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Precision agriculture’s role in improving forage production

Tyler Lund Published on 30 May 2014

It is clear that precision agriculture is quickly becoming conventional agriculture within grain production. You can find GPS in tractors, sprayers and spreaders across the country. Farmers can now precisely apply expensive and crucial inputs at varying rates across their fields.

The same yield gains, reduced inputs and increased profitability can be realized for alfalfa, hay and silage growers.

The question that both grain and forage growers need to consider as they venture into variable-rate applications is: What information and what map will I use to guide these applications?

As you make decisions about how to guide your variable-rate fertilizer, lime, irrigation and other products, keep in mind these four principles to make the most of this powerful new management tool available to growers.

Use the right map
The starting point for creating a variable rate prescription for your field is making a map that tells the planter or spreader to automatically change rates as you drive across differing soils.

There are many ways to make maps and it can be tempting to use the least expensive map or the map that is easiest to access, but like most things, if it’s a job worth doing – it’s worth doing right.

The starting point for many growers is the USDA soil survey because it is free and easy to use. The maps created by surveyors in the middle of the last century are quite impressive for the tools at their disposal and lack of GPS.

The maps can be helpful in assessing land suitability and gaining a general understanding of the range of soils in a field. However, the intention was never to guide the precise applications of varying rates of expensive and important crop inputs.

The USDA website provides this cautionary note when downloading the maps: “Warning: Soil map may not be valid at this scale.

You have zoomed in beyond the scale at which the soil map for this area is intended to be used … The maps do not show the small areas of contrasting soils that could have been shown at a more detailed scale.”

USDA Soil Survey

In Figure 1, data collected by on-the-go organic matter and electrical conductivity (texture) measurements reveal that the soil survey map for this Illinois field drew lines in the wrong place, missed soil types and failed to distinguish between significantly different levels of organic matter.

Aerial images of your field are exciting and can be a useful scouting tool. This is getting more attention because of the buzz surrounding UAVs.

These images can be a helpful way to identify that a problem exists, but at the end of the day it is simply capturing a symptom. Just like a doctor doesn’t rely on what he can see on the surface, a grower needs to look below the ground to see what is causing an issue in a crop.

Measure as precisely as you manage
Another way of creating maps is taking lab samples from fields with different patterns throughout a field and compositing them to determine a single rate for a field.

The information gained from these samples will not allow differing rates within a field, so this method works well for growers who do not have equipment that automatically changes rates of seed and fertilizer.

The next step up is breaking the field into grids or cells and taking lab samples from each of these grids and applying different application rates to each grid.

The most common size of grid is 2.5 acres – about the size of a football field including end zones. The benefit of this method is that a farmer gains great information about his soil at those points. In between the points, however, it is just a guess.

This is like taking a single measurement at the 50-yard line of a football field and driving back and forth, changing rates at each 10-yard line all the way to the back of the end zone but still basing the rates on that single measurement.

Farmers intuitively know that their fields don’t look like a checkerboard, and they don’t want their variable applications to look that way either. They want the contours of the soil properties to be captured and apply inputs to those zones instead of artificial lines.

There is extreme pH variability within 2.5-acre grids. The Soil Science Society Journal research confirms that “Data points from large grids (greater than or equal to 2.5 acres) were too far apart to provide much information about the nature of pH or lime requirement change between adjacent sampling locations.”

Measure as it varies
Not only have researchers found this, but I have heard from growers as well who can actually see the biomass improve at the grid sample spots and the drop-off in between the next grid location.

The natural conclusion is to take more lab samples. Unfortunately, lab analysis at grid spacing smaller than 2.5 acres becomes unfeasible.

Because soil pH can vary from 5.4 to 8.0 over distances of about 480 feet, researchers have concluded that a grid spacing of 11 times the density of the commonly used 2.5-acre grid would be required.

That ends up being five lab samples per acre or about $70 per acre for the sampling and analysis, which would not be a cost-effective way to manage pH.

On-the-go pH sensing becomes the viable, precise option for improving soil pH because of the 10 to 15 samples it takes per acre.

lime spread map

Figure 2 shows the difference between a lime spread map created by grid samples and one created by an on-the-go soil sensor. The areas of green show where a grower saved money on lime because the sensor found areas of neutral pH missed by grids.

The areas of red are even more important. These are areas of the field the grid samples said didn’t need lime, but because of the dense data from the sensor, it was found lime was needed. This will result in improved yields that otherwise would continue suffering from low pH.

Measure what matters
With each 0.1 of pH increase, alfalfa yields can be increased 0.125 tons per acre, according to research by the University of Wisconsin. Not only does nutrient availability increase, but crop establishment is improved and disease control becomes easier.

But this only happens when the right rate of lime is applied. Over-applying can be costly, not only because of unneeded expense, but high-pH soils can hurt crop health just like low pH. This can be avoided by taking the right amount of samples.

Despite the significant impact on water-holding capacity and nutrient storage, soil organic matter and cation-exchange capacity are two items often overlooked on soil test results.

Because the other properties reported on a soil test can be changed by applying fertilizer and lime, they get more attention. Now that growers are installing variable-rate irrigation pivots and adjusting the rates of seed populations on the go, they’re looking to these soil properties to be their guide.

As we move towards site-specific management of our operations, it is important to look for sensing and sampling solutions that measure as precisely as we manage, measure at the density it changes within a field, and measure what matters.  FG

References omitted due to space but are available upon request. Click here to email an editor.

Tyler Lund

Tyler Lund
Veris Technologies, Inc.