Weed Control in Ontario Horse Pastures


by Joel Bagg, Forage Specialist, Jack Kyle, Pasture Specialist, & Melanie Beech, OMAF and MRA

Controlling weeds in horse pasture can be challenging. Weeds grow well in poor, overgrazed horse pastures, so good pasture management (sacrifice paddocks, rotation, mowing, fertility, drainage, etc) goes a long way in controlling weeds. Overgrazing thins the grass stand and allows weeds to establish. When horses are turned out on to pastures too early in the spring, significant damage to wet soils and desirable pasture plants results. The weeds that become established are usually less palatable than forage grass species and therefore proliferate and can eventually dominate the pasture. Proper fertility, pH, drainage and other good agronomic practices also encourage vigorous forage grass growth that will compete well with weeds. Undesirable weeds reduce pasture growth available for horses. Some potentially poisonous weed species can result in horse health risks. (“Managing Your Horse Pastures” https://fieldcropnews.com/?p=7695

A good working knowledge of common horse pasture weeds is useful. Control options for the various horse pasture weeds depend on the characteristics of the specific problem weeds. These characteristics include whether the weeds are broadleaves or grasses, plant growth habits (annual, biennial, or perennial), reproduction (by seed, rhizomes, etc), and susceptibility to either mowing or herbicides. Being able to identify common horse pasture weeds is important, including those that are potentially poisonous or harmful. Useful internet tools to help identify weeds include www.weedinfo.ca.

Plant Growth Habits

  • Annual  weeds germinate, flower, produce seeds and die in one growing season. Examples include mustard, lady’s thumb, black medic, and ragweed. 
  • Winter annuals  germinate and produce a low, leafy rosette in the fall. The following year they form a stem, flower, produce seeds and die. Examples include stinkweed and shepherd’s purse.
  • Biennials germinate and produce a low, leafy rosette during their first growing season. The following year they form a stem, flower, produce seeds and die. Examples include wild carrot, burdock and bull thistle.
  • Perennials (herbaceous)  live a number of years, developing each year from underground roots, crowns or stems. They flower, produce seed, and then die back to the ground each winter. Some reproduce by these underground roots as well as set seed. Examples of herbaceous perennials include quack grass, Canada thistle, and dandelions.
  • Perennials (woody)  are weedy trees and shrubs that live for many years. These include European buckthorn and climbing nightshade.

Control measures for annuals, winter annuals and biennials are generally designed to eliminate the seed head or to destroy the plant before it goes to seed, and to minimize their competition to productive pasture species. Mowing and herbicides are the methods used. Eradication is challenging because weed seeds lie dormant in the soil without germinating for many years until conditions are favourable.

Perennial weed control is especially difficult because it is not only necessary to prevent their spread by seeds, but the perennials are persistent, and can sometimes also spread by underground root systems.


Regular mowing is very important in managing healthy, productive horse pastures. Access to mowing equipment is essential to good horse pasture management. Effective mowing removes seed heads before weeds develop viable seed. This prevents reproduction in species that reproduce only by seed, but is much less effective in perennial species that can reproduce by underground roots and rhizomes. Low growing, prostrate weeds are difficult to control by mowing because they can grow and flower below the mowing height. Mowing at a height of 4 – 6 inches is a good strategy to control many weeds, “even out” grass growth, and encourage the tillering of forage grass species to thicken the stand. Mowing should be done a minimum of 2 – 3 times per year. In rotational grazing situations, mowing should be done after each grazing rotation when the horses have been moved to another paddock (at the start of the rest period).

Herbicide Control

Refer to Chapter 10 of the “Guide To Weed Control”, (OMAFRA Publication 75, pp. 177 -179, 188 – 192 http://www.omafra.gov.on.ca/english/crops/pub75/pub75toc.htm), for pasture herbicide options. Be sure to consult the product labels for directions, including the period of time to keep livestock out of treated areas.

Herbicides used to control broadleaf pasture weeds include 2,4-D, dicamba, and MCPA. These herbicides will also injure or eliminate broadleaf legumes, such as white clover and alfalfa, so they should be limited to grass pastures. If a small amount of white clover is desired, it could be re-established by broadcasting.  Annual weeds are usually best controlled by spraying when smaller in size and actively growing in the spring. Biennials should be sprayed in early fall to control first-year growth, or in late spring to control second-year growth. Most perennials are better controlled with these herbicides when sprayed in late-spring (end of May to mid-June) when the weeds are actively growing. For more specific timing, refer to the “Guide to Weed Control”. More than one herbicide application may be necessary to control difficult weed species. Small-boom 12 volt sprayers are commercially available that are suitable for smaller horse paddocks and gates, and can be used with an ATV or small tractor.

Spot spraying of glyphosate with a back pack sprayer can be done to control thistles, burdock, milkweed, etc. Thistles and milkweed are best sprayed in the bud to full-bloom stage. Deep-rooted perennials (such as Canada thistle) are more susceptible to fall treatments because as they move nutrients into their root systems for winter, the herbicide moves with them. Glyphosate is non-selective, so will also kill desirable forage species. Where larger patches are inadvertently killed with glyphosate, weeds tend to germinate and regrow in these bare areas.

Ontario farmers must be certified through the Grower Pesticide Safety Course to purchase and use Class 2 and 3 Pesticides (which includes 2,4-D, dicamba, and MCPA) on their own farms. For information on certification, refer to www.opep.ca/.  Commercial custom spraying on someone else’s property requires that it be done with a Pesticide Exterminator Licence. (http://www.ontariopesticide.com/index.cfm/home-page/

Weeds Potentially Poisonous To Horses

There are hundreds of weeds that are potentially harmful or poisonous to horses. Naturally occurring plant toxins putting horses at risk include various alkaloids (ergot), glycosides, cyanide, mycotoxins (produced by fungi) and many others. Plant poisonings are dose dependant and can be very difficult to diagnose. Symptoms can be acute and severe, or chronic and very subtle. Horses are at the greatest risk of plant-induced photosensivity when grazing poor pastures that contain a lot of weeds. Most poisonous plants are not very palatable to horses, but do get consumed when there is little else to graze. Feed hay in a sacrifice paddock when necessary to prevent overgrazing. Be sure to keep pasture grasses, such as orchardgrass, before they become infected with ergot at the heading stage. Fencerows may contain red maple, wild cherry or other potentially poisonous shrubs and trees. www.omafra.gov.on.ca/english/livestock/horses/forages.html



Common Pasture Weeds

  1.     Annuals

Black Medic


Common Mallow


Lady’s Thumb




Prostrate Knotweed




Common Ragweed


Lamb’s Quarters


Wild Mustard



2.     Biennial Weeds

Sheppard’s Purse


Stinkweed (Field Pennycress)




Common Burdock


Common Mullein


Canada Fleabane


Bull Thistle


Nodding Thistle (Musk Thistle)


Scotch Thistle


White Cockle


Wild Carrot (Queen Anne’s Lace)



3.    Perennial Weeds

Smooth Bedstraw


Canada Thistle


Curled Dock




Daisy, Ox-Eye


Field Bindweed




Mouse-eared Chickweed


Plantain (Broad-leaf and Narrow-leaf)


Tufted Vetch





4.   Potentially Poisonous Weeds

Alsike Clover





Common Milkweed


Hoary Alyssum


Poison Hemlock


St. John’s Wort