Herbicides can help warm-season weeds | News

Drought years allow virulent invaders to gain ground in pastures with previously healthy grass stands. Getting ahead of weed advances and eliminating weed seeds before they get into the ground may give weeds a strong start to 2024.

During a recent agricultural herbicide conference in Oklahoma, Oklahoma State University agronomist Brian Pugh presented some pasture herbicides that can make a difference in the off-season.

“For most of the warm-season issues we deal with, we would put it in mid-February or early March,” Pugh said. “We’re trying to catch it before that plant comes out of the ground. That’s where most (herbicides) work either before the seeds open and sprout, or right after.”
Some herbicides, such as glyphosate, only work on weeds that are already growing above the soil surface with no residual effects. Pre-emergent herbicides can help eliminate weeds before they begin to compete with more suitable grazing options and can therefore help strengthen a forage stand.

“That’s where the pre can come in and it can give us a roadblock that those weeds can’t get through,” Pugh said. “Maybe if you have swampy ground in May, we can put on prey in February or March and still pick up weeds in the mud. It’s hard to find a good day to apply spray in May between the weather and the wind and things like that. Plus, The neighbors are putting out tomatoes at that time. So that’s probably why we want to use a pre for sensitive crops like those tomatoes and soybeans and things like that. And if you don’t have any major problems with perennial wheat.


Pendimethalin, the main ingredient in Prowl herbicide, is a pre-emergence herbicide with many years of application in the turf industry.

“It’s a meristematic inhibitor, so it inhibits seedling growth by interfering with cell division,” Pugh said. “It’s used on very small plants. We won’t get activity on tall plants with pre-type herbicides. But it’s really good on a lot of annual turfgrasses and some broadleaf weeds.”

Most warm-season perennial grasses tolerate pendimethalene, including native grasses, Pugh said. Herbicides are usually tried on weeds such as sand.

“If you live in fairly sandy soil and get very heavy rainfall, it’s better to use that higher rate. Because we tend to see a lot of that moving out of the soil when we get more rain,” Pugh said. You put that first application in the dormant season, which would be a late winter application like the February and early March application, you’re going to need to split the applications to get optimal control of all those weeds like foxtail or crabgrass in the hay meadow.”

Pendimethalin does not contain pre-harvest intervals for grazing or hay on its label, and the product is labeled for good control of many grass weeds currently plaguing pastures. Pugh said it’s important to read the product label and make sure you follow the ordering rules on it to the letter.

“Rounding is just pre-rolling,” Pugh said. “Use it perpetually warm season or cool season.” “We can put it on top of the fescue, and it won’t hurt that fescue,” Pugh said. If you have a mix of Bermuda and fescue, it is usually best to use two applications.


Indaziflam, the active ingredient in Rezilon, is a pre-emergent herbicide with a different approach to blocking weeds.

“It’s a little different in the way it works compared to pendimethalin because it actually needs to be in the soil water that’s been imbibed or soaked in the sea before it germinates,” Pugh said. “So, if this plant has already sprouted a little, we won’t be able to control it very well even though it may not have come to the surface yet.”

Bermudagrass, native grass pasture, or hay producers may consider indazeflame-type herbicides as a viable option for weed control, but fescue producers will likely stay away.

“We can use it on any warm-season grass but it is very difficult on a lot of cool-season grasses,” Pugh said. “In fact, some of its claims to fame in the Southeast have been in Bermuda hay meadows to control ryegrass. It’s very effective on brome and ryegrass and things like that that are very toxic to them.”

Pugh said pre-emergent herbicides seem to be a great option for annual weeds such as sand burr and foxtail, but producers need to be aware of whether the variety they are fighting is a perennial or true annual.

“What we do know is that a large portion of our sand buzzards in Oklahoma are overwintering, so they are not true annuals,” Pugh said. “They’ve come back from the crown and now we’re having problems with the foxtail as well. We’re killing the yellow foxtail, we’re killing the green foxtail. No problem. But now we’re having a problem with the gnarled-rooted foxtail, which is a perennial.”

Knotroot foxtail returns from the root crown year after year. Pre-emergent herbicides don’t have a good effect on perennial weeds, which can contribute to growing foxtail problems in hay lawns, Pugh said.

“A lot of times, what we use and see success with is using three ounces in that spring period, say in February, followed by another three ounces in late May or so, and that gives us good, broad control,” Pugh said. “If that plant you really want to fight is brome or ryegrass, that plant has a lot of activity on those who live along the way. But that’s something you need to think about before you make that commitment. As long as you stay at the three ounce level or less, you’re “Good for high yields. There are no grazing restrictions on this.”

These are just two of many good options for controlling weeds in pastures. Reading labels diligently and deciding what is best for a particular scenario is the first step to success.

(Tags for translation)Botany

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