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Weed management for pasture and fence line systems

Quick facts

  • Weeds reduce the yield, quality, and stand life of desirable forage plants.
  • Early control is key: small weed populations are much easier to control than large, established infestations.
  • Proper identification of the weed species and lifecycle will determine the best control strategy.
  • The key to long-term success is the promotion of beneficial species.
  • Controlling brush around fence lines will prolong the life of the fence and retain space for grazing.
  • A combination of grazing animals, herbicides, and mechanical control methods can remove weeds effectively and efficiently.

What is a weed?

A weed is any plant that is not wanted in its location, meaning any annual, biennial or perennial plant could be considered a weed.

It is important to remember there are several plant species considered weeds in row crop systems, but their characteristics are welcome in a grazing system, such as those with high grazing tolerance and nutritional quality. These types of weeds are the exception, not the rule.

Importance of weed control in pastures

Persistent weed infestations in pastures can:

  • Reduce the quantity of forage left available for grazing.
  • Decrease the stand life of desirable forage species.
  • Impair the functionality of fence lines.
  • Negatively affect nutritional quality and palatability.

You need to control plant species that are detrimental to the goals of your grazing operations. Certain weed species are potentially poisonous to grazing livestock. Often, undesirable plants are very aggressive and compete with forage stands for resources (light, water and nutrients). For both of these reasons, weed control in newly renovated or established pastures is a worthwhile use of time and effort.

Effect of weeds on forage yield and quality

The effect of weeds on a pasture is highly dependent on the species of weed, its competitive characteristics, the level of infestation, and the condition of the established forages.

In general, weeds are not as high-yielding as our selected forage species and can potentially reduce the overall quality of the forage provided by the pasture. The lower protein content and lower digestibility of common pasture weeds can cause reductions in forage quality and can lead to decreased intake by grazing animals.

Economic impact of weeds

In the United States, weeds in pasture and rangeland systems cause an estimated loss of $2 billion dollars annually. Controlling weed infestations that have spread throughout pastures is time-consuming and costly. As a weed population expands, stocking rates decline and land values can potentially drop. Fewer animals being fed per acre decreases productivity.

Early control is key

A small weed problem can be easy to ignore. But the costs to control them will only increase as the weed population spreads.

Frequent observation, early identification, and early control will minimize production losses and will ultimately decrease the negative economic impact.

Toxicity issues with poisonous plants

Some plants may be poisonous if consumed by livestock. The level of toxicity is dependent on the grazing animal species. Sensitivity to these toxins can also vary by the age of the animal and pregnancy.

Before allowing livestock to graze an unknown plant, identify the plant and research the anti-nutritional factors including toxicities. Some plants can accumulate excessive nitrate during times of plant stress like droughts, increasing the potential for nitrate toxicity.

Plant toxins can lead to abortion, weight loss, reduced animal performance, and even death. Consult a veterinarian if there are concerns.

Often, animals will not consume poisonous or toxic weeds if given a choice. In areas where there is little or no desirable forage, livestock may resort to eating these plants. Make sure proper forage is available for livestock to consume to limit the exposure to toxic plants.

Noxious weeds

Noxious weeds are harmful plants designated by the Minnesota Department of Agriculture to be detrimental to human or animal health or the environment. The two main designations are Prohibited and Restricted.

Minnesota regulates prohibited noxious weeds and assigns them to the Eradicate list or the Control list.

  • Plants on the eradicate list are not widely established in Minnesota, but the removal of above and below-ground plant parts is required.
  • Plants on the control list are established in Minnesota and must be controlled to prevent further reproduction.

Visit the Minnesota Department of Agriculture site to identify and report noxious weeds.

How weeds are introduced to a pasture

Weeds can be introduced into a pasture through many activities:

  • Purchasing feed or seed that contains weed seeds.
  • Seed distribution by wind or wildlife.
  • Manure spread on the pasture from another livestock owner’s herds.
  • Livestock sent to rental pastures that contain weeds may bring back weed seeds in their manure to the home pastures.
  • Purchasing new animals.

Being aware of how weeds can be imported from elsewhere can help producers take preventative measures.

Are livestock coming back from a weedy pasture? Try keeping them contained in a sacrifice area before allowing them to graze on your home pastures, and don’t spread the manure from that pasture.

Prevention is typically an easier task than treatment after weeds are established.

Life cycles of weeds

After identifying a weed species, identify the weed’s life cycle to effectively manage a weed population. It can help you make important and well-informed weed control decisions.

The timing for most effective weed control measures depends on:

  • Knowing when a plant is going to produce seed, and
  • If or when it is storing carbohydrates in the roots for overwintering.

Weed management guidelines based on lifecycle type

These species-specific characteristics will determine the best timing for weed control.
Lifecycle type Mechanical control Chemical control
Winter annuals Mow after the plants are tall enough to prevent seed production. Apply herbicide in the fall or early spring before seed set.
Summer annuals Mow when weeds are tall enough to be affected and before seed production. Apply herbicide in spring or early summer.
Biennials Mow after bolting but before seed set. Apply herbicides to rosettes in the spring or fall.
Herbaceous perennials Mow frequently to suppress and prevent seed production. Many can spread by seeds or rhizomes. Apply a systemic herbicide at bud to bloom or in the early fall.
Woody perennials Remove by cutting or remove by roots with equipment. Apply a systemic herbicide in the early fall.

Integrated weed management

The most effective weed management plans draw from a variety of weed control options. An integrated weed management program combines cultural, mechanical and chemical methods. This approach is adaptive and uses different management tactics to effectively control weeds with lower input costs.

Using only one method time after time and not analyzing all the options can lead to a waste of time, money and effort. Complete pasture management systems will include many management practices to be successful.

Considerations for selecting methods of weed control:

  • Current forage composition.
  • Intended species composition objectives.
  • Current and future forage needs.

Brush and fence lines

Fence lines that are maintained with limited weeds and brush (woody perennials) last significantly longer than those where the brush is allowed to grow. Brush can degrade fence functionality by shorting out an electric fence and pulling, stretching, and otherwise deforming non-electrified fence.


Common misconceptions about pasture weed management

Herbicides are the only way to control weeds effectively.

Herbicides are one of the many methods for preventing and removing weed populations. Using integrated pest management practices is the most effective weed management plan.

Applying non-selective herbicides on a fence line is the best way to chemically control weeds.

Using non-selective herbicides like Glyphosate or Glufosinate kill both broadleaf and grass plants. This controls the weeds for a short period, but eliminating all plants along a fence line opens the soil up for more seeds to establish. Consider using a selective herbicide to control broadleaf weeds and brush while keeping the beneficial grasses.

Mowing weeds is the cheapest way to control weeds.

Factor in the cost of labor, fuel, maintenance, and equipment depreciation when determining the total cost of a weed control treatment. The frequency and timeliness of mowing are critical for long-term control. On average, mowing costs $15 per acre per time, and since you should mow weeds two to three times a season over two or more years, it could cost upwards of $60 an acre.

If the livestock eat it, it must be fine.

Many annual weeds emerge early in the spring, which can provide early spring grazing for producers. However, as the summer progresses, the palatability of these annual weeds declines. During the spring, the annual weeds may have outcompeted the perennial forage and now those species are limited when they are most needed.

Author: Tarah Young, Extension educator, agriculture production systems, Carlton County

Reviewed in 2021

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