Your rake or merger may be your key tool for reducing yield losses from wheel traffic, says Ron Schuler.

“Combining two windrows of alfalfa with those lighter implements will cut the traffic made by heavier harvesting equipment by 50%,” says Schuler, a University of Wisconsin-Madison ag engineer.

Soil compaction caused by heavy equipment impedes root growth, decreasing plants' ability to take up nutrients and water. The impact on alfalfa yields varies with machinery size, soil type, moisture conditions and other factors, says Schuler.

Dan Undersander puts yield losses from wheel traffic in the 10-30% range.

“Those percentages include losses from compaction, damage to the plant crown and stem breakage,” says Undersander, University of Wisconsin extension forage agronomist.

Minimizing the number of field passes made with heavy equipment is one way to combat the problem. Schuler lists these additional steps:

  • Line up equipment wheels.

    Whenever possible, the tires on your tractor or self-propelled forage harvester should be lined up with the tires on the implement that it's pulling. Changing your tractor's wheel spacings can usually accomplish this.

    “This is so important because 70-80% of soil compaction occurs on the first pass of a tire,” says Schuler. “Subsequent passes in the same path cause less than 10% compaction.”

  • Don't work on wet soil.

    “Sometimes this becomes a trade-off and you have to make a decision: Do you want to take the cutting now and lose future yield? Or do you wait to harvest and suffer a yield or quality loss this year, but have fewer losses next year?”

  • Match the job and the tractor.

    “If you're raking or merging, don't use a bigger tractor than is necessary. Also, take off your front-end loader and any other unnecessary weight, which exacerbates compaction and uses extra fuel.”

  • Use a guidance system.

    “These systems permit the operator to follow the tracks from earlier cuttings,” says Schuler.

  • Use radial-ply tires. (See story below.)

    Schuler recommends regularly monitoring soils for evidence of serious compaction.

“One way of doing this is to look at the crop growth patterns in a field,” he says.

Soil cone penetrometers can evaluate compaction to 20". Those readings must be compared to areas where no compaction has occurred or when the moisture in the soil is at field capacity.

Driving a rod into the soil and observing the resistance as it moves downward is another method. The best way, though, is to grab a shovel and dig a trench to see how the crop roots are growing.

If you discover serious compaction, tillage is one solution.

“Prior to seeding, apply manure and then follow it with the appropriate tillage to loosen the soil,” says Undersander. “A chisel plow works very well, whereas a disk usually packs the soil vs. loosening it.”

Schuler recommends determining the depth of the compaction and then tilling 2-3" deeper.

“If the soil is compacted to a depth of 12", tillage should be done to a depth of 14-15" using a subsoiler with a 24-30" shank spacing,” he says.

Be Tire Smart

To cut soil compaction, put radial-ply tires on your heavy equipment. They have longer and wider footprints than comparable bias-ply tires, plus their recommended inflation pressures are lower. That larger footprint spreads out the weight over a bigger area.

“Inflation pressure is directly related to ground contact pressure,” says Len Wagner, Firestone's manager of global field engineering.

“If you have a high-pressure tire carrying the load, it's packing the ground. If you have a low-pressure tire carrying that same load, it's very ground friendly.”

Adds Ron Schuler, a University of Wisconsin-Madison ag engineer: “The amount of pressure in the tires is directly related to the load with radial-ply tires. With bias tires, you want the same pressure no matter what the load carries.”

Radials should have a bulge at the bottom when carrying a load, making for a larger contact area.

For an 18.4-34 tractor tire with an axle load of 5,500 lbs, Firestone recommends 6 psi of pressure for radials and 16 psi for bias-ply tires. With an 8,500-lb axle load, the recommended pressure jumps to 13 psi for radials, but stays at 16 psi for bias-ply tires.

Here are recommended radial tire pressures for various other axle load weights for the 18.4-34 tire: 6,000 lbs - 7 psi; 6,500 lbs - 8 psi; 7,000 lbs - 9 psi; 7,500 lbs - 10 psi; 8,000 lbs - 11 psi. The recommended psi of 16 for bias tires remains constant across the above load weights.