Identifying and treating weed infestations on native rangelands improve productivity

One of the questions University of Nebraska Range and Forage Specialist Mitch Stephenson gets quite often is what is and isn’t considered a weed. 

Stephenson tells producers they should consider any plant growing where it is unwanted or a plant having a negative value within a management system as a weed. 

Identifying weeds

Noxious, native and exotic plants, as well as poisonous plants, can all be considered weeds, he says. 

Noxious weeds are designated by the agricultural authority as one that is injurious to agricultural or horticultural crops, natural habitats, ecosystems, humans or livestock, he says.

Poisonous plants are “plants containing naturally occurring toxic compounds that when consumed by livestock, can cause biochemical or physiological changes resulting in death or reduced performance.”

Native weeds can alter the landscape in a way that is undesirable in terms of forage production, wildlife habitat, aesthetics or ecosystem function. Exotic weeds originate outside North America and are often highly invasive and out-compete native vegetation.

Defense

Stephenson tells producers the best defense for controlling weeds is developing a good offense, especially if it is done before the plants put on a seed set. 

“I would create a good strategy and define what my objectives are. It will help control my time and costs,” he explains. “Monitoring results is also important.”

He recommends recording changes over time, in pasture and rangeland, with photos, instead of relying on the eye to remember.

Noxious weeds, like Leafy Spurge, musk and Canadian thistle, Eastern Red Cedar and Diffuse and Spotted knapweed should also be controlled. 

Stephenson says some management options are herbicides, mechanical methods like mowing or cutting it down, fire, targeted grazing and bio-control. 

Using herbicides

“Herbicides can provide the most direct and complete control of invasive species,” Stephenson explains. He also tells producers about the “Nebraska Herbicide Guide,” which provides an overview of what herbicides are available and most effective. 

He says recommendations of when to spray may differ based on whether the weed is an annual or perennial. Producers should also keep in mind that most herbicides have a zero- to 14-day grazing restriction. 

“A lot of times, we treat when we can, but to get the most bang for the buck, we should treat when its most effective,” he says.

Limiting seed set

Mowing or cutting can also be effective to limit the seed set, Stephenson says. 

“It is most beneficial when applied with other treatments, like mowing, just before the seed sets in late spring, and then spraying herbicide later in the fall,” he says. 

If the plant is a perennial, meaning it has a lifespan of more than three years, management differs, as well as recommendations of when to spray and how to control it. Stephenson says the most effective time to control biennials is when they are in the rosette stage, which is late fall or early spring. 

Some native plants are able to compete with noxious weeds, as long as moisture is adequate. In the case of the native forb western ragweed, Stephenson says one study showed, as the density increased from five to 41 tillers, scientists observed more grass had developed in north central Oklahoma. 

Another study shows western ragweed would need to produce more than 1,000 tillers an acre to affect grass production on the clay uplands in Kansas. 

“Western ragweed is not a strong competitor in the presence of vigorous grasses,” he says.

The problem with using herbicides on forbs like western ragweed is most herbicides are non-specific, Stephenson explains. 

“If we spray western ragweed, it can also impact other native forbs. Annual forbs can serve an important purpose. They provide ground cover and allow grass to establish,” he notes. 

If producers are using herbicides to control invasive plants, Stephenson recommends focusing on localized areas or spot spraying to avoid non-targeted species.

Target grazing 

Even well-managed rangelands can have some species of weeds invade them, he says. One of the most common is cheatgrass or Downy brome. 

“Cheatgrass actually has high nutritional value early in the growing season,” Stephenson says, noting a current study underway looking at how often cattle will select cheatgrass early in the growing season, and if native perennials are harmed by trying to graze the cheatgrass before it matures. “We want to see how much they select cheatgrass, compared to native perennial forage.” 

In the first year of the study, the cattle were turned out April 20 to start grazing, and then they stopped for about a week. “

There was a cold snap, the cheatgrass stopped growing, and they had already grazed it out.

  “Then, we hit a flush and the amount of cheatgrass in their diet was substantial,” he says. 

The ongoing study will continue this spring.

Targeted grazing can be key to controlling some invasive species, like cheatgrass and leafy spurge.

“The key is picking the right animal,” Stephenson says. 

Producers also need to consider what season of grazing will give them the most benefit, as well as duration and intensity. Target grazing can benefit forage quality for livestock, since many weeds are high in nutrient quality, he says. Targeted grazing can also offset some of the cost by adding value. 

He explains, “We may have to stock those areas pretty heavily, because, in a case like cheatgrass, it can get away from us if you don’t have enough head to keep up with it.”

Gayle Smith is a correspondent for the Wyoming Livestock Roundup. Send comments on this article to roundup@wylr.net.