In spring as pastures begin growing again, weeds begin to take off as well. Whether through over- grazing or imported seed from purchased hay, weeds always seem to find a way to thrive. Many producers often ask whether they should spray their pastures to control these weeds.
The first step in deciding whether or not to spray your pastures is to determine which weeds are
present. This will be important in deciding when is the best time to spray and if spraying is the best option. Annuals such as common ragweed and spiny amaranth are easiest to control in the vegetative stage before they begin flowering. Biennials such as musk and bull thistle are best controlled in the rosette stage. Perennials such as horsenettle or curly dock can be controlled early in the season while the plant is in the vegetative stage or in the fall before it goes dormant. For some weeds such as ironweed, mowing two to three times per year can result in comparable control to herbicides.
Many producers do not like to spray their pastures because it will generally kill any legumes that are present as well. A study by the University of Missouri has shown that cattle prefer to graze weed-free pastures, even if legumes are not available. In a study where half of a pasture was treated with herbicides while the other half was left untreated, the cattle were nearly five times as likely to graze in the treated portion of the pasture, indicating that the presence of legumes is not enough to attract cattle from the weed-free portions of the pasture.
Several herbicides are available for pasture weed control, with most of them being in the growth regulator family. These herbicides include 2,4-D, dicamba, picloram and aminopyralid. As a rule, these herbicides will not affect grass growth, but will control any broadleaves present, legumes included. Products containing metsulfuron, an ALS inhibitor, are also commonly used in pasture settings.
For specific recommendations about pasture weed management contact your local University of Missouri Extension office.
Andy Luke is a field specialist in agronomy.