Can the harvest traffic affect your alfalfa field?

Beatriz Bizzuti, UMN Assistant Professor and Forage Extension Agronomist; Marta Kohmann, UW Assistant Professor and Extension Specialist; Brian Luck, UW Associate Professor and Extension Specialist

This article originally appeared in the Midwest Forage Association’s Forage Focus newsletter, August 2025 edition.

During harvest season, alfalfa fields experience frequent traffic from multiple pieces of equipment. Typically, four types of machinery (mowers, mergers, choppers, and trucks) enter the field, each contributing to soil disturbance and plant stress. Repeated passes of equipment across fields can significantly impact forage production, reducing both productivity and long-term stand health. Gaining a better understanding of how and why traffic affects alfalfa performance can lead to more effective management practices and improved productivity and persistence over time.

More harvesting, more traffic

Most alfalfa fields in the Midwest are harvested three to five times annually, depending on whether the field is newly seeded or established. Each harvest involves several field operations (mowing, tedding, merging, and either chopping or baling), followed by transport. A recent study from University of Wisconsin-Madison reported that around 50% of an alfalfa field is subject to traffic at least once during a growing season, with many areas experiencing multiple machinery passes.

How machinery traffic reduces productivity

Currently, researchers at the University of Wisconsin–Madison are investigating the effects of machinery traffic on soil and plant responses. We conducted a 6-month-long greenhouse study where we simulated traffic impact on soil only, plant only, and both soil and plant simultaneously. We also included a no traffic treatment for comparison. Traffic simulation was performed using a pneumatic piston that applies a pressure of 200 psi over 1 second to simulate the pressure exerted by a typical piece of equipment used in alfalfa production running at normal speed (e.g., a chopper). Traffic simulation was performed four times, occurring three days after each harvest. Early results indicated that, more than soil compaction, traffic directly over the plant causes significant stress and physical damage to alfalfa. In this study, we observed a reduction in both above and belowground biomass (18 and 23%, respectively) when traffic impacted only the plant, indicating that plant damage plays a more significant role than soil compaction in influencing alfalfa`s response to traffic. As shown in Figure 1, the alfalfa root system was also affected by traffic, particularly the fine roots, which showed a 26% decrease in biomass compared to plants that did not receive traffic simulation. This is relevant because fine roots are critical for nutrient and water uptake.

Figure 1. Alfalfa roots after 4 harvests, simulating a growing season. On the left, no traffic simulation; on the right, traffic simulation hitting the alfalfa crown.

A field study conducted at the University of Wisconsin–Madison also demonstrated that the decline in forage accumulation from harvest to harvest across the growing season can be more pronounced when the stand is subjected to machinery traffic (Figure 2). This effect is stronger in hay operations, where the merger and other harvesting equipment enter the field approximately 72 hours after harvest, damaging new shoots that have already begun to emerge.

The primary mechanisms for forage loss include:

• Soil compaction: Heavy equipment compresses the soil, reducing porosity, water infiltration, and oxygen availability.
• Crown damage: Tires rolling over the alfalfa crown can physically injure plants, increasing susceptibility to disease.
• Carbohydrate relocation: Plants under stress relocate the reserves from roots to regrowth, decreasing root reserves, which can impact winter survival and persistence.

Figure 2. Forage accumulation (FA) across time predicted (line) vs. observed (point) on a moderate-tilled soil operation.

Traffic coverage matters

It is not just the number of passes that matters, but also how much of the field is affected and what type of equipment is used (Figure 3). For instance, around 30% of the field may be trafficked by trucks equipped with road tires during forage transport. Road tires have a smaller surface contact area, so the narrow footprint concentrates the force exerted over soil and plants and exacerbates soil compaction and physical injury to the plant.

Figure 3. GPS map showing the movement of each piece of equipment on a commercial alfalfa farm. Red represents the mower, green represents the merger, orange represents the chopper, and blue represents the truck.

Practical recommendations

Machinery traffic is an unavoidable part of the alfalfa harvest, but there are some strategies that can minimize the negative effects of machinery traffic on alfalfa fields:

• Harvest in controlled traffic lanes: Designating traffic corridors can protect more of the stand.
• Limit harvest time traffic: Avoid driving on fields when the soil is excessively wet.

As you prepare for your next harvest, take a moment to reflect on your previous harvest management. Small adjustments can help preserve productivity and long-term field health.