- 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.
- 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.
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.
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.
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 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
|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.
The best way to prevent and control existing weed infestations is to promote the desirable species in a forage stand. Soil sampling on pastureland is just as important as it is in cropland. Sampling can help you reach and maintain proper soil pH and fertility levels to avoid over application or under application of soil amendments. Over application will not infinitely increase forage yield, and under application will decrease profitability per acre.
If the forage legumes and grasses can get enough nutrients from the soil to be at their maximum performance, weedy species have a difficult time infiltrating the pasture. Weeds are opportunists -- they will take advantage of open spots in the pasture very quickly.
Conditions such as low pH, low nutrient levels, and poor drainage can affect the prevalence of weeds. Select a plant that is more competitive in that environment or alter the conditions to benefit the forages.
For long-lasting results, use proper grazing practices to promote forage species. Concentrating livestock at some stages of growth and giving pastures rest periods is the key to grazing-based weed control. A poor grazing plan can undermine even the best weed control strategy.
How much residue should you leave? A plant’s leaves are its solar panels. Plants need some leaves left so they can photosynthesize to produce energy for regrowth. Removing all the leaves by overgrazing results in the plant drawing carbohydrates out of the roots. Over time, that leads to decreased plant vigor and a thinning of the stand. Thin stands create the opportunity for weeds.
Ideally, leave the grazing residue height at 4 inches. It is recommended to “graze half and leave half.” In environmentally stressful conditions, leave more residue to minimize the stress on the plants.
Proper stocking rates depend on the species, number of mature and young animals, forage composition and availability, and weed populations.
Short duration with high stocking rates is called “mob grazing.” As grazing pressure increases, livestock begin to be less selective. This results in animals eating more weeds that they normally would not graze, regardless of forage quality. Carefully manage this type of grazing so as not to defoliate the forages too frequently or aggressively.
In continuously grazed pastures, grazing intensity is high since there is no limit to the amount of time a plant can be grazed. This practice increases the abundance of undesirable plants, as intense grazing selects for the plants that can handle the frequent defoliation and regrowth. If given the choice, livestock will continue to graze the most palatable species and avoid the undesirable weeds. Typically, that selects for resilient weeds over forage grasses and legumes.
Rotational grazing is the practice of grazing one portion of the pasture at a time, giving the other paddocks time to rest. This allows the forages to build up carbohydrate reserves, instead of depleting them as in continuous pastures.
The specific length of time to allow a pasture to rest needs is based on forage species and environmental conditions.
Suggested rest periods for commonly grazed forage species
|Species||Suggested rest time|
|Smooth bromegrass||20-30 days|
|Tall fescue||20-30 days|
Differences in livestock species
Cattle, sheep, goats and horses are the most common animals used for grazing pasture.
- Cattle prefer grasses and legumes and tend to avoid shrubs and forbs.
- Horses are usually much more selective than cattle and they can pick out the tenderest new growth easily.
- Sheep tend to consume forbs over grasses and shrubs.
- Goats can control many of the woody shrubs that cattle, horses and sheep will not touch.
- Restricting the pastures to only one class of livestock can lead to increases of weed abundance.
Thin forage stands do not thicken once weeds are removed. Assess the stand to determine if there are sufficient desirable species to fill in the gaps. If weeds make up 50% or greater of the stand, it is a good time to renovate the pasture.
Interseeding or overseeding are practices that aim to plant species into a pasture without using tillage to prepare a seedbed. Interseeding with a no-till drill or overseeding by broadcasting seed over a freshly grazed stand can be ways to incorporate more forage plants without completely tilling up the pasture and reseeding.
Complete renovations by tilling and reseeding can help start the pasture with a near weed-free environment.
The goal of mechanical weed removal, as is true for any of the options, is to eliminate seed production. This is the key to prevent future re-infestations.
Mowing temporarily removes the top growth of the weed as well as the top growth of the forages, which can be a disadvantage or advantage. Frequent mowing can hinder the forage plants, but moderation can promote the regrowth of tender new forage stems.
This method requires a commitment to repeated mowing at the right times. Ideally, mowing should begin when the erect plants are in the stem elongation stage, which usually means the plant is about 12 to 18 inches tall. However, you should mow the plants before flowering or seed set because there is little benefit to mowing after the seed is mature.
Perennials can spread by rhizomes and stolons, thus a single mowing is not an effective strategy. Essentially, the goal is to prevent seed set and exhaust the carbohydrate storage in the roots, so the weeds become weak.
Herbicides, when properly used, can be a convenient, economical and effective weed control method. There are many herbicide options available to control weedy species, so it is important to know the various types of herbicides and their characteristics.
A great resource for herbicide applicators is the herbicide label. Always read the label before using an herbicide product. It provides information on the plants it can control, the plants that won’t be affected, rate guidance, and safety information. The herbicide label is always the final authority on herbicide uses.
When using any herbicide in a pasture, consider the beneficial plant species. Unfortunately, there is a lack of herbicide options for stands consisting of legumes due to the potential for the desirable legume species to be injured or killed. For example, does the pasture have a large stand of clovers with few broadleaf weeds in it? Perhaps the use of an herbicide would do more harm than good if the objective is to maintain the clover stand.
When selecting an herbicide, consider the type of weed. Is the problem weed an annual, biennial, herbaceous perennial or woody perennial? Is it a broadleaf or grass plant?
The age and size of the plant will also determine the rate to be applied and the herbicide’s effectiveness.
Weed predominance is described by the percentage the weed takes of the total stand space. If the weeds are uniformly distributed throughout the stand at an unacceptable percentage, broadcast spraying is the most efficient method. With hot spots, where the weeds are concentrated in areas, spot treatment is a more economical use of the product.
Be aware of the crops surrounding the pasture or fence line. Herbicide drift or volatilization can move the chemical off-target, especially in the cases of herbicides with high vapor pressure.
Proper timing is important when planning to apply an herbicide. Newly seeded pastures can also be sensitive to herbicides, even if the mature plant is tolerant to the herbicide.
In the case of annual broadleaf weeds, control is more effective earlier in the season when the weed is small.
Treat perennials with an herbicide in the late summer or fall. During that period, the weeds are storing food reserves for winter. An herbicide that can be translocated will travel down to the roots, which is the goal of controlling perennials.
Take temperature into account before spraying. Plants are most susceptible to herbicide toxicity when they are actively photosynthesizing, so make sure to wait for at least 60°F temperatures a few days before and after application.
When applying herbicide on pastures, you must wait the required time before allowing livestock to graze. The herbicide label provides a time interval DAA (Days After Application) that limits a producer from using forage during that time frame. Generally, the concern is not that the livestock would become ill from the herbicide compounds; but animals can retain the chemicals in their systems. Always follow grazing restrictions according to the label.
Herbicides can vary in the amount of time they are active in the soil.
- Some herbicides, like 2,4-D, have a very short residual of just a few days.
- Tordon (active ingredient is picloram) can leave a soil residual of up to 2 years.
Keep this in mind in case there is a plan to interseed legumes into the pasture or rotate to a different crop.
Chose herbicides based on the goals of the weed control application.
- Selective herbicides inhibit the growth of a specific group of weeds (grasses or broadleaves) while leaving the other plants unharmed.
- A non-selective herbicide kills all plants it contacts.
- Once absorbed into the plant, systemic herbicides can translocate throughout the plant in the xylem or phloem, affecting all plant parts.
- A contact herbicide does not move after it is absorbed into the plant and only affects the plant tissue it contacts. This is essentially a way to chemically “mow” perennials.
Common pasture herbicides
|Cimarron Max||Metsulfuron + Dicamba + 2,4-D|
|Cimarron Plus||Metsulfuron + Chlorsulfuron|
|Crossbow||Triclopyr + 2,4-D|
|Curtail||Clopyralid + 2,4-D|
|ForeFront R&P||Aminopyralid + 2,4-D|
|Garlon 3A and 4||Triclopyr|
|Grazon Next*||Aminopyralid + 2,4-D|
|Grazon P+D*||Picloram + 2,4-D|
|Weedmaster||Dicamba + 2,4-D|
*denotes restricted use products
Note: Stinger and Curtail have been shown to leave residual compounds in manure spread from treated pastures into susceptible crop fields. Read the label to address these concerns.
Many herbicides are pre-mixed, meaning they consist of more than one active ingredient. Including more ingredients that are active broadens the range of susceptible plants that could be killed by a single herbicide.
Learning how to apply herbicides safely is the best way to avoid unintended consequences.
- Make sure to wear all PPE (personal protective equipment) that is required by the label.
- Take care when mixing concentrated chemicals, as that is the action with the greatest risk to the applicator.
- Remove livestock from the area and keep them out for the period of time recorded on the label.
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.
Mechanical methods for brush control include chain saws, brush cutters, and bulldozer or skid steer buckets. Chain saws can selectively remove brush while larger equipment like bulldozers can unintentionally create excessive soil disturbance.
One disadvantage of mechanical brush control options is the likelihood of re-sprouting from cut stumps. To prevent this regrowth, apply a translocated herbicide to the cut stumps.
Another way to remove brush from a fence line is to kill the brush chemically first before removing it. This method ensures the death of the plant roots and allows for easy removal. Similar to other perennials, the most successful time of year to apply herbicides to woody perennials is the late summer.
Commonly used herbicide treatments to control woody plants:
- Require complete coverage of the plant foliage to be effective.
- When spraying large plants, there is an increased risk of the herbicide contacting sensitive vegetation compared to other methods.
- Apply before cutting the brush.
- Useful for trees that are too tall for foliage spraying.
- Mix an oil-soluble herbicide (check the label) with an oil carrier and apply to the lower 12 inches of stems and trunks.
- This method is most effective on woody plants that are less than 6 inches in diameter.
- Be sure to remove the tree before it falls on the fence.
- Can prevent re-sprouting after mechanical removal of the woody perennial.
- Apply herbicide shortly after cutting the plant.
- Paint these compounds onto the stump surface or apply with a sprayer.
- For larger trees, only apply the herbicide directly underneath the bark, in an area called the cambium.
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.
Reviewed in 2021