Dry conditions over the past month have reduced weed growth, so fall pasture herbicide application plans may need to be changed, says Mike Moechnig, South Dakota State University Extension weed specialist.

"Since herbicides generally work by inhibiting normal plant growth processes in such a way that the plant's own toxic chemicals cause its death, any environmental factors that slow plant growth will also reduce its susceptibility to herbicides," he says.

Canada thistle has been particularly nasty this year, but the dry conditions may reduce its susceptibility to herbicides.

"It’s difficult to speculate what the effect may be, but perhaps control could be 10-20% less than a spring application," Moechnig says. "Higher herbicide rates and the use of surfactants may improve control slightly, but likely not enough to completely negate the drought effects."

He recommends using higher rates and surfactants if applications are still planned for this fall. Using a potent herbicide such as Milestone or ForeFront could minimize reductions in control compared to a general-maintenance herbicide such as Grazon or 2,4-D.

To assess the potential for good control yet this fall, growers should look for two things:

Regrowth: “The success of fall applications is very dependent on fall thistle growth. We have been monitoring Canada thistle shoot emergence at one location in Brookings and have not seen many new shoots emerge since the end of August. Therefore, the combination of dry conditions and thick grass canopies could inhibit fall herbicide activity. Areas mowed or grazed earlier this year provide the best opportunity for fall thistle growth,” Moechnig says.

Condition of thistle leaves: “Herbicides may be effective as long as the thistle leaves are still green. Frost has been light so far in many areas, so older thistle growth is still somewhat green in many areas, which will allow herbicide uptake even if the fall growth is limited. In general, Canada thistle control will begin to decline as thistle leaves become more brown from frost and drought. However, some of the more potent herbicides will have some soil activity so you will likely get some control (about 50%) regardless of the thistle shoot conditions.”

If landowners have the flexibility to wait until next spring or summer, they will likely get better Canada thistle control for each dollar spent, he adds.

"In late June, soil moisture is often more plentiful and Canada thistle plants are often taller than the grass canopy, making them better targets for herbicide applications. Generally, it’s best to control Canada thistle in mid- to late June – just prior to seed production – but some herbicides may also be effective anytime in July or August."

Dry conditions may affect control of other pasture weeds as well. Landowners may see more emergence of biennial thistles, such as musk and bull thistle, in the spring if dry conditions this fall inhibited normal fall germination, Moechnig says.

"In my opinion, fall applications of herbicides such as Milestone, ForeFront or Tordon/Grazon will not provide much residual activity to control spring-emerging biennial thistles. Absinth wormwood control may be fine yet since it has such a large taproot. We had great wormwood control this year from late-fall applications last year, so there may be plenty of time to wait on the absinth wormwood if people would like to wait for more moisture.”