Tread lightly

WISCONSIN DELLS, Wis. — Alfalfa, one of the key components to healthy feed and healthy cows, has its own battles to stay healthy.

Some of those battles include pests or drought, but another is entirely dependent on how it is harvested. Battling tire treads and their effect on alfalfa persistence was recently covered by Brian Luck, an associate professor in biological systems engineering at the University of Wisconsin-Madison.

In his presentation at the Midwest Forage Association’s hay symposium in February, Luck described the biggest detractors in alfalfa persistence pertaining to tire treads. 

Luck shared the differences between road tires compared to agriculture tires in the field, the effects of tire pressure and how and where tires go in the field matters.

“The biggest takeaway we had was that road tires are bad on alfalfa,” Luck said. “But changing road tires on a full fleet of equipment is often beyond the budget.”

In 2018, Luck and a team of students observed how machines moved through a field and how many machines were involved during the harvesting of alfalfa. They totaled 1,300 passes made by two choppers, 12 trucks, two mowers and two mergers.

Measuring in pounds per square inch, Luck and his team measured the pressure difference pieces of equipment put on the field. They measured the effect of a baler, chopper, windrower, semi-truck, merger and a hay wagon.

“The biggest difference we saw was the ag tires compared to road tires,” Luck said. “Ag tires are radial, flexible and bouncy. The road tires because of how they’re built, increased the ground pressure applied by the machines.”

When compared, the ag tires applied 220-230 pounds per square inch while the road tires generated 510-800 PSI.

The next step for Luck and his team was determining the traffic patterns during alfalfa harvests. Using GPS positioning of the machines, his team determined there were seven different levels of compaction.

The biggest difference the team observed was between the 3-pass areas compared to 5-pass areas and the level of damage incurring on the alfalfa plant.

“We broke it down into areas of compaction,” Luck said. “The first three inches, then 3-6 inches, 6-9, 9-12, all the way down to 18 inches.”

Using these zones, it was determined that soil resistance is less affected further down in the soil meaning less compaction in the deeper areas. At the end of the year, the team observed higher soil resistance possibly due to dry soil and some of it due to the compaction of the machines. As the process went on, the numbers went down towards the spring of the second year.

“We saw an increase in soil moisture in the spring,” Luck said. “The other part was the soil relaxing over the winter. The freeze/thaw cycle is actually doing a little movement of that soil and reducing the compaction.”

To determine the effect the level of compaction had on the crop, Luck discussed with the farmer what the traffic strategy was for his field. The farmer asked his drivers not to drive on the same spot twice, attempting to distribute weight across the field.

Using the GPS tracking of every vehicle in and out of the field, the worst-case scenario saw 61% of the field saw a tire at least once, with the average being 49%, give or take 9%.

To reduce the amount of the field that saw a tire or limit the damage that would be done because of tire treads, Luck and his team conducted a new experiment with tires at 9, 16 and 25 PSI.

“We borrowed pressure mats from the University of Minnesota,” Luck said. “We were able to map out what the tire looks like at those three different pressures.”

Using a 28,000-pound tractor, the study showed that with tires inflated to 9 PSI, it affected 6.32 square feet, compared to 25 PSI, a more road-worthy tire pressure, affected 5.35 square feet. 

Ten days after the harvest, the regrowth rate was different depending on the field. The no traffic control group represented the best regrowth rate, with other fields seeing similar yields but likely utilizing stored resources in the plant to compensate for the damage caused by the wheel traffic.

With these trials, Luck and his team developed a tool to determine what would play the biggest factor in alfalfa persistence. They developed the Badger Alfalfa Bashing System. This system featured a pneumatic cylinder that could apply different levels of pressure to better measure the impact on the alfalfa.

The program was written to affect three areas, the plant, the soil around the plant and the entirety of the plant cylinder.

Through the testing with BABS, it was determined that plants that were not directly hit showed better signs of regrowth. It was also determined that the difference in tire pressure, accompanied by traffic patterns, played a big role in the alfalfa’s persistence.

Minimizing the amount of area in a field that sees a tire is better for regrowth and persistence. Although it was undetermined if the difference was the impact on the plant or soil compaction, best practices included making more efficient trips into the field.

Semi-trucks were determined to be the most efficient vehicle because of their ability to haul larger loads, making fewer trips more feasible.

“The biggest takeaway we had was if you can switch from road tires to ag tires, and pulling tractor towed carts will help,” Luck said. “Managing air pressure to put less force on the plant also helps limit the damage.”

The final practice Luck said to help limit damage caused by tires is to control the traffic. Communicating with drivers about how to drive through the fields, avoiding crossing over rows, traveling with the field and even maintaining similar traffic paths.

“The similar traffic paths will damage the alfalfa, but we can reduce the 49% of the field that sees a tire,” Luck said. “We don’t need to go as far as making tram lines, but it limits the amount of the field that can be used for transportation, which will help with regrowth and persistence.”