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Plan Now to Control Weeds With Grazing Next Season

Updated November 09, 2020
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Pete Bauman

SDSU Extension Range Field Specialist

Exposed soil in pastureland with an emerging Canada thistle problem.
Figure 1. Canada thistle infestation on exposed soil in a pasture.

South Dakota Invasive Plants

Canada goldenrod is a native flowering plant in South Dakota, but it is generally considered as a grazing ‘increaser’ and is relatively undesirable by landowners at high densities. Canada thistle and perennial sow thistle are both common noxious weeds in South Dakota requiring that managers control the production and spread of seed.

S.D. Distribution Maps:

    Watering area near pastureland with trampled grass and exposed soil.
    Figure 2. Areas that concentrate livestock impacts, such as water points and gates, can often result in weed infestations.

    In all three cases, poor grazing management, such as season-long grazing or heavy soil impacts from livestock, are often the culprit for infestations to start or persist (Figure 1 and Figure 2). A common assumption is that chemical application is the only solution to weed issues stemming from a lack of understanding of the interaction of grazing and plant biology. The use of targeted grazing can help control these plants in pastures once managers understand the importance of timing, intensity, and livestock habits.

    Livestock will graze Canada goldenrod, Canada thistle, and perennial sow thistle, and the plants are nutritious at certain times of the year. A review of several reports and our own research and observations confirm that at certain times these plants have crude protein, total digestible nutrients, and invitro dry matter digestibility concentrations similar to alfalfa and other common forages.

      When and How to Graze These Plants in South Dakota

      Mature cow eating Canada goldenrod in a pasture.
      Figure 3. A mature cow consumes Canada goldenrod in SDSU Extension trial.

      Canada Goldenrod

      Our work here at SDSU Extension suggests that Canada goldenrod plants contain high nutritive value, with the tops of the plants consistently similar to alfalfa. We found that we can train cows to eat Canada goldenrod and that mature cows with calves naturally utilize Canada in early to mid-June, nipping plant tops. This use is nearly imperceptible as the plant continues to grow and flower after the bud is nipped off, leading many to assume cattle do not utilize the plant. A small high intensity – short duration trial also confirmed that cattle will forage on goldenrod extensively under restricted grazing in August, but it is unclear as to the long-term impact on the goldenrod plant community (Figure 3). Grazing managers should target goldenrod plants in early to mid-June, prior to the onset of flowering (usually late July and August).

       

        Bar graph displaying Canada thistle response to grazing systems to three different grazing systems.
        Figure 4. Canada thistle response to grazing systems in Alberta. SL = Season Long; SD = Short Duration; HILF = High Intensity – Low Frequency. At all sites, HILF resulted in the best overall control of Canada thistle. Adapted from Bruijn and Bork (2006).

        Canada Thistle

        Nutritive value of Canada thistle is perhaps the most well documented of the three plants discussed here. The primary issue for Canada thistle management is when and how to target the plant.

        Research from Alberta compared three grazing systems for Canada thistle control: 1) season long, 2) low intensity - high frequency, and 3) high intensity - low frequency (Figure 4).

        They found that season long grazing where livestock are turned out and not rotated or managed resulted in increased Canada thistle populations and reduced overall forage yield.

          A Canada thistle plant with missing leaves and stem due to grazing.
          Figure 5. Canada thistle grazed in mid-June.

          Conversely, high intensity - low frequency grazing reduced Canada thistle shoot density, biomass, and flowering and resulted in greater weed suppression.

          Two ‘intense’ defoliations of Canada thistle during the growing season for 2 to 3 years in succession dramatically reduced the Canada thistle population, and the plants that remained stayed vegetative (did not flower) and had higher forage quality. This system proved better for Canada thistle control than did low intensity – high frequency (Figure 4 and Figure 5).

          Short Duration – High Intensity Grazing

          Short duration - high intensity grazing can control Canada thistle over time. The near portion of the grazing cell in Figure 6 is likely impacted too much, and it may result in additional weed issues due to exposed soil and lack of residual vegetation.

            Pasture managed with short duration - high intensity grazing.
            Figure 6. Pasture managed with short duration - high intensity grazing.
            A group of cattle resting in pasture. They appear uninterested in the Canada thistle growing throughout.
            Figure 7. Cattle may avoid use of Canada thistle in September, even though it would appear plants are green and palatable.

            General Observations

            As a grassland manager, I have spent years manipulating and observing livestock for Canada thistle control, and have come to a few general observations that are also supported in reports:

            • Canada thistle has the potential to invade anywhere there is exposed soil. Minimizing livestock soil damage is key in preventing new infestations.
            • Mature cows will forage on Canada thistle buds in mid-June and may consume a great deal of the plant at certain times and for about 7 days between about June 10 and July 1. However, the exact timing of this period of more intensive use is not always predictable in this three-week period.
            Left: A yearling heifer grazes on Canada thistle after a mid-October snowfall. Right: A mature thistle grazed in mid-October.
            Figure 8. A) A yearling heifer grazes on Canada thistle after a mid-October snowfall. B) A mature thistle grazed in mid-October.
            • Cows will teach calves to forage on thistle and other plants.
            • Yearling cattle will learn to utilize Canada thistle if given the opportunity.
            • The key to Canada thistle control with livestock is to first stop the grazing practices that promote thistle expansion (season long grazing, heavy impacts to soils). The second step is to concentrate animals for high-intensity-short duration grazing during bud stage before thistle plants flower.
            • Finally, I’ve also observed that livestock often do not re-graze Canada thistle in September, even though the plants might appear green and palatable (Figure 7). Interestingly, yearlings turned out into a stockpiled pasture in early October were observed to select both dead and decadent and younger green Canada thistle plants, presumably for their higher protein and nutrition content compared to surrounding forages (Figures 8).

            Perennial Sow Thistle

            Perennial sow thistle is very palatable and is will be selected by cattle if they are exposed to the plant at the correct time of the year. Largely, this information is based on observation over 10 years with different groups of yearling cattle. I’ve observed consistent targeting of perennial sow thistle from late July through mid-August during the bud and flowering stage of the plant. My observations suggest that yearlings (and likely cows) will generally avoid the plant if grazing in a pasture with perennial sow thistle before this time. And, if they do forage on the plant in early July, the plant may continue to grow and flower. However, if targeted in the late July to mid-August time period, livestock often consume the entire plant, not just the flowers, and thus there is very little opportunity for the plant to rebound, especially if repeated for several years in a row. Targeted grazing may be necessary if there are large infestations, but generally cattle appear to seek out the plant during this period. Finally, once flowers start maturing, grazing selection drops off dramatically.

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