By Trevor Lennox, PAg, Range Management Extension Specialist, Swift Current
June 2024
Canada thistle can be a problem on pastures, particularly in high rainfall areas such as the black soil zone. It is a prolific seed producer, has a very aggressive creeping root system and can out-compete favourable pasture species resulting in significant forage yield declines.
pastures.
Can pasture management impact the amount of Canada thistle in a pasture?
Several years ago, researchers in Alberta looked at three different cattle grazing systems and their impact on Canada thistle: season long grazing, low intensity-high frequency rotational grazing and high intensity-low frequency rotational grazing. The four sites of the experiment were in the black soil zone. Predominant grasses were smooth brome, Kentucky bluegrass and quackgrass.
Season long grazing allowed severe Canada thistle infestations to consistently reach advanced maturity stages of flowering and seed set. Repeated grazing of palatable grass patches further helped Canada thistle to invade or increase in these patches.
Low intensity-high frequency treatments grazed the herbage to about six inches within two to three days. The experimental paddocks were then rested for four to six weeks until the grass growth had approximately doubled and then regrazing occurred. Suppression of Canada thistle was not significantly different than season long grazing. Grasses were more competitive, but it was not enough of an advantage to achieve more than a partial reduction in Canada thistle. Cattle avoided Canada thistle at this conservative grazing pressure.
High intensity-low frequency rotational grazing treatments in the experimental paddocks kept the cattle grazing until the Canada thistle was heavily impacted, regardless of how much of the desirable forage was consumed. As a result, most herbage was grazed to about three quarters of an inch. There was an extended recovery period of eight to nine weeks between grazing periods. This system reduced Canada thistle flowering, shoot density and biomass. Two intense defoliations, annually over two to three years, nearly eliminated Canada thistle stems either due to trampling or eating by the animals. It appeared intense competition for forage during a short time forced cattle to utilize the Canada thistle, averaging 72 per cent defoliation. This reduction remained after the experiment when the electric fences were removed from the experimental paddocks and the season-long grazing system returned.
This grazing project demonstrated how pasture management can influence cattle grazing activities to reduce their selectivity for specific plants within a paddock. Obviously, there will be situations where grazing pressure alone will not reduce Canada thistle and additional tools such as herbicides may also need to be used to help reduce this weed on pastures. To discuss possible solutions to reducing weeds such as Canada thistle on your pastureland, reach out to your local range management extension specialist or call the Agriculture Knowledge Centre at 1-866-457-2377.