Research »   Weed Control

Weed Control

Weed control is an important aspect in forage and cattle production as it affects forage yield and quality. Weed management can be accomplished through cultural, mechanical, chemical and biological controls. A more effective and sustainable weed management program should integrate two or more of these methods and consider long-term impacts. Over 95% of the weed control in a healthy forage crop comes from the competition provided by the existing forage stand.


Weeds on Farmed Land

top 10 weeds in irrigated perennial crops alberta 2009In 2009, a survey that focused on weeds was conducted of common irrigated crops including annual cereals (spring wheat, barley, corn), annual broad-leaved crops (canola, sugar beets, dry beans, potatoes) and perennial crops (alfalfa, grass hay) in Alberta. For the 135 fields of irrigated perennial crops, the top 10 most abundant weeds were reported as: Dandelion, Canada thistle, Lamb's-quarters, Kochia, Green foxtail, Wild buckwheat, Redroot pigweed, Foxtail barley, Smooth brome, and Quack grass.

Weeds on Pasture

Other than plants that are poisonous to livestock, individual plants growing on a pasture are rarely a problem. However when large populations of a single species spread over a wide area, choking out and replacing the pasture grasses that provide nutrition to livestock, weeds become an economic issue.1

If weeds become a problem, they can compete or interfere for light, nutrients, water and space, directly influencing forage yield. A study by Chad and Bork (2004)2 assessed herbage yield losses within eight central Alberta pastures due to Canada thistle from 1999 to 2001. The study shows that yield losses due to Canadian thistle can be substantial, peaking at 2 kg/ha for each kilogram of standing thistle biomass and 4.3 kg/ha with each additional thistle stem per square meter. Demonstrated yield losses were variable among sites, likely due to factors such as heterogeneity in soils, available moisture, and variation in disturbance history or pasture vegetation composition. Research also shows that if enough weeds are present to cause a reduction in forage production, there is a gain of at least 1 pound of grass for each pound of weeds controlled. Some work shows a return of up to 7 pounds of grass for 1 pound of weed controlled (Donald et al.2011).3

Weeds and Harvested Forages

Since weeds are often harvested along with the forage crop, it can reduce forage quality by resulting in lower protein content and feed digestibility. Dandelions have nearly equal amounts of protein and total digestible nutrients as alfalfa, so control of dandelion may not improve the quality of hay, but it may increase the time necessary to dry the hay, since dandelion dries more slowly than alfalfa. Increased drying time may mean greater harvest losses due to untimely rainfall. Grassy weed quality can be similar to that of the forage. In general, weedy grasses have about 75% of the quality of alfalfa. However, controlling quackgrass in alfalfa can increase forage protein levels 4% to 7%. Weeds with woody stems or flower stalks, such as yellow rocket, white cockle, rough fleabane, curly dock, and broadleaved dock, have lower protein levels (about 50% of the quality of alfalfa), so controlling them is even more important.4

While most studies focus on weed control and its economic impact during the current growing season, K.G. Beck5 pointed out that weed management must be applied and evaluated over an extended time to be successful. It is important to develop a comprehensive weed management plan and incorporate that plan into a long-term land management program.

Methods of Weed Control

Weed control can be accomplished by cultural, mechanical, chemical and biological methods. Cultural controls are methods that suppress weed growth and production, while promoting the development of the desired plant, such as proper grazing management, irrigation, and seeding vigorously growing, competitive, desirable plant species. Mechanical controls manage weed populations through physical methods that remove, injure, kill, or make the growing conditions unfavorable, and include such methods as tillage, mowing, mulching and burning. Chemical controls are the use of herbicides. Biological controls are the use of an organism to disrupt weed growth. Classical biological control uses natural enemies of weeds, such as insects or disease organisms. Biological control also may include use of sheep, cattle, goats, or other large herbivores to control weeds. A good weed-management plan integrates two or more control measures into a management system.6

Cultural controls

Over 95% of the weed control in a healthy forage crop comes from the competition provided by the existing forage stand.7A good weed management strategy should focus first on cultural practices. According to Green et al. (2003)8 weed control is more critical during the first year than any other period of forage production as forage seedlings grow slowly and are easily overcome by rapidly growing weeds. The timing of seeding is also important. As a general rule, the summer complex of weeds tends to overcome spring seedings; whereas, the winter weed complex tends to outcompete forages seeded in the fall. Therefore, for optimum establishment of most forage crops, one should consider fall seedings in fields that have a history with such weeds as large crabgrass, foxtails, or lambs quarters. Spring seeding should be considered in fields that are potentially infested with common chickweed, henbit, and yellow rocket. In addition, using weed-free seeds will also prevent weeds in the first step. Proper fertility during the establishment phase and throughout the life of the forage stand is important to achieve a competitive forage stand that can suppress weed emergence and growth (Green et al. 2003). Nevertheless, increasing forage species diversity in pastures could also be an effective cultural control.  A study by Tracy et al. (2004)9 found consistent negative relationships between forage species diversity and weed abundance. The results suggest that maintaining both productive pasture communities (>150 g m−2 of above ground biomass) and an evenly distributed array of forage species should be combined to effectively reduce weed invasion.

Mechanical controls

Mowing or clipping can be an effective option for controlling some weeds. This removes leaves and lateral buds that develop new growth and helps to stop seed production. Annual broadleaf weeds have buds that develop above the soil surface; they are more easily controlled with clipping or mowing than grasses, which have crown buds near the soil surface (Green et al. 2003). Undesirable annual grasses should be mowed after the seed stalk has elongated, but prior to seed formation. Mowing perennial weeds once usually reduces seed production; repeated mowing reduces vigor and slows spread (Lemus & Weirich. 2010)10. In grazed forages, livestock often selectively graze and may leave particular weed species. Mowing soon after livestock have been removed from the field can help control these weeds and prevent seed production and further spread of infestations (Green et al. 2003).

Chemical controls

Herbicides control many annual, biennial, or perennial broadleaved weeds and woody plants in grass pasture. Chemical control costs may be lower than mechanical methods. Clary et al. 2008 compared the costs of mowing pastures with a 40-horsepower tractor and 6-foot rotary mower versus spraying pastures with the same 40-horsepower tractor pulling a 30-foot boom sprayer applying herbicide. The results show that herbicide and application totaled $11.57 per acre compared with $15.24 per acre for one mowing. However, it is important to note that once the weed has begun to flower, herbicide applications are much less effective. Therefore it is important to spray at the right time with the right rate. Generally speaking, the best time to spray weeds in new pasture is 4 - 8 weeks after sowing, prior to the first full grazing when the weeds are still small and there is about 70% or more ground cover. The herbicides should not damage the newly established clover while providing effective weed control. For established pasture, the best time to use herbicides is during fall, early winter or early spring when weeds are actively growing. The approach will depend on the targeted weeds species and sometimes, different applications throughout the year might be necessary. (Information on herbicide control on specific weed species can be found at Saskatchewan Agriculture’s website). A producer should consider price, method of application (spot spraying vs. broadcast), rates, and time of application. (A comparisons of herbicides used in forage legumes can be found in Green et al. 2003.)

Biological controls

Previous studies on weed control have generally focused on herbicides and mechanical methods. However, grazing can be a key cultural component to weed control within the forage stand. Olson (1999)11 described three grazing strategies for managing weeds:

      1. moderate grazing to minimize the physiological impact on desirable plants and reduce soil disturbance
      2. intensive grazing to counteract inherent dietary preferences of cattle such that the physical impact on weeds and desirable species is equal; and
      3. multi-species livestock grazing that distributes the impact of grazing more uniformly among all plant species.

Recent work shows that a high intensity, low frequency (HILF) grazing system can be used as a weed control tool. A study conducted by Sue et al. (2006) tested three cattle grazing systems - (1) continuous or season-long grazing (SL), (2) short duration (SD) (or low intensity-high frequency) rotational grazing, and (3) high intensity-low frequency (HILF) rotational grazing - for their ability to reduce Canadian thistle (CT) and release non-thistle herbage within permanent pastures of central Alberta, Canada. The results showed that season-long grazing maintained or increased severe CT infestations and reduced forage yield. In contrast, the HILF rotational system reduced CT shoot density and biomass, as well as flowering, and resulted in greater weed suppression than the short duration system. The implication is that prescribed grazing with an HILF system can be an effective biological control tool for CT, particularly in areas where other control options, including the use of herbicides, are not possible due to environmental restrictions or inaccessibility to equipment.

Bork et al. (2008) compared the HILF and continuous grazing systems’ effectiveness in reducing Canada thistle in the Aspen Parkland.12 Over a three-year test period of short duration, heavy grazing, the paddocks in the HILF grazing system had Canada thistle reduced to nearly zero, while under continuous grazing the Canada thistle continued to grow vigorously. However, it is not recommended that such intense heavy grazing treatments be applied every year because heavy spring grazing of the same field each year for many years will reduce the health and forage production of the desirable perennial species, the amount of forage will decline, and other weeds will invade the fields.

For classical biological weed control, a growing range of beneficial insects are becoming commercially available to control thistles and some other perennial weeds. A study by Stephen Crozier (2006) assessed the current effectiveness of biological control insects that had previously been introduced in Canada, focusing on field studies in Nova Scotia. Following up on insect biocontrol introductions carried out in the 1980s and 1990s, the study surveys of 18 insects on 10 target weed species. The five insects showing the greatest potential to control weeds were: Hadroplontus litura (a European weevil) for Canada thistle control; Urophora stylata (a gall fly) for bull thistle control; Longitarsus jacobaeae (a flea beetle) for Tansy ragwort control; Deloya guttata (a leaf beetle) for bindweed control and Galerucella calmariensis (a European beetle) for purple loosestrife control.13

Studies on using of fungus as a biological herbicide have also been conducted. Examples include using Phoma macrostoma to control for several broadleaved weeds and Colletotrichum truncatum for scentless chamomile. While biological controls can be effective in weed management, it is not always easy for producers to adopt these new technologies. It requires changing their current weed control plans and potentially an investment that has not historically been a part of their operation. However, significant advancements have been made in the selection of biological control agents, their mass-production and method of delivery that could make these options more attractive in the future (Boyetchko et al. 2009).14


Crozier, Stephen. 2006. Rearing, Release and Distribution of Insects for Biological Control of Pasture Weeds. accessed on May 26, 2014

Chad W. Grekul and Edward W. Bork (2004)  Herbage Yield Losses in Perennial Pasture Due to Canada Thistle (Cirsium arvense)1. Weed Technology: July 2004, Vol. 18, No. 3, pp. 784-794.

Donald J. Dorsett, 2011, Pasture Management for Beef Cattle Production,  accessed on May 26, 2014

Penn State Extension, Managing Weeds in Legumes  accessed on May 26, 2014

K.G. Beck Range, Pasture and Natural Area Weed Management, Colorado State University Extension Fact Sheet, 2014 on May 26, 2014

Olson, B.E., 1999. Grazing and weeds. In: Sheley, R.L., PetroV, J.K. (Eds.), Biology and Management of Noxious Rangeland Weeds. Oregon State University Press, Corvallis, OR, pp. 85–96.

Stephen Crozier, Rearing, Release and Distribution of Insects for Biological Control of Pasture Weeds, BCRC project 2006, accessed on May 26, 2014

Green, J. D., MV Marshall, and J. R. Martin. Weed Control of Alfalfa and Other Forage Legume Crops. University of Kentucky, US, Cooperative Extension Service, AGR-148, 2003.  http://www. ca. uky. edu/agc/pubs/agr/agr148/agr148. pdf  accessed on May 26, 2014

Tracy, Benjamin F., and Matt A. Sanderson. "Forage productivity, species evenness and weed invasion in pasture communities." Agriculture, ecosystems & environment 102.2 (2004): 175-183.

Rocky Lemus and Jason Weirich, Making Pasture Weed Control Decisions, Mississippi State University Extension Service, 2010. accessed on May 26, 2014

Olson, B.E., 1999. Grazing and weeds. In: Sheley, R.L., PetroV, J.K. (Eds.), Biology and Management of Noxious Rangeland Weeds. Oregon State University Press, Corvallis, OR, pp. 85–96.

Bailey, Arthur W., Duane McCartney, and Michael Peter Schellenberg. Management of Canadian prairie rangeland. Ottawa, Canada: Agriculture and Agri-Food Canada, 2010. Page 36

Stephen Crozier, Rearing, Release and Distribution of Insects for Biological Control of Pasture Weeds, BCRC project 2006, accessed on May 26, 2014

Boyetchko, Susan M., Karen L. Bailey, and Rosemarie A. De Clerck-Floate. "Current biological weed control agents-their adoption and future prospects." Prairie soils and crops scientific: perspectives for innovative management 2009 (2009).

Learn More:

Weed Control on Pasture
Ontario Ministry of Agriculture Food and Rural Affairs

Pasture Production
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This topic was last revised on March 9, 2016 at 08:03 AM.

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