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Increased CO2 in the Atmosphere Increases Weeds. Weed-Eating Livestock Could Be a Solution

The atmospheric concentration of CO2 has varied greatly over time, from a low of about 180 parts per million (ppm) during several periods of glaciation over the past 650,000 years to this year’s high of 419 ppm. The atmospheric concentration of CO2 was relatively constant at about 280 ppm for about 1,000 years before 1750. Since 1750, the CO2 concentration has risen, reaching ~377 ppm in 2005. The inset (small graph in upper right) of figure 1 shows the atmospheric levels of CO2 for the last 50 year that produce the sharp spike at the far right of the main graph. Although the cycles of CO2 levels (approximately 100,000 years apart) are naturally on a rise from the last ice age, this spike is dramatically higher than any levels seen over the last 650,000 years.

Some years back, I went to a meeting in Denver sponsored by the Colorado Cattlemen’s Association. I don’t remember the exact title, but it was something about how ranchers could adapt to a changing climate. Scientists explained what their research was telling them about plant responses to increasing CO2 in the atmosphere, and they shared computer models of how the weather might change in different parts of the country. As is normal for scientists, their talks were laced with the words, “probable,” “possible,” and “further research is needed.” That’s because, as we all know, the more you learn, the more you realize that there are always a lot of factors affecting what’s happening on a day to day basis, making it difficult to be “sure” of anything. During breaks the cattlemen shook their heads and laughed. It’s hard to make decisions about such long-term potentials when you’re trying to solve today’s very real problems.

I was reminded of that meeting when I came across a 2008 presentation to the Northeastern Weed Science Society, by USDA-ARS scientist Lewis Ziska describing his research on how increasing levels of atmospheric CO2 might affect agriculture and the weeds that so many folks combat. For those who think of pasture weeds as problems, the results of research aren’t good news. Yes, CO2 does increase your pasture weeds, and the weeds seem to do better than our grasses.

In his presentation, Ziska shared results of research showing that:

• Initial evidence indicates that warmer winters are a factor in the northward spread of Kudzu. His work shows that it is likely to spread as far north as the Great Lakes region by 2019. He said that “Data suggest that the link between global warming and the spread of an invasive species is real, not hypothetical.”

I’ve since checked to see if his prediction came true, and it did. Kudzu is now in Michigan, though it’s populations are still somewhat small. Still, given it’s nickname “Mile-a-Minute-Weed,” that’s not good news.

• For those of us in the West where cheatgrass is expanding its range, data shows that higher levels of atmospheric CO2 make cheatgrass more combustible. This lowers the point at which cheatgrass becomes a fire fuel and increases fire intensity. Though additional research needs to be done, Ziska said that, overall, the likelihood of fire in cheatgrass has increased as a function of recent increases in CO2. Increased CO2 also reduces the digestibility of many grasses, including cheatgrass. In regions where the spring flush of cheatgrass is the primary forage for cattle, this is not good news.

Again, checking his work, I found that he’s correct here as well.

• Ziska’s work with Canada thistle and CO2 indicates that Canada thistle does quite well as atmospheric CO2 increases. The plant, which can spread by roots (rhizomes) increases root growth as CO2 increases whether nitrogen is present in the soil or not. However, this doesn’t have to be a huge worry for graziers. It is my favorite forage after all.

RoundUp-and-CO2• Finally, increasing CO2 appears to reduce the effectiveness of glyphosate (Roundup).

He concluded by saying that we need new management strategies for weeds and that “We can no longer assume that what worked in the past will work in the future.

Oh No! What Can We Do?

None of that feels like very good news if we keep on thinking of pastures as we’ve always thought of them. But if we change our ideas about pastures and think of “weeds” as forage, the future could still be pretty bright.

In 2004, I developed a method for teaching cows (or any of our livestock) to eat weeds. It’s simple and takes just 8 hours spread over 7 days. In addition, weed-eating can increase forage available by about 43% and weeds often more nutritious than the grass we typically ask our livestock to eat. The good news is that if you have weed-eating animals you can have more of them and they’ll gain weight more quickly too.

Apparently this sounds like a crazy answer because 18 years later, something I created to save graziers time and money is still not common practice. For ten years I went from one end of the country to another, and even spent time up in Canada doing workshops and training projects. That was tiring and took me away from home too much, so I started On Pasture as another way of spreading the word. My success is still questionable. 🙂 Yet, I still look forward to the day when everyone will have weed eating livestock and it will be so common that no one will even remember the crazy gal that put together the training method.

How Can You Teach Your Livestock to Eat Weeds?

For resources on training livestock to eat weeds and what weeds are edible, check out the 2-ebook set and other articles and links here.

Head over here where you’ll find many of the On Pasture articles I’ve written on the topic, links to videos for more information AND my new, downloadable ebook set: Cows Eat Weeds and Edible Weeds & Training Recipe.

In Cows Eat Weeds I include all the background you need, along with problems and solutions to them from my ten years of actively working in the field training livestock across the U.S. and Canada. I supplement that with Edible Weeds & Training Recipe. You’ll find a a list of all the weeds I’ve trained animals to eat, or that I’ve looked up for graziers along with information on their safety as forage. The “Recipe” takes you through the training process step by step so you can get going quickly.


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Kathy Voth
Kathy Voth
I am the founder, editor and publisher of On Pasture, now retired. My career spanned 40 years of finding creative solutions to problems, and sharing ideas with people that encouraged them to work together and try new things. From figuring out how to teach livestock to eat weeds, to teaching range management to high schoolers, outdoor ed graduation camping trips with fifty 6th graders at a time, building firebreaks with a 130-goat herd, developing the signs and interpretation for the Storm King Fourteen Memorial trail, receiving the Conservation Service Award for my work building the 150-mile mountain bike trail from Grand Junction, Colorado to Moab, Utah...well, the list is long so I'll stop with, I've had a great time and I'm very grateful.


  1. Last summer in 3rd yr of holistic grazing I first noticed a very definite increase in patches of smooth Brome and Canadian T may also be expanding. Our cattle love both, and I attribute this development to our grazing program, which might be described as “mini-mob, relaxed Holistic version of Greg Judy’s” in style.
    This spring the Brome patches are even more prominent, and they get grazed down to 1″ within an hour or two Happily they are regenerating noticeably within days. If it weren’t for winter, I’d be worrying whether my Ky31 toxic fescue will get pushed out.
    After reading the above, it occurs that the story may be more complicated than first thought. As you know Cheat is a cousin of Smooth Brome. Moreover I noticed a patch of very lush Brome just outside our perimeter which has never been grazed. Maybe CO2 is the prime mover here too?

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