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Making sense of sensors

Agriculture.com Staff 03/11/2008 @ 12:24pm

One of the newest techniques for managing nitrogen (N) is to use sensors to let the crop tell you whether it has enough N to meet its yield potential. These sensors use reflected light to compare corn in N-rich reference strips to the rest of the corn in the field when the corn is several feet tall.

The reference strip has more commercial N than the crop is expected to use, while the rest of the field has less commercial N than the crop is expected to need. Supplemental N is applied according to the sensor readings. If the sensors are mounted on an applicator, variable rates can be applied.

Ankeny, Iowa, farmers Sean Harmon and Bruce Johnson experimented with N sensors in 2006 and 2007. Harmon farms with his dad, David, and brother, Mike. Johnson farms with his uncle, Allen.

Both families have John Deere applicators they use for spraying their own fields and for custom work. They teamed up to buy GreenSeeker nitrogen sensors (made by NTech Industries), which they mounted on a Yetter toolbar with injection coulters for liquid N. In the fall of 2006, both operations laid the groundwork for comparing their customary N rate to rates determined by the GreenSeeker sensors.

Johnson typically puts 150 pounds of anhydrous ammonia (NH3) per acre on his best fields in the fall. He used the GreenSeeker system on three fields. He applied 200 pounds per acre to soybean stubble for the reference strip. Where he planned to use the GreenSeeker to determine the rates for additional N, he applied 70 pounds per acre in the fall.

On the field where he felt he got the most precise comparison, the GreenSeeker system called for rates that averaged out to an additional 80 pounds per acre, which took him right back up to his typical fall rate. However, that was a variable rate, so some parts of the field got more than 80 additional pounds; other areas got less. Theoretically, that should lead to more efficient N use.

Yields, however, didn't vary. "There was absolutely no difference," says Johnson. "We put the same amount of N on and got the same yield."

Johnson is impressed with the GreenSeeker technology. But based on what he saw last year, he doesn't think the economics are right for his operation because of the cost of the second trip and the wide price spread between NH3 and liquid N.

Johnson paid 26 cents per pound for the NH3 he put on in the fall of 2006. He paid 50 cents per pound for the liquid N he applied with the GreenSeeker system last summer. So, his N ended up costing about $19 more per acre. He values the trip at $5.50 per acre.

"We needed to see some benefit to justify the expense," he says. "And we just didn't find that benefit. But we learned a lot. We think maybe our old program is better than we thought it was."

One of the newest techniques for managing nitrogen (N) is to use sensors to let the crop tell you whether it has enough N to meet its yield potential. These sensors use reflected light to compare corn in N-rich reference strips to the rest of the corn in the field when the corn is several feet tall.

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