'Combine chasing' has compaction drawbacks

Unloading grain on-the-go is efficient, but it can create long-term soil compaction.

Dick Wolkowski has a pet peeve that bugged him each harvest during his career as a University of Wisconsin Extension soil scientist.

“It’s not even anything I lose sleep over at night,” Wolkowski says. Still, he notes some tossing and turning could result if the consequence of unloading grain on-the-go – or “chasing the combine,” in his words – is yield-slicing compaction. That’s especially true today, considering the heavy weights of modern tractors, grain carts,
and combines.

An increasing number of tractors, full combines, slurry tankers, and grain carts can weigh between 18 and 40 tons per axle, says Jodi DeJong-Hughes, a University of Minnesota (U of M) Extension educator. This equipment, whether equipped with tires or tracks, can compact the soil to
3-foot depths. 

“Axle arrangement, tracks, and dual axles can compensate (weight distribution) some, but that’s still a lot of weight going on a field,” DeJong-Hughes says. 

Time Efficiency vs. Compaction 

till, farmers do it for a good reason. Unloading on-the-go enables farmers to harvest more quickly during time-squelched autumns.

To weigh these two variables, Gregg Carlson, a South Dakota State University precision farming agronomist, compared unloading on-the-go vs. unloading in one field area. From a time standpoint, there’s no debate. Unloading in one spot, the field center, was only 80% as time efficient as unloading on-the-go in Carlson’s trial. He used these parameters.

  • Corn grown in 30-inch rows that’s harvested with a 12-row head at a speed of 3 mph in rows a  1/2 mile long. 
  • Corn yields of 150 bushels per acre unloaded in the grain cart at 3 bushels per second.
  • One pass covering 1.82 acres that is unloaded on-the-go or center of the field at a 273-bushel-per-pass rate. 

The parameters led to these results: 

Each on-the-go unloading pass took 10.5 minutes for an acre-per-hour rate of 10.4.  

Each field center unloading pass took 13 minutes for an acre-per-hour rate of 8.4.

Losing in the Long Run 

From this data,Wolkowski created a partial budget for harvesting corn on a 160-acre field. He calculated that unloading on-the-go had an edge of $594 per 160-acre field – $3,135 vs. $2,541 – over unloading in one spot. This was due to efficiencies gained in time, combine rental, and labor. 

Conversely, heavy weight from equipment can spur compaction, says Wolkowski, but yield losses from compaction are difficult to calculate because they hinge on many factors. In 2010, DeJong-Hughes and Minnesota crop consultant Frenchie Bellicot performed GPS analysis on seven pairs of ruts and neighboring nonrutted areas in four fields near Clarkfield, Minnesota. The duo found the rutted areas had a 17% yield decrease that was fairly consistent across the seven sites. 

Wolkowski reasons that if compaction in Carlson’s example squelched yields by only 1 bushel per acre priced at $3.80 per bushel across 160 acres, the resulting loss would be $608. This would eclipse the on-the-go unloading efficiency for the field of $594.  

“We want to do things faster, and we want to do them cheaper,” he says. “But any time we lose more than 1 bushel per acre due to compaction from unloading on-the-go, we can actually lose money over the long run.”

This can be particularly true for compaction spurred by large loads on wet soils, Wolkowski says.

“Think of the soil as a sponge that we compress as we apply force,” Wolkowski points out. It moves soil aggregates together, reduces porosity, and increases soil density. That can be done from the standpoint of wheel traffic, heavy loads, and all the different operations we might do on a field.”

What to Do? 

ranted, each farm differs, and weighing efficiencies gained by unloading on-the-go prompts many farmers to risk increased compaction. Still, it’s easier to nix compaction in the first place. Ways to do it include: 

  • Staying off wet fields. There are times when field operations need to go ahead, but waiting for weather to clear has long-term benefits, says Wolkowski.

“The times we’re in the field with heavy equipment like manure tanks, fertilizer
spreaders, large tillage equipment, loaded combines, or grain wagons that have the most potential for compaction if they’re on fields near field capacity for moisture,” says Wolkowski. “A good job of managing compaction can be destroyed by feeling the pressure to go out and do something.”

  • Controlling traffic. Many European farmers establish “wheelways” where they intentionally compact a small part of a field and stay off the rest with their implements, Wolkowski says.

“If you don’t manage traffic, you may be driving over 50% of the field in a year with all the different field operations,” he says. 

Controlled traffic can preserve this soil strength over most of the field via controlled traffic patterns, he says. Meanwhile, roots can better penetrate untrafficked soils in search of water and nutrients, he says.

  • Diagnosing compaction severity and its extent. Farmers may not have their own penetrometer, but they may work with a crop consultant who does and who can use this tool. This tool reveals where compacted layers lie. It works well to compare these areas with uncompacted ones, such as in a fence line. 

Subsoil Responsibly

To subsoil an inch or two below the compacted layer, Wolkowski recommends using an in-line subsoiler vs. a deep ripper. 

Still, Wolkowski says subsoiling is akin to taking a decongestant for a sniffly nose. 

“It treats the symptom,” he says. “It doesn’t cure the problem. At least leave some check strips just to see if whatever it cost you was worth it. Subsoiling is expensive. You need a large tractor, and you will also use a lot of fuel.”

Tracks vs. Tires

Tracks on tractors won’t worsen surface compaction compared with tires. Still, if tractor tires are properly inflated, tracks won’t give an edge either, says Jodi DeJong-Hughes, a University of Minnesota Extension educator.

Properly inflated tractor tires and tracks create a similar amount of surface compaction, she says.

Tracks exert a ground pressure of approximately 4 to 7 pounds per square inch (psi) depending on track width, length, and tractor weight, says DeJong-Hughes. Radial tires exert a pressure 1 to 2 pounds higher than their inflation pressure.

Thus, a radial tire inflated to 6 psi exerts a pressure of 7 to 8 psi on the soil, she says. Assuming that the loads and soil pressures are similar, both tracks and tires will exert similar stress onto the soil, she says.

Cover Crops

Pactices like cover crops can help alleviate compaction, says Jennifer Hahn, executive director of the Minnesota Soil Health Coalition. They have other perks, too, including:

  • Increased organic matter over time. 
  • Better competition that helps suppress weeds. 
  • Nitrogen fixing.
  • Increased water infiltration.
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