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5 Ways to Control Pasture Weeds

Identification is a start. After that, mowing, proper fertilizer, and herbicide applications all play a part.

Manage pasture weeds as aggressively as you do weeds in corn and soybeans, says Kevin Bradley, University of Missouri weed scientist. He offers the following five tips.

1. Know them.

Start by identifying your pasture weeds, says Bradley. “We have a smartphone app and a booklet to help,” he says. (Order the weed booklet or download the app at extension.missouri.edu/p/ipm1031.)

Over the past two summers, Bradley’s graduate student, Zach Trower, has walked across 46 Missouri pastures every 14 days to record weed species, estimate densities, and sample soil. “Every pasture had horse nettle, and almost every pasture had common ragweed,” says Bradley. “Ragweed had the highest density, at an average of over 5,000 per acre.”

Other common weeds identified were nutsedge, fleabane, yellow foxtail, and dandelion.

2. Mow them.

It may be a low-cost option to reduce weed populations, says Bradley. Some herbicide programs can cost up to $30 per acre, and mowing has the bonus benefit of little harm to desired forages. Make sure you mow weeds ahead of seed shedding.

One research project showed that ironweed, found in 72% of pastures in Trower’s survey, can be 80% to 90% controlled with three timely mowings per year over two years. “That’s good control,” says Bradley. “All you may have to do after that is some spot treatment.”

3. Fix fertility.

Trower’s survey found that 80% of the pastures were low or very low in soil phosphorus (P) and 37% were low in potassium (K). Average soil pH was 5.8, also very low.

The survey correlated pasture fertility to weed density. As fertility moved closer to ideal, weed density declined. Each one-unit increase in soil pH (going from 5.8 to 6.8 pH, for example) resulted in 4,100 fewer total weeds per acre, and 2,454 fewer common ragweed plants.

P and K level increases also resulted in fewer weeds.

4. Time herbicides correctly.

If you use herbicides to control pasture weeds, carefully think through the timing, says Bradley. For instance, metsulfuron-containing herbicides (Chaparral) can give excellent weed control but also can suppress tall fescue yield if used in the spring.

Weed-growth habits are also important, as herbicides are more effective when plants are small and actively growing.

For instance, 43% of pasture weeds are annual broadleaves, and they tend to peak in June, July, and August. Foxtails usually emerge in July and August.

5. Simplify.

“Identify the one weed you want to control the most and concentrate on it,” says Bradley. “You may just end up controlling some others in the process.”

Killer weeds

Some weeds need to be controlled because they’re poisonous to animals, says Kevin Bradley.

  • Perilla mint. Many animal deaths, at least in Missouri, from poison weeds involve this one. It tends to grow in shady areas and can be controlled easily with most pasture herbicides when actively growing.
  • Poison hemlock. It moves from roadside ditches into pastures, says Bradley. A low rate of ingestion can kill livestock. Several herbicide options, including Grazon and Remedy, can control hemlock.
  • Nodding spurge. It, too, is on the increase. Metsulfuron gives excellent control at emergence in July and August, and 2,4-D and dicamba give good control.
  • Horse nettle and thistles. Not poisonous, these prickly weeds are very undesirable nonetheless. Several chemical options will control horse nettle at the prebloom stage. Musk and bull thistles are best controlled at the early rosette stage, says Bradley.
  • Canada thistle. Mostly seen north of Missouri, it is best controlled with prebloom applications of Grazon, Milestone, or Tordon products, he says.
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