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New Mexico State University

New Mexico State University

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NMSU Range Scientists Battle Invading African Weed

LAS CRUCES - An invasive rangeland weed that first appeared in North America in the 1920s has established an 11-county foothold in southern New Mexico and is looking for more space to roam.



Bright green with pretty white five-petal flowers, African rue hardly looks like a dangerous invader. But NMSU researchers say the little range weed is tough to kill thanks to a tap root that reaches more than 10 feet deep. (03/25/2002) NMSU agricultural communications photo by Norman Martin

African rue, a poisonous basketball-sized plant which was first discovered near Deming, has drawn the attention of a team of New Mexico State University range researchers who are determined to unravel the biology and ecology of this invading species and turn it back.

"For being a relatively small plant on the surface, African rue occupies a very large volume below ground." said Laurie Abbott, an NMSU assistant professor of range science. "I excavated a root system of this plant, and the tap root reached at least 13 feet deep, at which point I decided it was time to stop digging and come up for air."

Bright green with pretty white five-petal flowers, the half-shrub hardly looks like a dangerous invader. The perennial weed is bushy and branched, growing a foot tall when fully mature, said Richard Lee, a weed specialist with NMSU's Cooperative Extension Service. But the little plant contains at least four poisonous alkaloids, plant chemicals that are toxic to cattle, sheep and horses.

African rue's effects on livestock range from loss of appetite and listlessness to trembling and lack of coordination, said Lee, who has led several invasive weed programs in New Mexico. Acute poisoning, usually caused by eating seeds, can cause hemorrhages in an animal's heart or liver, he said. The seeds and fruit of the plant are the most toxic.

The good news is that African rue apparently tastes terrible and smells worse, Lee said. Animals generally eat it only if they're starving or suffering from severe mineral deficiencies. If poisoning occurs, it's usually in spring and summer, so one basic livestock management plan has been to simply avoid infested pastures during those times. Meanwhile, efforts to control the African invader are still a work in progress.

"We believe that [African rue] tends to dominate in areas where it occurs, but that not much is known about either its biology or ecology," Abbott said. To learn more, NMSU's range science team last year began field studies, laboratory and greenhouse projects.

NMSU has two field sites under evaluation. Located southeast of Deming and on Holloman Air Force Base rangeland near Alamogordo, the experimental plots have different soil and environmental conditions, Abbott said.

"By studying different populations, we can see which environmental parameters have the greatest influence on sprouting, flowering and germination," she said. "We need this basic research because we don't actually know what its primary mode of establishment is." New plants can develop from seeds, but the shallow lateral roots also produce new sprouts, so it's important to understand the environmental conditions that favor either mode of reproduction.

Native to the deserts of North Africa and Middle East, African rue slowly spread east, eventually crossing Pakistan and India. But no one is precisely sure how African rue first arrived in New Mexico, specifically Deming.

"One plausible mode of delivery was camels," Abbott said. "I've heard that people were attempting to introduce camels from the Middle East to the area at that time. We don't know for sure, but it's a good possibility." Another theory tells of the rue's arrival on airplane tire treads at a former military airport at Deming.

No matter how it arrived, researchers do know that apparently it stayed fairly localized in southwestern New Mexico for decades, but as infrastructure and agricultural markets expanded, so did its boundaries. Today, it's been reported in 11 counties, primarily in southern New Mexico; Hidalgo, Grant, Luna, Socorro, Sierra, Doņa Ana, Otero, Chaves, Eddy, Lea and San Juan. Meantime, the plant has been identified by researchers in Texas, Arizona, California, Nevada, Oregon, Washington and Montana.

"Just look at that broad range of states and you can see it has large ecological amplitude, at least in terms of temperature tolerances," Abbott said. "It's found in northern Montana, and believe me it gets cold there in the winter time."

African rue apparently likes to establish itself in disturbed soil. It is often found in parking lots, near cattle watering facilities and along roadsides. In fact, it's likely that mowing along roadsides help the weed's spread by keeping competitive vegetation down, Abbott said.

Like a true alien aggressor, African rue has a variety of adroit survival methods. In addition to producing a seed capsule containing between 50 and 75 seeds during May and June, the plant dies back during the hot, dry summer months. With late summer rains, the plant resprouts and resumes growing until a killing frost in the fall.

The plant also spreads with a tenacious, two-layered root system. The first part stays relatively close to the surface, sending out woody tentacles laterally. A second root system dives deep into the earth, searching for water.

Once the plant has a foothold, it usually reduces the availability of other kinds of forage plants, changing the plant species composition in the area. Kirk McDaniel, a range management specialist with NMSU's Department of Animal and Range Science, has found that because of African rue's elaborate root system, the plant quickly grows back after mowing or burning, and deep cultivation only divides and spreads the roots.

"Numerous herbicides have been investigated in NMSU field tests, but only chemicals that are able to move through the soil profile and deep into the plant's root system have been effective," McDaniel said. As a result, he said, herbicidal control is unusually slow, requiring a year or more to kill a plant that's been individually or broadcast sprayed.

For the moment, NMSU's primary research plan is gathering additional data on the plant's biology and ecology. "It's difficult to plan a management strategy without fully understanding how the organism works," Abbott said. "If we can understand how this plant gets established and how it spreads into new areas, then we can make recommendations to ranchers and to farmers about how to keep it from becoming a problem on their land."