Wet harvest traffic could cost Midwest farmers 10% to 20% in yield

Wet-field wheel traffic could cost Midwest farmers 10% to 20% in yield, while 60% to 80% of soil compaction can occur during the first wheel pass.

The Iowa State University guidance, dated November 4, 2026, follows above-average precipitation in Iowa during July, August and September. Wet conditions weakened the soil’s load-bearing capacity as harvest and grain handling moved into the fall.

Delaying harvest can increase grain losses, reduce grain quality and add weather risk. Some wheeling on wet soils may nevertheless be unavoidable this fall.

Mehari Tekeste, an Iowa State University associate professor in Agricultural and Biosystems Engineering, and Arora, an ISU Extension and Outreach field agricultural engineer, wrote that "60% to 80% of soil compaction can occur during the first wheel pass." They also wrote: "The best way to manage severe compaction is to prevent it."

Modern harvesting and grain-handling systems move grain quickly, but their capacity places substantial weight on two-axle pneumatic-tire or track systems.

ISU measurements made under field conditions show harvest and grain-equipment loads exceeding 10 tons per axle, the recommended limit for minimizing deep soil compaction.

A 12-row combine with a 400-bushel grain tank can place about 22 tons on the front axle without the header and about 31 tons per axle with the header. A single-axle, 1,100-bushel grain cart is listed at about 26 tons per axle.

Those figures describe equipment configurations, not predicted yield losses for a particular field. Load alone does not determine damage; the degree of soil wetness is critical. Near field capacity or under saturated conditions, topsoil and subsoil have less capacity to support machinery without excessive deformation.

The 10% to 20% yield range comes from Iowa and Midwest research on random traffic and compaction in wet soils. It is a potential loss, not a harvested result from this fall. Actual effects vary with soil, weather, crop, traffic pattern and compaction severity, while deeper soil compaction can persist for years.

Compaction increases penetration resistance, restricts air and water movement, and creates layers that limit crop root growth. It also changes pore size and continuity, slows infiltration and restricts internal drainage.

Even modest visible rutting can conceal stress reaching subsoil horizons. Damage may remain for years and extend below the usual tillage operating depth. During drier seasons, restricted roots reach deeper nutrients and moisture less easily, extending the yield penalty into subsequent crops.

What should farmers do before harvest?

Start with a field check rather than a repair pass. In wet conditions, limiting where the heaviest equipment travels may do more than trying to alleviate compaction across the entire field afterward with deep tillage.

Tekeste and Arora recommend assessing moisture with the ribbon soil-molding method:

• Take soil from about 4 to 6 inches deep and try to mold it into a ribbon.
• Low compaction risk: The soil fragments before rolling into a ribbon about 1/8 inch thick.
• Too wet: The ribbon reaches roughly that thickness and begins to crack. The soil is near the plastic limit and susceptible to compaction.

If harvest must continue, select the appropriate tire size and set inflation within the tire manufacturer’s load and speed recommendations. Larger tires operated at appropriate lower pressures increase the tire footprint and reduce ground-contact pressure at the soil surface.

Low-ground-pressure track systems also spread the load over a large footprint. The goal is to carry the required load with the largest practical footprint within the soil’s allowable bearing capacity.

Plan where the combine, grain cart, tractor and grain-handling equipment will travel. Dedicated grain-cart lanes concentrate repeated passes, confining damage to a smaller area while leaving much of the field untouched.

Avoid carrying full loads through the wettest parts of the field when practical. Keep grain-handling and semitrailer activity near the field exit rather than bringing heavy traffic farther into the field. Minimize unnecessary passes.

Deep tillage is not a universal remedy. Disk-ripper equipment and subsoilers rupture compacted soil better when conditions are dry. Tillage on wet soil can smear soil along the tools, creating additional compaction while adding fuel, time and field passes.

When soil conditions allow for rupturing, setting the maximum tillage-tool depth to one inch below the hardpan could help break the compacted soil layer.

The ISU harvest checklist is direct:

• Check soil moisture before committing heavy equipment to the field.
• Perform the ribbon soil test.
• Plan where combines, grain carts, tractors and grain-handling equipment will travel.
• Match the ground-drive system, tire or track size, and inflation pressure to actual field loads.
• Concentrate the heaviest traffic in controlled lanes and minimize unnecessary passes.

Fall fieldwork will test how much wetness the soil can bear. Controlled lanes, appropriate ground pressure and fewer passes can keep this season’s traffic from depressing yields for years.