Treat pastures for biennial thistles now, not during bolting or flowering. Minnesota has nearly one million acres in pastureland providing forage to livestock throughout the growing season. Some of the more problematic weeds for NW Minnesota pastures are plumeless, musk, and bull thistle (Figure 1). These plants are not consumed by cattle and the thorny plant structures quickly train animals to avoid heavily infested areas. The result can be areas of pasture left relatively ungrazed. This lost forage productivity is unnecessary.

Biennial thistles are noxious and invasive weeds located statewide and commonly found in pastures, CRP, and wastelands. Management of these species must focus on reducing seed production in the attempt for long term control. Unlike Canada thistle, they are relatively easy to control with herbicides. Our research has indicated that the timing of herbicide applications can commonly be more significant then the thistle herbicide selected. Table 1. Illustrates the effect of properly and poorly timed herbicide applications for the control of plumeless thistle. Applying herbicides in May while the 1st and 2nd year plants are still in the vegetative (rosette) stage (Figure 2), has provided greater than 90% control. Waiting until 2nd year plants transition into reproductive growth (bolting), places much more importance on the herbicide selected and rate applied. A factor that often influences control decisions is the desire to preserve legumes in mixed grass/legume pastures. This is a difficult problem. Each of the herbicides and rates we evaluated and found to be effective on plumeless thistle also eliminated legumes from the pasture.

Mowing has traditionally been used to remove flowers and seed heads. Most mowing efforts have brought landowners into compliance with state weed laws but have proven cosmetic as apposed to effective in overall control. Biennial thistles are reproductive from late May through October with seeds becoming viable seven to eight days post flowering. Mowing can work if cutting heights are used to remove the terminal growing points and repeated throughout the season as lateral branches develop in response to mowing. Once bolting occurs and prior to flowering, a near soil surface cutting can be effective (especially so for bull thistle).

While immediate control of weeds is commonly our goal, often it is merely a band aid. Considerable thought must also continue in an attempt to correct the factors leading to the original infestation. In a pasture system, this includes management to incorporate proper soil fertility, maintaining the presence of desirable plants, and managing animal grazing pressure to insure a competitive natural system. Management of biennial thistles needs to be thought of as a longer term process not as a single event. Part of the overall strategy should include 1) a plan on treating the infested areas over a three to five year time frame to wear down the seed bank, 2) treat small patches of thistles before they are able to spread, and 3) avoid overgrazing. Grazing restrictions of selected herbicides, labeled for pastures, are listed in Table 2.

Carlyle Holen, IPM specialist and
Doug Holen Regional Ext Educator—Crops