Pasture and Forage Weed Control; Mow or Spray?

Clif Little, OSU Extension Educator (Discussion of herbicides in this publication is strictly for educational purposes)

It is important to remember that each herbicide varies in terms of target weed response. In other words, herbicides vary in ability to kill specific weeds and always refer to product labels prior to use. Always wear the personal protective equipment recommended on the product label. 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, utilizing surfactants may improve the effectiveness of herbicide application. Be aware of water quality issues that may affect herbicide performance and spray product soon after mixing since solution pH may change reducing formulation effectiveness.

Other useful tools are University pasture and hay weed response tables that rate overall product response to various weeds. In addition, weed response can vary based on stage of plant growth and timing of herbicide application. Prior to selecting a herbicide be sure to review the restrictions relating to; replanting, hay harvest, slaughter withhold, milk discard, grazing, manure usage, and selling the hay, etc.

Residual activity of herbicides can vary greatly restricting the use of the forage. Herbicide active ingredients such as; picloram, aminopyralid, and clopyralid, belong to a class of herbicides known as “pyridines” and may have a relatively long residual activity. Products that have long residual activity should only be used on sites where manure or hay will remain on-farm, other restrictions apply. The life-span of these products can vary from several months to a year or more. Please refer to the product label for specific information related to residual activity. Legumes within a pasture or grass hay field may be damaged as a result of a broadleaf herbicide application.

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

Biennials are best controlled during the seedling and rosette stage with most broadleaf herbicides. When biennials bolt control can be significantly reduced.

Perennials are best controlled during the early-bud stage (the 2 weeks before flowering), 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 and grazing can be tools utilized to set back plant maturity and consequently may improve weed response to herbicides.

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.

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.

While perennials are best controlled when weeds are in the bud to bloom stage, woody brush found along fence rows and in pastures is best controlled when actively growing and fully leafed. Specific information for tough to kill perennials can be found in the OSU Weed Control Guide, Bulletin 789.

Prior to spraying for weeds it may be helpful do some investigation into what contributed to weed establishment. If hay and pasture management doesn’t change we may likely end up in the same situation later.