What about Western Ragweed?

by Jimmy D. Wright, Rangeland Management Specialist
Natural Resources Conservation Service (NRCS)
Greensburg, Kansas

One of the most common and often maligned rangeland plants is western ragweed (Ambrosia psilostachya). Western ragweed is distributed throughout the lower 48 states and 4 Canadian provinces. It was used by Native American tribes for multiple medical purposes. Western ragweed provides a high protein, livestock forage in early spring and furnishes food for songbirds, small mammals, and game birds throughout the year.

Grazing livestock at numbers higher than optimum can result in the native grasses being kept at a lower height than expected. This will result in western ragweed appearing to greatly increase, while in actuality it is only responding to the direct sunlight and reduced competition from the grasses.

In the early spring, western ragweed is a very diminutive plant that is palatable to livestock and good for them as well. These small plants have a protein level of around 20 percent and livestock use during this period will have a tendency to reduce plant vigor and allow grasses, under a light-to-moderate stocking rate, to compete more successfully for moisture and nutrients in the later stages of the growing season. The light-to- moderate stocking rate should allow enough grass growth to keep the western ragweed plants shaded which further reduces their vigor and keeps growth rates down.

When is it economical to control western ragweed? First, look at the reason(s) the stand is proliferating. Is there evidence that overgrazing is the culprit? If so, attempting to control the western ragweed will not be successful unless livestock stocking rates are reduced over the growing season or intensive early grazing is used. Economically, a threshold of greater than 30 percent western ragweed composition should be present. Control can take many forms including mechanical, chemical, or biological. Keep in mind that control should increase livestock performance enough to see that monetary returns exceed the costs of treatment.

Mechanical control would include mowing and prescribed burning. Mowing is inefficient and results are tenuous. Not only are you setting the western ragweed back, but you are also removing the majority of the grass as well. In order to have any effectiveness, continuous mowing throughout the summer would be needed reducing the amount of grass available for livestock use at the same time. Prescribed burning is not a good method of controlling western ragweed. Western ragweed tends to decrease following fire on the short-grass prairies, but in tall-grass or mid-grass prairies it tends to increase or have no effect on these plants.

Chemical control would include 2,4,D applied when air temperature is warm, soil moisture is good, and the western ragweed plants are 2-4 inches tall and actively growing (generally early May). Spraying 2,4,D under these conditions at about 1 pound per acre can result in 80-100 percent control. Banvel (dicamba) plus 2,4,D and Grazon PC (picloram) plus 2,4,D can provide higher levels of control, but they are not as cost effective as properly applied 2,4,D. Keep in mind that livestock are attracted to treated areas. They can over use these areas or nitrate toxicity could develop in the western ragweed plants causing further problems. Another thing to keep in mind is that this control will also take out any other desirable forbs present. Light-to-moderate stocking rates under a proper grazing system should follow this treatment for best results and cost effectiveness.

Biological control is generally using a grazing system designed to use livestock in controlling pest plants as much as possible. Examining the current operation is the first step. Light-or-moderate stocking with rest during the growing season, especially the last half, is the best and simplest means of control. Ways of obtaining better livestock distribution (additional watering sources, movement of salt/mineral to under-used areas, cross fencing) can reduce concentrated grazing areas which lead to western ragweed proliferation. Intensive early stocking with stockers can also play a large role in control of western ragweed by setting back the plants’ growth rate and removing apical buds allowing grasses to get a quick competitive start and shade the western ragweed during the latter part of the growing season.

Grasses tend to have a competitive edge over western ragweed in normal growth situations. The western ragweed becomes a problem when the grasses are not maintained in good condition where they can compete. The best way to keep grasses in good condition is by using a well-designed rest-rotation system stocked at a light-to-moderate rate. For assistance in setting up such a grazing system, contact the Natural Resources Conservation Service (NRCS) or conservation district office located at a nearby USDA Service Center.

For more information about NRCS programs, visit the Kansas NRCS Web site at www.ks.nrcs.usda.gov.

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Last Modified: 12/10/2008