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Why Are There Weeds In My Pasture?   
Bob Hartzler

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October 03, 2007 - Weeds reduce the value of pastures by reducing the quantity and quality of forage, and in some situations the weeds may be poisonous. When weeds invade pastures, too many people look for the quick fix and simply apply a herbicide to kill the offending weeds. A more effective approach is to identify the factors that have allowed the weeds to become established, and adjust management practices to create a less favorable environment for the weeds. A paper in Weed Research (Suter et al. 2007) described a survey conducted in Switzerland to determine factors that favored the establishment of tansy ragwort (Senecio jacobaea). The researchers identified 31 pairs of pastures, one infested with tansy ragwort and one not, and then looked for differences between the sites that might be responsible for the absence/presence of tansy ragwort. The distance between the two pastures did not exceed 1000 ft.

The primary finding was that good management greatly reduced the establishment of tansy mustard. Pasture that were fertilized with nitrogen were 5 times less likely to have tansy mustard than unfertilized pastures. Pastures that were continuously grazed were 11 times more likely to be infested than pastures that were either mowed or rotationally grazed. Continuously grazing resulted in uneven use of the forage. Areas that were overgrazed created openings in the canopy that favor establishment of tansy mustard. Tansy mustard also was more likely to be found on steep slopes since there was a greater likelihood of open areas due to mechanical damage to the sod from cattle or tractors.

Tansy mustard was occasionally found in well managed pastures. In these situations the pastures were adjacent to areas with high infestations of tansy mustard. Movement of tansy mustard seed from adjacent areas created large seedbanks that increased the likelihood of tansy mustard seedlings surviving in spite of strong competition from a healthy sod.

This research reinforces the importance of cultural practices in reducing weed problems. A healthy sod created by sound cultural practices is able to prevent the establishment of most weeds and greatly reduce the need for the use of herbicides. In addition, management of weeds in adjacent areas (roadsides, waterways, etc.) will reduce the encroachment of weeds into pastures and other managed areas.


Suter, M., S. Siegrest-Maag, J. Connolly and A. Luscher. 2007. Can the occurrence of Senecio jacobaea be influenced by management practice?

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
Submit questions or comments here.  

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Common chemical and trade names are used in this publication. The use of trade names is for clarity by the reader. Inclusion of a trade name does not imply endorsement of that particular brand of herbicide and exclusion does not imply nonapproval.