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PRAIRIE DU SAC, Wis. - Large farm equipment can damage roads. But there are things farmers can do to lighten the impact.
Kevin Erb, University of Wisconsin-Extension conservation training coordinator, looked at some of those things during the recent North American Manure Expo, held this year at the U.S. Dairy Forage Research center, near Prairie du Sac, Wis. Erb also talked about the results of a just-completed four-state research project that examined the impacts of certain types of large equipment on roads. The equipment included manure tankers that hold 4,000 to 9,500 gallons, along with overloaded semis - those carrying 102,000 pounds - and grain carts.
Simple changes, said Erb, can mean big differences. The distance machinery travels from the edge of the road, the time of day heavy machinery is on the road, and where a manure tanker or grain cart is loaded all factor into road damage.
Erb began by pointing out that Wisconsin's cows produce twice as much manure as they did during 1970. The state's 1.2 million dairy cows are responsible for 12 billion gallons of manure and wastewater a year. As their milk production has risen, so has their manure production, because cows generally eat more today than they did 42 years ago, Erb said.
Problems with roads and bridges are among the top three issues identified by the Professional Nutrient Applicators Association, according to Erb. One problem is that many town roads and bridges are not designed for heavy loads, Erb said. Compounding the problem is the fact that farm equipment of all types is getting larger and larger.
What's more, regulations regarding farm equipment on public roads have generally not kept pace as machinery has evolved, Erb said. Rules written for a 1950s-era tractor and manure spreader are not appropriate for a modern, four-wheel-drive tractor and multi-axled manure tanker.
Machinery designs keep changing, too. Erb said the impact of innovations in axle spacing, tire design and type on rural roads is largely not known. And, conflicting results from studies hasn't helped clear things up.
Two questions, in particular, are still unresolved. First, is it better for roads and bridges if a farm hauls more smaller loads, or fewer large loads? Second, which is more important: the total number of pounds per square inch on the pavement, or the axle weight?
The Professional Nutrient Applicators Association of Wisconsin (PNAAW) wanted to know, so it initiated a study. Others involved included manure applicator groups in Minnesota, Iowa, Ohio, Indiana, Michigan and Pennsylvania, the Wisconsin Towns Association, Minnesota county highway engineers, and transportation departments in four states.
Industry groups also participated. They included farm equipment makers AgCo, Case New Holland, John Deere, Houle, and Husky, and tire manufacturers Firestone/Bridgestone, Michelin, and Titan. In all, industry and agencies contributed $610,000 toward the study.

Two objectives
Researchers had two main objectives, Erb said. One was to determine pavement responses to selected agricultural equipment, using instrumented pavements. The second goal was to see how different kinds of pavement held up under an 80,000-pound load - the kind a typical, five-axle semi would carry.
Testing took place during early March, when frost was coming out of the roads, in August, when professional applicators had time to operate the equipment on stretches of roads, and in November.
Five types of road construction were tested: 3.5 inches of hot-mix asphalt on an eight-inch gravel base; 5.5 inches of asphalt on nine inches of gravel; six inches of asphalt on 14 inches of gravel; 7.5 inches of Portland cement on 12 inches of gravel; and five inches of cement on six inches of gravel.
Commenting on the method using six inches of asphalt over 14 inches of gravel, Erb said, "That's how our roads really should be constructed. A lot of times, we find two inches of asphalt on two inches of gravel, and mud underneath that."

Equipment
Researchers looked at the impact from a range of equipment. It included an 80,000-pound semi, a grain cart with 57,000 pounds on one axle, two manure tankers that each held 6,000 gallons, and a 9,500-gallon manure tanker, along with two smaller ones.
Data from 1,198 vehicle passes over a concrete road was collected. Researchers also gathered numbers from 3,626 passes over asphalt roads. Loads ranged from zero pounds to maximum capacity.
Some combinations of vehicles and load weights exceeded the legal limit. One 9,500-gallon manure tanker, when full, tipped the scales at 134,000 pounds. Erb pointed out that the legal maximum for farm equipment on public roads in Wisconsin is 80,000 pounds.
"So this sucker is illegal to go down the road," Erb said. "But quite honestly, the tractor by itself, with no diesel or hydraulic fluid is illegal to go down the road, because of axle weights."

Three keys
The study turned up three key things to lessen road damage, Erb said. They are: the weight over an axle, the distance the equipment is operated from the edge of the pavement, and the road's construction.

Axle weight
"Per-axle weight is a critical factor - much more so than total vehicle weight," Erb said. Smaller vehicles can cause more damage than larger ones if there's more weight over the axles.
"That equipment also needs to be properly set up and adjusted when it's used," Erb said.
He noted that one 6,000-gallon manure tanker rolled out of the factory with a rear axle several thousand pounds heavier than the front one. Two other tankers could haul manure, but they did less damage to roads than the lighter one because the weight was spread over more axles.
"Axle weight is huge," Erb said. "Going to a smaller tanker doesn't eliminate all the problems."

Stay away from edge
The second key factor to lessening damage is to keep the centers of the tires at least 16 inches away from the road edge. It's also important to avoid hauling manure or other heavy loads at certain times of the year - when the road's subgrade is wet and when frost is coming out of the subgrade.
Time of day plays a role, too. Erb said, "We saw much less damage to the road in the morning than in the afternoon. As the pavement warms up, we lose some strength in the asphalt and in the subgrade."
If manure needs to be hauled, do it between 6 and 7 a.m., Erb recommended. "We saw no damage before 11 o'clock," he said.

Pavement construction
The type of pavement construction, along with the quality of the subgrade and asphalt, is critical, Erb said. If the subgrade was only 2.5 inches thick, instead of 3.5 inches, damage appeared quickly, Erb said. No damage was seen if the subgrade was 5.5 inches thick.

Other solutions
Erb also touched on some of the measures being used in several Wisconsin counties, most of them in the eastern part of the state, where there are more large dairy farms. In some places, stretches of roads are designated as one-way for a few days while manure is being hauled. That lets the equipment travel down the middle of the road instead of along the edge.
In addition, some counties are finding ways to place drag hoses under roads instead of on them. The manure-carrying hoses run through culverts or through special pipes.
Farmers and local highway departments are also building pull-offs along roads, so heavy farm machinery can be parked without being on the pavement or the shoulder. Longer culverts - as much as 60 feet long - are being installed at field entrances, to prevent road edges from being eaten away by turning farm machinery.
Road damage by farm machinery is a continuing problem.
"A long-term solution," Erb said, "needs to involve farmers, towns and counties, and the industry."
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