If fields were rated like highways, those containing alfalfa would instantly earn freeway classification because of the repeated equipment traffic of multiple harvests.

Current research shows it's time to take aim at reducing the stress of 10,000 lbs. of heavy metal and rubber that break down alfalfa plants trip after trip. Why? Research plots have shown yield losses from equipment traffic of 30 to 70%.

“I was shocked at the amount of yield loss due to wheel damage that we've seen in our first two years of study,” says Dan Undersander, extension forage agronomist, University of Wisconsin — Madison. “That's why there is a cooperative research effort under way across six states in 2002 to collect more data on causes of alfalfa yield loss by specific traffic patterns.”

Variety choice matters

Will “traffic tolerance” climb to the top of the list of choice characteristics of alfalfa varieties, ranking up there with yield potential, pest resistance and seed price?

Jim Moutray, director of research of America's Alfalfa, believes that traffic tolerance should be a standard characteristic of an alfalfa variety. “Traffic occurs on every alfalfa field, and yield loss from wheel damage occurs with every variety, so this is valuable information for every alfalfa producer,” he says.

Granted, Moutray's company is currently marketing several varieties as “traffic tested,” and other companies are certain to follow. But America's Alfalfa discovered the potential of a high-tolerance-to-traffic trait more than 11 years ago while working on grazing tolerance. “Grazing tolerance led us to cooperative work with the university community to develop traffic data,” Moutray says.

And it didn't take long for Undersander to realize how important this work on yield loss to wheel traffic could be, and to witness differences among varieties. “In our two years of trials, it already appears that there are some big genetic differences between varieties and how they handle traffic,” he says.

Undersander says he has seen enough consistency among the limited number of varieties he and Moutray have tested to be comfortable releasing current varietal information. The chart at the end of this story, provided by America's Alfalfa, shows varietal differences based on average university cooperator data from four locations (Wisconsin, Iowa, Nebraska and New York).

One commonsense belief is that varieties with strong grazing tolerance will be best at handling the stress of wheel traffic. “But we're not just researching grazing-type varieties,” Undersander says. “I would say as a general rule that maybe the grazing types will do better in this type of test than others, but that's not necessarily the case.”

Plant traits not identified

Although more companies are beginning to tout the traffic tolerance of their alfalfa varieties, Undersander advises asking the company for research to justify the claims.

“We really don't have enough data at this point about which plant traits help a variety better survive traffic,” he says. “I think maybe there are certain characteristics such as low crown height that seem logical, but frankly we have not yet identified the traits that are important. This year's six-state project will help us gain this information.”

How alfalfa varieties stand up to traffic
Variety Average annual yield under traffica (tons/acre) Percent yield reduction from wheel trafficb Brand
Ameristand 403T 5.45 -05% America's Alfalfa
Rebound 4.2 5.24 -13% Croplan Genetics
Geneva 5.22 -17% Novartis
FQ 315 5.05 -19% Mycogen/Cargill
GH 757 4.96 -10% Golden Harvest
DK 140 4.83 -24% DeKalb/Monsanto
Magnum V 4.74 -22% Dairyland Seed
54Q53 4.63 -17% Pioneer
a Wheel traffic consisted of a 9,000-lb., 90-hp tractor driven over the entire plot with both wheel tracks five days after each cutting. It was repeated three times in an attempt to simulate driving over the field with a tractor, chopper and wagon, or a tractor, baler and wagon. Some plots were cut every 21 days, some every 35 days.
b Two-year results from University of Wisconsin — Arlington Research Station.