Managing Weeds in Pastures

Friday, August 22, 2025

‘If You Don’t Change Management, The Same Weed Problem Will Come Back.’

By Emily Mckinlay

Maintaining productive pastures is key to many livestock producers’ operations. Weeds can limit the yield and quality of forage crops and can be more difficult to control if the problem is ignored for too long.

“Weed pressure can decrease the productivity of forage crops,” says Sheila Taillon, range management extension specialist for the Government of Saskatchewan’s Ministry of Agriculture.

“Weeds often have early emergence, giving them a competitive advantage for resources like water, space, and nutrients.”

Christine O’Reilly, a forage and grazing specialist with the Ontario Ministry of Agriculture, Food and Agribusiness, believes that managing forages to be competitive and avoiding bare soil is the basis of weed prevention.

“Bare soil enables weed seeds to germinate. Usually in established pasture, it’s the grazing management that has created a situation where there is bare soil,” says O’Reilly.

“This occurs often in high-traffic areas, or it could be the result of overgrazing. It could be from pugging damage, if there was wet spell and livestock were in a field when the ground can’t carry them. These can all cause bare soil and that’s a space for weed seeds to germinate.”

How can management changes avoid the losses in pasture productivity caused by weeds?

Limiting productivity

There are a few ways that weeds can limit the yield and quality of forage crops.

“Weeds can reduce yield either by being low-yielding themselves or by being unpalatable and taking up space,” says O’Reilly.

“Dandelions are palatable but take up space. They don’t yield very well because they just have a little rosette, so if a field was completely taken over, it would be fairly low-yielding.

“There are other weeds that livestock don’t eat. One example would be thistles, especially when they are big and mature and stemmy. They take up a lot of space and are not contributing to the yield of pasture because livestock don’t eat them.”

Both low-yielding and unpalatable weeds use up resources and compete for space with more productive pasture species.

Taillon says that some weeds may also release compounds that are harmful to pasture species.

“This is called allelopathy. These weeds release compounds that limit the growth of other plants around them. This allelopathic property can further help the weed compete in pastures,” explains Taillon.

In some cases, weeds can also be toxic, causing illness, reduced performance, or death in livestock. Taillon says that in the Prairies, leafy spurge is one example of a weed that can cause diarrhea in livestock.

O’Reilly notes that cases of toxicity are more common when livestock run out of pasture.

“Plant poisoning generally only occurs if livestock run out of pasture and are not promptly given supplemental forage,” says O’Reilly.

“They might get hungry enough to try something they shouldn’t. If you keep an eye on how much forage is available for your animals to eat, you can generally avoid that challenge.”

To avoid the costly limitations caused by weeds, preventative management is the best option for eliminating weed challenges.

Prevention

The strategies used to manage weed challenges in pastures before they start often also lead to a healthier pasture stand.

Taillon believes that a healthy pasture is a more resilient pasture.

“Prevention is best way to reduce the harm from weeds,” says Taillon.

“Keep pastures healthy. If we haven’t been taking care of ourselves, we have a harder time fighting off illness. It’s the same with pasture. If it’s not healthy, it is more susceptible to having weeds in that condition.”

O’Reilly says that producers should identify areas in their management that give weeds an opportunity to take hold in a pasture.

“Producers should be grazing in a way that encourages grass to tiller and keeps the stand thick, to make sure there’s not space for weeds to encroach,” says O’Reilly.

She recommends starting grazing when pasture is around 18 inches tall and letting it be grazed down to four inches.

“If the pasture grows taller than a foot and a half, the lower leaves won’t get sunlight and the pasture stand can actually start to thin out,” explains O’Reilly.

“You want to utilize as much sunlight as possible to produce the sugar that the plants need.”

Preventing areas with bare soil maximizes ground cover for increased plant photosynthesis, optimizing the available resources and avoiding areas for weed seeds to germinate.

“We often see bare soil in high-traffic areas. There are a few different strategies for preventing having so much traffic that it starts to cause a problem,” says O’Reilly.

O’Reilly lists five recommendations for reducing animal traffic and minimizing bare soil.

1. Rotational grazing: As livestock are moved around, the areas they like to congregate in will shift.


2. Moving landscape features that livestock like to congregate around: This would include water sources, mineral feeders, and gates. If they have more than one gate and can change where they enter and exit the pastures, that will change traffic in those areas.


3. Alternatively, you can build up the ground around high-traffic areas to prevent bare soil: Producers can use gravel or a concrete pad if they can’t move high-traffic features. How can you engineer the area so that it won’t become weedy?


4. Maintain soil fertility: Take a soil test and get the necessary nutrients on the field. If pastures have enough nutrients, pasture plants can often outcompete a lot of weeds.


5. Reduce overgrazing: Overgrazing puts stress on plants. Reduce overgrazing to help keep pasture plants alive and productive. Don’t leave your livestock somewhere for too long and don’t bring them back too soon.

“If you can keep soil covered with species that are productive and palatable, you can often keep weeds out successfully,” concludes O’Reilly.

Taillon suggests that producers also consider how their equipment might introduce new weeds.

“Keep equipment clean so weed seeds are not transported into your pastures,” recommends Taillon.

If the weed problem has progressed beyond prevention, producers can also use management to get a patch or field of problematic weeds under control.

Dealing with problem weeds

Weeds that are already present in a pasture inhibit the productivity of desirable pasture species. Fortunately, one of the most convenient management options is grazing.

“Grazing can be a helpful tool to manage weeds,” says Taillon.

“Grazing management, such as rotational grazing, can help provide pasture with time to rest and regrow, keeping it healthy and resistant to weeds. Producers can also use targeted grazing. This would use a specific type of livestock, at a specific time of year or season, at a set duration and intensity for a defined goal.”

As an example, Taillon says that sheep and goats have been used to control patches of leafy spurge.

In addition to grazing, Taillon lists four other types of weed management: Cultural, mechanical, biological, and chemical.

Cultural management includes measures of prevention such as keeping equipment clean to avoid weed seed transport.

Mechanical management could involve hand-pulling smaller patches or using a mower on larger patches.

O’Reilly adds that the efficiency of this method will depend on the life-cycle of the weed.

“Mowing tends to work best on annual weeds, particularly if they are mowed after they have flowered and before they have set seed,” says O’Reilly.

“At this time, annuals have put all their resources into growing and flowering and are unlikely to regrow. If they are mowed before they set seed, they are not adding more weed seed into the seed bank.

“Mowing perennial weeds takes a lot more patience. It doesn’t tend to work as well because they have large root systems with greater energy reserves to help them regrow. For perennials, mowing can help, but is not as effective as mowing and a secondary strategy.”

O’Reilly says that tillage is another means of mechanical management, but it should be confirmed that the weed doesn’t spread through root fragments.

As a method of biological management, Taillon says that insects like beetles have occasionally been used to manage patches of leafy spurge.

Chemical management of weeds in pastures typically involves herbicides.

“You have to look for a product labelled for use on the weed species in your pasture. There can be a couple challenges with using herbicides in pasture,” says O’Reilly.

“The main challenge is that most selective herbicides are either grass safe or safe for broadleaf plants, including legumes. If you have a broadleaf weed and have legumes in your pasture, your herbicide will also take out trefoil, clover, and other legumes.

“If you go the herbicide route, you may need to overseed with the species you want that the herbicide took out, and sometimes herbicide residual can last a long time.”

An integrated weed management system, using two or more options, offers a higher chance of success in managing a weed challenge.

O’Reilly cautions that if control measures are successful, producers should still review their management to identify and address the source of the problem.

“If you only mow, use tillage, or apply herbicide and don’t change management, you will have the same weed problem come back within the next few years,” says O’Reilly.

“Management change might involve changing your rotations by shortening your grazing period or lengthening rest time or changing the time of year your livestock are in that paddock.”

Good grazing management can help minimize challenges with weeds in pastures, keeping your forages healthy and resilient. BF