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How to Manage Cattle to Reduce Leafy Spurge, Spotted Knapweed and Canada Thistle

Editors Note: This is the second in a series on managing grazing to get the forage you want. (Here’s the first article if you missed it.) Folks are sharing questions with us, and we’re going to find some experts with answers. In the meantime, Kathy shares what she’s learned about the first three weeds she taught cattle to eat when she was developing the training process at Grant-Kohrs Ranch National Historic Site in Deer Lodge, Montana.

Once animals are grazing a target weed, our goals for that weed along with the time and money we have will determine how we manage our animals’ grazing.

ChangesinPlantPalatabilityandSusceptibility“Timing” and “Intensity” of grazing are our primary tools for managing vegetation.  Timing of grazing means choosing the time of year when the target weed is most susceptible to damage from grazing and preferred species are least susceptible.  As the picture below notes, a plant’s palatability and susceptibility to grazing changes over the growing season with both palatability and susceptibility to control decreasing after seed set (Launchbaugh 2006).  Since palatability is based on the nutritional value, it is obvious that we will have best luck with grazing when the plant’s nutritional value is higher.  By paying attention to the growth stage of the target weed, we can pick a time before seed set to graze for maximum impact.

Intensity describes the frequency, or how often a weed is grazed, and the stocking density used when targeting a weed.  We know that grazing a plant more than once during a grazing season can reduce its vigor and its population over time.  Intensity of grazing is also increased when using a higher number of animals (a heavier stocking density), resulting in higher impact on the target weed.

The Fewer Weeds You Want, the Harder You Have to Work

NoneofthisworksifyourcowsarenteducatedEven with proper timing and intensity, eradication of an invasive species is unlikely based on our past experience.  In spite of concerted efforts over the last 50 years, weed populations continue to grow at about 14% per year. However, our historic experience with reductions in native grasses throughout the west, along with more recent targeted weed grazing research indicates that a plant can be severely reduced with grazing.  How rapidly and to what degree we want to reduce a weed’s population will determine how we manage our animals.

If speed and near eradication is the goal, we must spend a great deal of time and money managing our livestock.  Since herbivores do not eat every part of every plant unless they are confined or forage is limited, and because rapidly reducing an invasive species population requires that every target plant be consumed, the closer we want to come to eradication, the more time and money we will need to spend on managing our grazing animals.  Animals must be focused in smaller pastures, we must move them to new pastures to prevent damage to preferred species, and then move them back again to hit regrowth of the target weed.

The balance required between protecting preferred species and adequately injuring the target weed is difficult to achieve and requires daily and even hourly attention to changes in the pastures being grazed.  My suggestion would be to keep in mind that these plants are great forage, and so, in moderation, they can be a useful part of your pasture. So instead of working yourself to death to eliminate them from your pasture, take a slower approach with a longer-term commitment to the process.  You’ll reduce your labor costs, your cattle will gain weight on an inexpensive forage, you risk less damage to preferred species, and if you do make a mistake, it will be less dramatic and easier to recover from.

Here are suggestions for developing a targeted grazing program to address the first three weeds I trained cattle to eat at Grant Kohrs Ranch National Historic Site in Deer Lodge Montana.  These plants are all high in protein which makes them a tasty, beneficial forage if your cattle have learned to eat them.

Leafy Spurge (Euphorbia esula)

Researchers recommend that 95% of top growth be removed.  After the first treatment, regrowth should be regrazed.  Grazing can occur in the growing and flowering stages. In moist areas, leafy spurge may be more resilient, so it may require more intense management.

Work with sheep indicates that animals first learning to eat leafy spurge may prefer younger plants. However, when I’ve worked with cattle, they ate the plant well in August.  Variety seemed to be the limiting factor in how much leafy spurge cattle would choose to eat. The more variety, the more spurge they ate. When their only choices were grass or leafy spurge, they ate much less leafy spurge.

Potential Effectiveness

Grazing by sheep and goats has been very effective at reducing biomass on an annual basis when leafy spurge is moderately to severely grazed. We should be able to expect similar results from cattle that have been trained or have learned from herd mates to eat the weed.

Researchers note that grazing effectiveness can appear to be low after the first year because plants might produce a flush of new growth the spring of the following year.  The pictures here are from a grazing project in the Deer Lodge Valley in Montana, showing that after four years of consecutive grazing, this high density infestation of leafy spurge was suppressed (Launchbaugh 2007).



Spotted Knapweed (Centaurea maculosa)

Reducing the number of seeds a plant puts out is our goal anytime we’re trying to get rid of it and in the case of Spotted Knapweed, scientists have spent a lot of time trying to figure out how to reduce the volume of seeds it produces. A 2008 clipping study (Benzl) found that clipping during bolting stage reduced viable seeds by nearly 90% compared with no clipping, and that removing 100% of flowers at full bloom reduced seed viability by 100%.  Unfortunately there is a great deal of difference between clipping and grazing.  The clipper will always remove 100% of the flowers regardless of plant palatability.  The cow, on the other hand, will never remove 100% of the flowers.

The 2007 Launchbaugh study below indicates that target weed palatability is maximized, while impact on other species is minimized in July.  Animals choose more of the target weed, and less of the grass during this month than the months of June and July.   This coincides with what I observed in 2004 when the herd first grazed in trial pasture.  Cattle in pasture grazed every knapweed plant to some degree, but all tops were bitten off.  It is also what we would expect based on crude protein values of pre-flowering spotted knapweed (as much as 18%) and flowering knapweed (6.6 percent).  Keep in mind that cattle require a minimum of 8% protein to function adequately.


Jim Gerrish has turned to grazing to manage spotted knapweed in his pastures as he describes in this On Pasture article. Click!
Jim Gerrish has turned to grazing to manage spotted knapweed in his pastures as he describes in this On Pasture article. Click!

Based on this information, I weigh the probability of reducing viable seeds by nearly 90% based on my animals’ willingness to eat all bolting plants, versus the lesser likelihood of achieving 100% seed reduction when asking my animals to graze a plant that does not meet their nutritional needs.  I also keep in mind impacts on desirable species. For me, this means that grazing Spotted Knapweed in late June and July is my best bet. That is not to say that grazing at this time doesn’t have it’s challenges.  While our first year of grazing in trial pastures at Grant Kohrs Ranch in 2004 showed that grazing the grasses to 4 to 5 inches produced no negative effects, changes in precipitation can and will affect grass response.  Thus, it is critical for range managers to take this into consideration when deciding timing and duration of grazing in any area.

Canada Thistle (Cirsium arvense)

CanadaThistleEasiestToEatCanada thistle is a perennial plant that has a vigorous and spreading root system making it more difficult to control whether with grazing or herbicide.  From another perspective, that makes it an especially good forage because it’s very nutritious and extremely resilient.  Still, you don’t want just Canada thistle in your pasture, so to reduce the quantity, graze it early and often.

Researchers suggest beginning grazing when rosettes first appear, but since these are quite low to the ground, livestock may have difficulty removing rosettes.  Our work at Grant-Kohrs Ranch and at 6 different ranches in the Ruby and Madison Valleys in 2008 demonstrated that trained cattle will graze this plant throughout the grazing season, that they will eat stemmier plants with equal gusto, and that they will graze it to the same height as the grass in the pasture.  Ranchers in other areas have indicated that this kind of repeated grazing has eradicated Canada thistle from their pastures. Thus, animals should be moved from a pasture when necessary to protect preferred species, and returning to regraze new Canada thistle sprouts if grazing will not harm preferred species.

Want more?  Here you go:

Benzel, K.R. 2008. Defoliation effects on spotted knapweed seeds production and viability. Thesis for Master of Science in Animal and Range Sciences, Montana State University, Bozeman, MT.

Launchbaugh, K.L. 2006.  Targeted Grazing:  A Natural approach to  Vegetation Management and Landscape Enhancement.  An American Sheep Institute Publication.  Available online at:

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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.

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