Identification is the first step for controlling weeds in pastures

DUBUQUE, Iowa — Weeds don’t always impact pastures and animal performance.

“Weeds can impact desirable forage quality, quantity, palatability, animal heath and performance, but there are a lot of factors that determine what that impact is,” said Mark Renz, professor of agronomy and weed specialist at University of Wisconsin-Madison.

“One of the big factors is the weed species,” said Renz during a presentation at the Driftless Region Beef Conference, presented by University of Illinois Extension, Iowa State University Extension and Outreach and UW-Madison Division of Extension.

“Dandelions don’t have any impact on the yield of pastures or utilization. However, brush is highly detrimental to forage quality, so the impact is specie specific,” he said.

The impact on pastures also varies by how the pastures are grazed.

“Continuous grazing, rotational grazing or high intensity grazing can really change the impact on weeds,” Renz said.

Brush is a very different story compared to weeds.

“We should have a low tolerance of brush because it severely impacts forage availability,” Renz said. “Try to keep brush below 5% cover — if you take 20 steps, you shouldn’t be standing next to a shrub.”

Annual broadleaf weeds such as pigweed and lambsquarters have OK forage quality if the animals eat them when they are vegetative.

“That’s when the forage quality is good to great,” the weed scientist said. “Once they start getting stemy and they develop a flower head, the forage quality goes down and they become unpalatable.”

Annual grass weeds are not desirable in pastures, Renz said.

“They have lower forage quality, less protein and more fiber, but they don’t produce a lot of biomass, so we don’t worry about them too much,” he said.

“The goal for weeds should be to keep them to a minimal component of our pasture,” he said. “We don’t want them dominant — 15% or less is the point where they won’t impact forage quality.”

Weeds impact utilization and palatability of pastures.

“Depending on the system, we can get reductions in utilization greater than 72%, but that is highly dependent on the weed species that are there, the stage of development and the grazing patterns,” Renz said.

In a continuous grazed system almost none of the Canada thistle is eaten by cattle.

“But in a rotationally grazed system, our data suggests one-third to three-quarters of the Canada thistle is eaten by animals,” Renz said.

“The utilization will vary based on the time of the season,” he said. “In the spring, they eat less when the stems are present, and later in the summer when it is not as spiny, there is pretty good utilization and the forage quality sometimes goes up.”

Graziers need to be concerned about animal health and toxic plants.

“We also need to be aware of plants that can injure animals like foxtails that can cause injury to eyes,” Renz said.

“We don’t have a lot of toxic plants in the Midwest and most of the plants are low in palatability,” he said. “Jimsonweed is very bitter and not palatable. Wild carrot and wild parsnip have some mild toxicity issues, but a large percentage of forage consumed needs to have these plants for the animals to show symptoms.”

Poisoning can occur when there are no other forages available or when there is a drought.

“When you apply herbicides sometimes that increases the palatability so keep the animals off the pasture for a while after application,” Renz said. “When you make a nitrogen application, some annual weeds like lambsquarters can accumulate nitrogen and that can cause nitrogen poisoning.”

Never put yard clippings or waste in a pasture, Renz said.

“When moving animals into a new pasture, keep an eye on them because they may eat something that causes problems,” he said.

If there are weeds in a pasture, Renz said, the first step is to identify the weed species.

Once the weeds are identified, the next step is to monitor the pasture and determine if the animals are grazing the weeds.

“If the weeds are not toxic and they are being used as part of the forage, maybe you shouldn’t spend the time managing them,” Renz said. “If the weeds are not being eaten and they are expanding, that’s when you should manage them.”

Weed management is much more important in continuously grazed systems.

“Animals have a choice of what they can eat so they will always choose desirable clovers and avoid weeds,” Renz said.

“Maximize grazing for weed control by using rotational grazing,” he said.

“Make sure the cattle graze the pasture prior to the weeds producing viable seeds and if during rotational grazing your cattle are not eating a weed species, consider increasing the stocking rate,” he said.

It is important to avoid overgrazing because weeds will take advantage of bare soil.

“Many weeds need light to germinate so we need to avoid overgrazing particularly in the spring because that’s when most of our weeds are germinating,” Renz said.

“We recommend leaving residual forage of six inches or more in the fall because it reduces the following spring germination of many of the weed seeds,” he said.

“Mowing is a good option to prevent seed production and that works better on annuals and biennials, but it is not that effective on perennials,” he said.