Controlling Weeds in the Fall
Fall weed control can contribute to better pasture growth next year; it depends, though, on which weeds, and how many of them, are growing in your pasture.
“Some perennial broadleaf weeds can be controlled pretty well in the fall before the plants start going dormant,” Dr. John Jennings, professor of animal science at the University of Arkansas, told Ozarks Farm & Neighbor. “However, summer annual weeds like ragweed, pigweed and woolly croton all die in the fall so attempts at weed control in the fall really don’t do any good, and the herbicides that you apply wouldn’t have any effect on next year’s weed growth.”
Even on the perennials, herbicides will be less effective if the weeds are so stressed they can’t take up the chemicals. It’s also valuable to conduct a weed inventory before spraying; Jennings said if there are just a few scattered weeds, an herbicide application will only help keep your fields clean. If there’s a tremendous weed encroachment, though, it can make a difference.”
To conduct an inventory, walk a line across the field. Stop at every fourth or fifth step, and record a tally mark on the sheet next to the category corresponding to what is at the end of your right toe, whether it is a grass, legume, weed or bare ground. Continue walking across the field until you collect at least 50 and preferably 100 such data points. Record the data for each pasture on different tally sheets.
“Our rule of thumb is if it’s 20 percent weeds, you can do some control,” Jennings said.
University of Missouri Extension Forage Agronomist Tim Schnakenberg said fall is a good time to address biennials like thistle, poison hemlock (carrotweed) and spotted knapweed.
“Usually, the biennials will be in a rosette stage starting in October, so they’re very susceptible to herbicides,” he said. “These species can also be sprayed in the early spring. Schnakenberg said, “If we have a mild winter, sometimes there will be biennials that will continue to germinate throughout the winter. If you wait until early spring and spray the rosettes then, you can probably even catch a few more.”
It’s particularly important to catch buckbrush (coralberry) in April, when its leaves are just emerging, because the weed can be very difficult to control in the summer.
As of mid-October, producers were still applying brush control. Schnakenberg said MU research has found blackberry control is enhanced two to three weeks prior to the first frost.
“We’ve also found that some brushier species, like oaks, persimmons and locusts, can be sprayed as long as their leaves are green and healthy,” he said. “When they yellow or turn shades or red and start to die, we’re not going to get anything done…The rule of thumb for most brush species is to spray when the leaves are fully expanded and new; the plant needs to be happy and healthy, which means it’s had some rain recently, and it’s still in a young vegetative stage.”
Winter annuals and some of the thistles start to emerge in the fall, and Jennings said they should be controlled from late October into November and early December.
“If we can kill them while they’re still small we can have a good effect on next spring’s production, “ he said. “Most people don’t get excited about trying to control those until next March, but March is usually too windy or too muddy so they can’t get into the field.” You can also apply a lower rate of herbicide when the weeds are just emerging in the late fall.