Are cows the ultimate weed seeders?

  • Cows on the Oregon rangeland

    Bureau of Land Management photo
  • WEED SPOTTING: Joy Belski

    photo courtesy Oregon Natural Desert Association

Note: This article is a sidebar to this issue's feature story.

When Joy Belski thumbs through the dozens of federal weed-management plans now circulating throughout the West, she almost always finds one thing missing.

"The agencies will mention that trucks, hikers, ORVs and roads contribute to the spread of exotic weeds," she says, "but they almost never mention cattle."

The omission is one that Belski, who spent 10 years studying African grasslands before landing a job as the staff ecologist for the Oregon Natural Desert Association, hopes to rectify. In a new report, Livestock Grazing and Weed Invasions in the Arid West, Belski and co-author Jonathan Gelbard make the case that cattle and sheep grazing have greatly accelerated the alien plant invasion in the Interior West and that the only way to really control the problem is by reducing or eliminating livestock grazing.

"Over and over again, the research shows that where you have grazing, you have much higher numbers of weeds than in ungrazed areas," says Belski, who reviewed more than 150 scientific papers in preparing the report.

The report points to several obvious ways cattle spread weeds: Sticky weed seeds will often take a ride to unexploited territory on furry hides and in hooves. Cows also carry in their guts loads of seed, which they deposit wherever they wander. In one study cited in the report, a single cow in a pasture in Alberta, Canada, redistributed over 900,000 viable seeds in a single season. Other reports found that cattle and sheep carried viable seeds of such aggressive exotics as leafy spurge and houndstongue.

The report says, "Given that livestock graze 70 percent of the land area of the West, including 94 percent of BLM's 165 million acres, and carry viable seeds for as long as 10 days, they are undoubtedly major vectors of non-indigenous plant seeds."

But more important than the ability to transport weeds, says Belski, is the cow's ability to alter terrain.

"One of the keys to whether exotic weeds will take root is whether they can find the right seed bed," says Belski. "The 20 million cattle and sheep in the West are the major disturbers of soils."

The churning action of cattle hooves has broken up the delicate soil crusts that have historically excluded seeds from moisture and nutrients, Belski says. Without broken-up soil, plants have a lot tougher time getting established, which is why exotic weed outbreaks in national parks and other ungrazed landscapes are often confined.

"You can find weeds along trails in Yellowstone where people have disturbed the soils, but in untrammeled parts of the park you won't find them," she says.

In some places, humans have become the major soil disturbers. In Utah's Arches National Park, for example, hikers have crushed soil crusts, giving exotic species a leg up on natives, says Belski. And cattle may not be nearly as important a factor in the spread of weeds in the Great Plains, where the soil and plants are adapted to large herds of grazing animals.

But West of the Rockies, Belski says, cattle are the culprits. In their report's conclusion, Belski and Gelbard say that land managers need to quit treating cattle grazing and invasion of exotic plants as separate threats to native ecosystems.

"Most of the current ... management plans ... focus on preventing landscape-level introductions of weed seeds by washing vehicles and using weed-free livestock feed. Although useful, these strategies are similar to rearranging deck chairs on the Titanic. ...

"Not until plant communities and soils are allowed to recover their natural defenses, such as healthy, deep-rooted native plants and intact microbiotic crusts, will the spread and dominance of nonindigenous weeds in the American West be reduced or reversed."

The report is available free on the Oregon Natural Desert Association website,, or for $5 by writing to ONDA, 16 NW Kansas, Bend, OR 97701, 541/330-2638.

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