Kansas State University researchers will soon wrap up a four-year study that has shown positive results in controlling a noxious weed common to the state’s pastures.

KC Olson, a beef cattle scientist for K-State Research and Extension, said the incidence of sericea lespedeza has fallen dramatically in research plots where the pasture was burned in August or September, as opposed to the normal practice of burning pastures in April.

“We chose those off-season fire timings for a couple reasons,” Olson said. “We thought that August or September prescribed burning would be compatible with intensive early stocking, as is practiced in the Flint Hills.

“And we were looking for a point of vulnerability with sericea lespedeza. Many people know that it flowers and sets its seed in about mid-August and completes that process in late October. Weedy plants are usually most susceptible to control at that point in their maturation.”

The researchers hit the mark on that hunch. After just one year of burning, sericea lespedeza failed to make more seed. After the third year of burning last year, the number of plants in the field fell dramatically, according to Olson.

An analysis completed in mid-July shows that the incidence of sericea lespedeza in pastures burned in April is about 8 percent of the basal cover. That drops severely for pastures burned in August (3½ percent basal cover) and September (1½ percent basal cover).

“That’s just from fire,” Olson said. “We have not used any herbicide.”

He added that the research fields even pass the eye test: “You can go out and look at those plots today, without having any prior knowledge of what treatment has been applied, and you can visibly see the difference.”

Sericea lespedeza was classified as a noxious weed in Kansas in the late 1990s. The plant is known to out-compete native plants for water and nutrients, and it contains high levels of condensed tannins that make it undesirable for cattle grazing the land – which means they eat less.

Tannins are naturally-occurring compounds that bind protein in the gut of ruminant animals, preventing the animal from digesting protein.

Aside from controlling sericea lespedeza in pastures, Olson said that Kansas State University’s research shows that late-season burning is not harmful to the state’s tallgrass varieties, nor to bare soil or the field’s overall plant cover. Two plants that ranchers struggle with – Baldwin’s ironweed and western ragweed – also seem to be weakened due to late summer burning.

“I’ve talked to many people over the last year that are at a point of desperation dealing with sericea,” Olson said. “They’re tired of repeated herbicide applications, and the expense and the time that goes into that. Burning is a whole lot more inexpensive option and a whole lot more time effective option, but you can’t expect miracles with one year of treatment. It’s the result of a change in management and will take about three repeated applications for a satisfying effect.”

Burning in the summer, “takes some of the smoke out of the month of April; some of our neighbors might like that,” Olson said. And, he said, it could free up some time for ranchers during spring, which is typically a very busy time.

“With our August burn treatment, we’re giving them the capability to create a much more nutritious forage base, that’s based on post-fire regrowth going into fall and the winter months,” Olson said.

Olson said future work could include studying the effects of fall burning on livestock performance.