Invasive weeds challenge livestock, degrade ecological integrity

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By Craig W. Anderson

A wide variety of invasive plants and weeds threaten both livestock and the ecological balance of areas needed to feed livestock and humans. Weeds alter the structure, organization and function of rangeland plant communities and impact rangeland more than all other pests combined.

The rate of introduction of invasive plants has increased significantly over the past several years and San Joaquin County Agriculture Commissioner’s Noxious Weeds Surveillance Program has been funded and repurposed, researching the impact of invasive weeds in the county and conducting outreach about what’s happening.

“The program’s funding has been renewed over the last two years,” said San Joaquin County Agricultural Commissioner Tim Pelican. “The program’s just getting started with some rangeland research.”

Weeds defined

Invasive and noxious weeds are defined by California Food and Agriculture code as “any species of plant that is… troublesome, aggressive, intrusive, detrimental or destructive to agriculture, silviculture (sound forestry practices) or important native species, and difficult to control or eradicate.”

Weeds tracked

When harmful weeds are found, their locations are mapped via GPS for reference regarding treatment. “It’s essential their location and degree of infestation be determined to help in control measures,” Pelican said. “Our county staff conducted weekly surveillance during the vegetative and reproducing growth stages in January and February.”

Included among the noxious invasive weeds found in San Joaquin County are Artichoke Thistle, Italian Thistle, Russian Knapweed, Alligator Weed and Yellow Starthistle.

Worthless weeds

Linden diversified farmer and SJFB First Vice President Andrew Watkins said one very troublesome weed is Medusa head that “pushes everything out, is all silicate and of no feed value. It keeps creeping in and takes more and more of the range.”

Watkins speculated that it arrived in 1976 in imported hay and some forage was planted in it. “It became established and the land had to be worked up and reseeded, all of which added additional costs to growers overhead,” he said, adding, “People don’t really know it’s here and the county’s gathering information so its threat can be addressed.”

Recognizing weeds

“Farmers need to be educated about paying attention to the various weeds and catching them early by being alert to any changes in rangeland, anything that looks different,” said Theresa A. Becchetti, UCCE Farm Advisor-Livestock & Natural Resources for San Joaquin and Stanislaus counties. “It’s likely due to an invasive weed gaining a foothold.”

Resources available

Becchetti said, “We have resources for different weeds and the Cooperative Extension, along with agricultural commissioners, have created an education booklet about our weeds with photos and information regarding what to look for and when.” She said a booklet is being developed for the North California Valley.

Weed population growing

A Natural Resources Conservation Service (NRCS) white paper noted that, over the past several years, “The rate of introduction of invasive plants has risen significantly.” Many invasive weeds took over the county’s green spaces, reducing economic productivity and upsetting ecological integrity, said the NRCS.

Weeds travel

Also, humans, usually unknowingly, transport invasive species into new areas on cargo ships, aircraft, cars “and other modes of transportation, including their bodies.”

“When I move from one site to another I check my shoes and clothing to be sure no weed seeds are hitchhiking on me,” Becchetti said. “If I’m on rangeland I don’t want to carry Medusa head, barbed goat grass, tarweed or fiddleneck – which is toxic for cattle – to a new site that could have its forage base reduced by them.”

Weeds are hardy and major reducers of natural habitats while decreasing thatch and “they’ll out-compete annual grasses. They create a thatch where nothing grows but weed,” she said.

More weeds

Other invasive weeds that show up in irrigated pastures include Johnson grass, Baltic rush, barnyard grass, smut grass, yellow foxtail, bur buttercup, curly dock and smartweed. At least some forage value is provided by various weeds such as mustard – as long as the grazers don’t devour too much of it; tarweed is valuable because livestock find it unpalatable, thus inedible, and it can foster pollinators and concentrate selenium.

Sneaky weeds

Weeds such as Italian thistle can indicate more nitrogen in the soil.

Researchers are developing best practices for accurately identifying the major weeds and ascertaining if they’re annuals or perennials, native or invasive and whether or not they’re toxic to the animals that could eat them.

Some weeds are physically damaging and toxic for cattle but sheep and goats can nosh toxic weeds cattle won’t touch.


Some varieties of weeds have, over time, become resistant to materials that once effectively and efficiently killed them.  “There are other herbicides available that remain effective against weeds,” Becchetti said. “When sprayed at the proper time the materials are still effective.”

“Weather patterns affect weed growth,” said SJFB President David Strecker. “If we don’t get a good frost, a good environment for weeds is created.”

More weed management methods

Mowing weeds can be effective depending on when it’s done. Strecker explained that multiple cuttings of alfalfa coupled with herbicide use can curtail weeds.

“Or you can get out there with hoes and weed eaters if it’s a small infestation,” Becchetti said. “Of course, targeted grazing with cattle, sheep and goats also works, depending on weed species and environments, the grazers being used and the toxicity, palatability and stage of the plant’s growth.”

“Weeds can now be burned as permitted in Senate Bill No. 332 recently signed into law by Gov. Newsom,” Becchetti said. The bill allows burns for wildfire hazard reduction, ecological maintenance and restoration, cultural burning, silviculture, or agriculture. “A certified burn boss will conduct the burn in compliance with a written prescription.”

IPM works

Integrated Pest Management programs are effective when used at the most opportune times for weed killing. Tansy ragwort is susceptible to the cinnabar moth which enjoys a good ragwort feast and the weed is controlled. “The IPM approach affords a whole suite of approaches, including bio controls. For example, there was a puncture vine explosion a few years ago and two different kinds of weevils were used to eradicate it. There are a lot of naturally occurring bio controls out there,” Becchetti said.

She said roads “are the earliest indication that weeds are on the way because they’re growing on the shoulders after hitchhiking on cars.”

UCCE expertise available

“Farmers can use the UCCE expertise if something pops up you thought you were treating,” Strecker said. “Noxious weeds surveillance, materials used on weeds, a cutting schedule designed to reduce weeds and herbicides. Using animals for targeted grazing. It’s amazing how nature, with our help, takes care of challenges.”