Problem Pasture Weeds

– William S. Curran, Professor of Weed Science and Dwight Lingenfelter, Program Development Specialist, Penn State University

Every so often we get questions regarding herbicide considerations and restrictions for grass forage situations. Over the past few years, the options have remained about the same. See Table 2.6-7 and Table 2.6-8 in the Penn State Agronomy Guide for a listing of products choices and comments.

For most of the products that contain plant growth regulator type herbicides (2,4-D, Clarity, Crossbow, ForeFront, etc.) remember to allow the seedling forage grasses to get established (4-5 inches tall, have a good secondary root system, and show good vigor) before making a herbicide application. Cimarron Plus also has some necessary establishment periods for different grass species before herbicide application that should be observed. Also, most herbicide for use in grass forage crops will severely injure or kill legumes. Don’t use these products if loss of legume species can not be tolerated. In many cases, weed control is very important during the grass establishment period since weeds can become very competitive.

As thistles continue to develop, a good time to spray is during the bud to bloom stage. Keep in mind that biennial thistles such as bull, plumeless, and musk are usually more susceptible to plant growth regulator herbicides (group 4; 2,4-D, dicamba, Stinger, Milestone) as compared to Canada thistle, which is a perennial.

Woody species such as multiflora rose, autumn olive, honeysuckles, brambles, and black locust can be very aggressive at this time of year as well. For most of these woody perennials we usually recommend a product such as Crossbow (or generic versions such as Crossroad or Candor) since the active ingredient, triclopyr, is usual pretty effective on these species. Metsulfuron or Cimarron products can provide control of multiflora rose, dewberry, and honeysuckle. In general, 2,4-D and dicamba are not as effective on woody species; however, if they are tank-mixed and used at higher rates, weed suppression can be obtained. Also remember that as these woody weeds such as multiflora rose begin to bloom in late May/early June, this is typically a good timeframe to apply an effective herbicide. Other tactics can be used to control these weeds but usually a combination of methods are necessary to obtain adequate control.

Mechanical control methods include mowing, which requires repeated mowings per season for several years, and excavating, which involves pulling individual plants from the soil with heavy equipment, can be costly, time-consuming and laborious. However, these are viable means for management. Also, specific to multiflora rose management, techniques which include biological controls have been used. Two of the more prominent biocontrol agents are rose rosette disease (RRD) and herbivores such as goats or sheep. RRD is a virus which is slowly spreading in our region of the country. Multiflora rose plants infected with RRD usually die within two years. Though RRD may not eradicate the multiflora rose problem, it should help reduce it over the long run. If managed properly, goats and sheep can help control multiflora rose. Research has shown that initially 8 to 10 goats and/or sheep pastured with compatible livestock (cattle) can help reduce rose and other brushy infestations.

No matter which control tactic is used, follow-up maintenance practices are a must for long-term control. Removal of dead brush, annual mowing and adequate soil fertility are examples of practices that should be used to maintain control of woody weed species and in turn, will encourage pasture growth.