The effect of wheel traffic on alfalfa yield

By Dan Undersander, Ph.D., University of Wisconsin-Madison professor emeritus

The reality of harvesting forage fields is that we must drive over them. However, wheel traffic causes both surface and deep-soil compaction, damage to the field and the forage stand, and crop yield reductions. To reduce damage, producers should implement several practices.

Studies have shown that machines equipped with agricultural tires apply ground pressures ranging from 100 to 220 pounds per square inch (PSI) while machines equipped with on-road tires apply ground pressures ranging between 520 to 820 PSI.

Tire pressure can also cause significant damage to alfalfa. One researcher noted that the ground pressure increases to 7.2 PSI for every 1 PSI of tire inflation pressure. This is due to reduced ground contact as inflation increases. Whenever possible, try to use agricultural or flotation tires in alfalfa fields. Research has shown that plots with machinery traffic applied by tires operated at lower inflation pressures showed more regrowth 10 days post-cutting (Figure 1).

Another portion of the crop yield loss can be due to cracking or breaking of crowns, which will reduce the shoots produced and may allow entry of disease into the crown. However, the major damage and cause of yield loss for alfalfa harvested more than two days after cutting is breaking shoots that have begun to regrow.

Yield loss to alfalfa regrowth for the next cutting is greater as the traffic occurs longer after mowing. The yield loss has generally ranged from 4% to 6% per day after mowing. For example, traffic five days after mowing creates a yield loss of about 22% (Figure 2). As a result, haylage fields tend to yield more than hay fields. It is also justification for making baleage rather than waiting for the forage to dry for hay.

Wheel traffic will cause some soil compaction and associated yield loss for grasses but will not break stems. Yield loss for grass will be significantly less than alfalfa.

Overall, it is important to minimize driving on the field. Identify “in-field roads” during harvest and limit field travel to those roads as much as possible. This may look like taking the shortest route to the edge of the field or using a bale accumulator when removing bales to reduce traffic. One study showed 5.3 miles driven on field to remove bale accumulations versus approximately 20 miles when bales were dropped randomly and then driven to and individually removed.

The following management practices can reduce the impact of wheel traffic:

1) Plant alfalfa varieties that are more tolerant to wheel traffic.

2) Use flotation tires when possible or radial agricultural tires to reduce ground pressure. Minimize use of on-road tires.

3) Check the tire pressures of all machines entering the field to ensure that they are on the low end of the manufacturer’s recommendations.

4) Use small tractors when possible (i.e. for raking).

3) Drive over the field as soon as possible post-cutting.

• Raking 24 hours post-cutting causes less damage than raking 48 hours post-cutting.
• Merge swaths into large windrows so harvesting equipment has less driving time on the field.
• Making haylage one to two days post-cutting causes less yield loss than making hay three to five days post-cutting.

4) Avoid unnecessary trips across the field when harvesting.

• Get full wagons and trucks off the field with as little driving as possible.
• If bales are dropped and collected, consider whether the process could be done with less driving.
• Do not drive on an alfalfa field when harvesting the crop of an adjacent field.

5) Consider using larger harvesting equipment.

• There is some debate about this; while less area is affected by wheel traffic when using large equipment, the affected area has greater weight applied to it.
• This could also be another benefit of contract harvesting.

6) Use of duals on harvesting machinery is not recommended unless necessary to avoid ruts.

Dan Undersander, Ph.D., is a professor emeritus at the University of Wisconsin-Madison College of Agricultural and Life Sciences. During his tenure, his research was focused on factors affecting alfalfa plant health and survival, best management practices for harvested forages, optimum management practices for intensively grazed pastures, and developing equations for the Infrared Reflectance Spectroscopy for commercial testing laboratories.