Tackle weeds in pastures, forages


To manage forage weed control, you have to know what factors contributed to weed establishment in the first place, before spraying.

If hay and pasture management does not change, it is likely the same situation will arise later.

Once management issues are addressed, you could opt to use herbicides, but it’s important to remember that each herbicide varies in terms of target weed response, and in their ability to kill specific weeds.

For this reason, it is important to correctly identify problem weeds, and always refer to product labels prior to use.

Timing is everything

Annual weeds are best controlled during the seedling and early vegetative stages. As annuals mature tissues harden, and the plant becomes less responsive to herbicides.

Biennial weeds are best controlled during the seedling and rosette stage. When biennials bolt, control can be significantly reduced.

Perennial weeds are best controlled during the early-bud stage, which is two weeks before flowering. This is when sugar direction is moving toward root structures; there is adequate leaf area to take in herbicide, and the plants are at their lowest energy level.

Remember, the brush hog or grazing can be tools to set back plant maturity and consequently may improve weed response to herbicides.

Summer annuals such as horseweed, common ragweed, giant ragweed, lambs quarter, yellow foxtail, and velvetleaf can be partially controlled through regular clipping or mowing.

If herbicide applications are used, control is best when annuals are actively growing in the spring or fall.

Problem biennials such as bull thistle, musk thistle, burdock and poison hemlock will respond to herbicide treatments similar to annuals.

Fall can be a good time to control problem weeds in pastures and hayfields. However, during extremely dry weather or periods of slowed plant growth, effectiveness of herbicide applications may be reduced.

Woody brush found along fence rows and in pastures can be controlled when actively growing; fully leafed, or per a basal bark application depending on the product selected.

Learn more

Specific information for tough-to-kill perennials can be found in OSU Agronomy Fact sheet 306 and the OSU Weed Control Guide, Bulletin 789.

Use university weed response tables to investigate the effectiveness of herbicides on your particular weeds while reviewing product restrictions.

Follow this up with a review of product labels, prior to selecting herbicides. The label will provide specific information related to product use that will help assure the application is effective and safe.

Right tools

Always wear the personal protective equipment recommended on the product label, and be aware of product restrictions and recommendations relating to the environment, sensitive crops, and bees.

To properly apply herbicides, it is important to calibrate the sprayer and utilize the correct nozzle.

When a label permits, using surfactants may improve the effectiveness of herbicide application. Be aware of water quality issues that may affect herbicide performance; spray product soon after mixing, as solution pH may change and reduce formulation effectiveness.


Do your own research when selecting and applying herbicides. Prior to selecting an herbicide, be sure to review the restrictions relating to; replanting, hay harvest, slaughter withhold, milk discard, grazing, manure usage, and selling the hay, etc.

Legumes within a pasture or grass hay field may be damaged as a result of a broadleaf herbicide application.

Residual activity of herbicides can vary greatly restricting the use of the forage. Some products with long residual activity can restrict manure and hay utilization. The life span of these products can vary from several months to a year or more.

Again, refer to the product label for specific information related to residual activity.

Bottom line

To effectively manage weeds within your forage and pastures; identify problem weeds, investigate management contributions, review weed response tables and review the complete product label prior to herbicide application.

Use all tools, such as livestock and equipment, to keep problem weeds from reproducing and time the herbicide application for maximum response.

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