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Variable rate cuts costs, pushes plants toward once-over harvest

JACKSON, Tenn. – Making variable rate applications of cotton inputs based on the infrared reflectance of the crop can save producers a good chunk of money. But integrating three or four permanent productivity zones into the formula could save even more, according to Tim Sharp, precision agriculture professor, Jackson State Community College.

Reflectance data - which can be obtained through aerial images or through ground based sensors - indicates how vigor is distributed over the field. The higher the reflectance value, the more vigorous that part of the field is.

Productivity zones correlate to permanent low-, medium- and high-yielding areas within the field, according to Sharp. No matter what the season throws at you, the low zone is still the low zone, the medium zone is still the medium zone and the high zone is still the high zone. The only difference is the expression of the zone.

Each zone needs to be managed differently, according to Sharp, as if it’s a field by itself. For example, in a low productivity zone, “you follow the management practices you would follow that would delay maturity.”

Conversely, in the high zones, you would manage for earliness, according to Sharp. “It wants to be late. It wants to be too tall. So lower the nitrogen rate and increase your Pix application frequency, but don’t do high rates.”

Incorporating these productivity zones into a variable rate plan based on reflectance data is a good way to fine-tune applications, according to Sharp.

For example, Sharp noted that a 0.7 reflectance can trigger a plant growth regulator application in a high zone, while the same reflectance in a low zone would never trigger a PGR application because you’re managing the low zone for delayed maturity.

Getting the right shot

Establishing productivity zones in your cotton fields should be the first step to implementing a variable rate or VR application program, according to Sharp.

To create an image of your productivity zones, acquire infrared aerial imagery of the field or fields at between 650 heat units and 800 heat units after NAWF-5, he recommends.

At this time of the year, reflectance data from aerial imagery will most closely correlate to eventual productivity. In other words, reflectance data will likely pick up cutout in your low zones, the onset of cutout in medium zones and the lushness of high zones.

The next step to implementing a VR program on your farm is to invest in a yield monitor. “After harvest, I have a yield layer that will quantify yield and a image layer that will quantify vigor,” Sharp says.

Software programs such as SST Toolbox can convert the infrared image into three to four permanent productivity zones, and show the number of acres in each, which you can use to assess whether you can make VR pay.

The decision is simply whether or not you would expect to spend less with VR than you would with blanket applications, and if the savings offset the cost of the technology, according to Sharp.

The next step is to assess total fertilizer availability through grid sampling. The idea is that over time, the grower will draw down fertility levels to exactly what is needed to produce desired yields each year.

“From then on, I’ll keep up with my removal rates with my yield maps and make annual assessments on where fertility is based on that.”

Time to buy stuff

Now it’s time to go out and buy stuff. Fortunately, the cost of precision agriculture technology is coming down all the time.

“You may already own a Raven 440 or better on your sprayer,” said Sharp. “Then all you need to add is a GPS unit and an I-PAC, (which runs the flash card containing the VR mission plan). If I have to start from scratch, I may buy a Raven Viper (which holds a flash card) or an equivalent controller.”

Sharp estimates the costs of startup at around $15,000, “but at this time, you should already know if it’s going to cash flow, because the economics of the system is done on cost savings. Any yield increases are extra.”

During the winter, sit down with your consultant and set up the options for each field - the rates you intend to apply. “The pre-planned options should be organized in a file structure on the flash card,” Sharp said. “That way, when you receive instructions from your consultant on rates, you open up the card, open up the field, open up the folder, open up the mission, batch the tanks and spray.”

It may take some time to work these things out, so do it during the winter, not on the turnrow, Sharp says.

As the season progresses, your scout or consultant monitors the progress of the crop and continue to execute the VR plan.

Once over harvest

Sharp has developed a power index for monitoring the development of cotton plants, which can be used to enhance the opportunity for a once-over harvest for VR technology users.

The idea is that managing for earliness in the high zones and lateness in the low zones will push both zones toward the middle, or medium zone.

Sharp uses data from a Greenseeker (made by N-Tech) to take weekly snapshots of the crop. The Greenseeker’s sensors have their own light source, meaning reliable readings are possible whether it’s cloudy or sunny.

“What we want to do is manage that power index (the relationship between plant height, reflectance and node number) all season, hold it as tight together as we can. The end result is all the productivity zones mature at the same time and we have a once-over harvest.”


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