hence, it would take about 1,000 acres with similar results to pay for the sensors.

Although, some farmers may already have some of the needed equipment, such as the monitor or controllers.

“The key is to get nitrogen delivered to the plant before it makes its yield determination,” Helmke says. “Unfortunately, by the time plants show stress that’s visible to the human eye, that yield determination has been made. That’s where you see the true benefit of the OptRx system.”

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