AMES, Iowa (AP) — On a hot and humid May day, more than 50 workers busily clear honeysuckle and buckthorn from Ada Hayden Heritage Park in Ames.

These aren't your typical summer temps — college students riding out break on top of lawn mowers.

These workers have four legs and an appetite for Iowa's invasive weeds.

Iowans are increasingly turning to goats to fight the state's widespread problem with invasive species.

In Ames, the herd was hired to chew back the broad leaves of woody vegetation that has invaded what once was an oak savanna on the northern edge of town.

The low-tech approach is a more environmentally friendly, and some say a more effective way, to deal with the unwanted plants. Grazing goats reduce reliance on chemicals and gas-powered equipment.

"They are a great alternative," said Aaron Steele, co-owner of Goats On The Go, a goat rental company based in Ames. "They consume the stuff that no other livestock would."

The goats move with almost zen-like precision from west to east across the Ames park as walkers and bikers whiz by on the trail. After an hour, the herd stops to digest. Then, they get up and do it all over again.

The goats work for about two weeks to clear the 2 ¼-acre plot. They'll come back in a month to make another pass at weeds that have grown back.

The goal is to disturb the notoriously hard-to-kill vegetation enough that it will die off in the winter. Without healthy leaves, the plants can't grab enough energy from the sun during the summer to make it through a frost.

Removing the invasive honeysuckle, buckthorn and white mulberry from the ground level will provide enough sunlight for native species to thrive, said Erv Klaas, retired professor of animal ecology at Iowa State University and president of the nonprofit group Friends of Ada Hayden Heritage Park, which brought in the goat herd. He expects to see insects, birds and snakes native to the oak savanna return once the ground cover is gone.

"We hope to restore a native ecosystem of Iowa before European settlement, before we had agriculture and mining," Klaas told The Des Moines Register (http://dmreg.co/2qZ35BM ). "Savanna is probably one of the most endangered ecosystems in the Midwest, but it's an important ecosystem along the edges of prairie and woodland."

The Center for Invasive Species and Ecosystem Health identifies 448 species of invasive plants in Iowa. Some, like curly dock and Canada thistle, are found in all 99 of Iowa's counties. Three varieties of honeysuckle live in Iowa, one of which is in 40 counties. Buckthorn is in 45 counties, the center said.

Goats are being used by conservation groups, governments and private individuals across Iowa. Companies that rent goats say business has exploded in the past five years as people become more aware of grazing as a natural disturbance technique.

Goats On The Go was started by two families with 17 goats in 2013. It now has more than 200 goats and a network of affiliates in Cedar Rapids, Iowa City, the Quad Cities and in Ardmore, Oklahoma, each with 20 to 40 goats.

"Our herd grows by 150 percent every year and we're booked earlier every year, even though we've added that much more capacity," Steele said.

Its customers are a mix of private acreages, parks departments and county conservation groups, he said. Steele's goats recently completed a project at The Harvester golf course in Marshall County.

Jen and Dustin Wall, owners of Twin Pine Farm in Manchester, have grown from 62 goats four years ago to more than 200 today. The company got its start contracting with Delaware and Delhi to control weeds and grasses along the cities' lagoons. It now works mostly with private property owners.

The Walls' goats cleared 110 acres of land in five Iowa counties last year. They cleared just nine acres their first year with private owners.

"It's definitely taken off," Dustin Wall said. "There's a couple of properties that look like a park now, you'd never know how thick of brush it was."

There is little data showing how well grazing compares to other weed-control measures such as herbicides, pulling or burning. But several groups, including the Iowa Department of Natural Resources and Polk County Conservation, are testing its long-term effectiveness.

Polk County Conservation used goats three years ago on a 10-acre site at the Chichaqua Bottoms Greenbelt in Maxwell. The experiment was part of a grant from the Leopold Center for Sustainable Agriculture.

Loren Lown, parks and natural areas administrator, said the conservation board is monitoring Chichaqua Bottoms to see if its buckthorn infestation returns. Some shrubs have grown back, but it's clear the problem has been managed, he said.

The clearing has allowed native species such as cardinal flower and Canada anemone to grow back. Woodland birds like woodpeckers, wild turkeys and robins can now be seen in the area, Lown said.

"We wanted to look at the long-term effect of exhausting the plant's root systems," he said. "As a maintenance tool I think they're really helpful. They're a viable option for people who don't want to use much herbicide or can't use fire for some reason."

The Iowa Department of Natural Resources is monitoring the goats' effectiveness managing weeds near a trout stream in southern Clayton County. The department wanted to manage woody invasive vegetation to provide better access for anglers, said Mike Steuck, regional fisheries supervisor for the DNR's northeast Iowa office.

The area was previously managed by a mowing crew, which costs about $2,000 per year in staff time, equipment maintenance and fuel, Steuck said. The department pays a similar price for goat grazing, but the goats free up employees to work on other projects along the 60 streams it manages.

"We're running around quite a bit trying to keep up given our staffing levels right now so we're looking for innovative and alternative ways to help us take care of some issues," he said.

Mowing was not needed at the trout stream last year, Steuck said. The DNR will make a decision on whether to continue using goats later this year.

Story County Conservation partners with a local farmer to use 10 goats to manage county properties, said Amy Yoakum, natural resource specialist. The goats most recently were working at the Robison Wildlife Acres, a 13-acre prairie the conservation group is trying to restore near Nevada.

Yoakum provides the goats with water and moves them around to where they're needed. The farmer benefits by saving money on animal feed. The partnership is in its seventh year.

"It's kind of a win-win for them and for us," Yoakum said.

Groups surveyed by the Register said the animals are an effective management tool, but they cannot be the only line of defense against invasive species. All of the groups still use herbicides, mowers or fire to control weeds, but they all said they've reduced their reliance on chemicals and fuel.

"The scale of the problem is huge," said Steele, from Goats On The Go. "We understand most agencies won't be able to use goats year in and year out for a decade. They need to use them really strategically with their other methods."

There's no consensus on potential cost savings, either. The DNR and Polk County plan to measure that data as part of their test sites.

It costs about $900 to $1,200 per acre on projects within a half hour drive of Goats On The Go's five locations. Projects more than 10 acres are significantly less money.

Steele said those costs are comparable to traditional methods of weed removal, "if all costs are realized," such as maintenance wear-and-tear and worker safety.

Goats could have a positive effect on another segment of the agriculture economy.

Demand for goat meat is rising as the county's ethnic population changes, according to NC State Extension. But more than half of the United States' supply of goat meat is imported.

Goats' diets aren't supplemented with food or hay during the grazing season. The animals rely solely on the woody vegetation they clear at project sites.

Steele said as a result of the weed management contracts, he is able to fatten his herd longer than traditional meat goats. Oftentimes he can sell his goats at 100 pounds instead of the average 65 pounds, he said.

"Feeding those goats with what is essentially a nuisance becomes high quality feed for goats," Steele said. "And so we're producing a really sustainable food product as well."

At Ada Hayden Heritage Park, the goats captured the attention delighted onlookers who stopped to watch them work. Several classrooms of students paid visits to learn more about Iowa's natural habitats.

"It's a great educational opportunity because these little guys just capture people's attention and hold it," Steele said.

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Information from: The Des Moines Register, http://www.desmoinesregister.com