Pastures and Hayfields: Poisonous Weeds, Multiflora Rose, and Other Suggestions

Usually, drought, other poor environmental conditions, and summer slump reduce forage growth, and are causes for concern for poisonous weeds in pastures and hay. However, even with plenty of rainfall, these types of weeds continue to grow. Whatever the situation, here are some considerations about toxic plants in forages. Livestock may be forced to graze on weeds that they normally would not, or they may eat weeds out of curiosity. Also, as severe summer storms roll through, cherry tree (black, wild, choke, etc.) limbs surrounding pastures may have broken off into the pasture, and the leaves become wilted and more desirable to livestock. Scout your pastures and remove these weeds (or broken limbs and leaves) before they cause livestock health problems. Keep in mind that there are numerous poisonous plants that could invade an area or pasture. Many plants contain potentially poisonous substances that may be toxic to livestock if consumed in large enough quantities. In addition, certain plants may be problematic because of mechanical irritation when eaten, photosensitization, and disagreeable tastes or odors in meat, milk, or milk products. Your local extension educator can assist with plant identification and control, but if you suspect livestock poisoning, call your veterinarian immediately. If death occurs, the stomach contents should be examined for consumed herbage. Identify the suspected plants and remove livestock from the grazing area until all poisonous plants have been removed or destroyed.  The table below lists some common plants and their poisonous properties, but many other plants (weeds, trees, ornamentals, etc.) can be toxic to livestock.

Key points about weed forage quality and poisonous plants:

• Some weeds have excellent nutritive quality.
• Weeds in the vegetative stage of development are usually more nutritious than more mature weeds.
• Regardless of weed quality, livestock may avoid grazing certain plants because of taste, smell, or toxicity.
• Some plants contain potentially poisonous substances that may be toxic to livestock if consumed. Properly identify potential problem weeds and consult with a veterinarian if necessary.
• A productive pasture is important to reduce the potential incidence of toxic weed exposure to livestock. Remember to soil test and maintain the proper lime and fertility levels. If possible, routinely mow or spray to manage weed problems within and around pasture areas.
• Recently, there has been some research that suggests that for every pound of weeds present in pastures, available desirable forage is reduced by one to one and a half pounds! So, if a pasture is really weedy, there is a lot of forage that is not being consumed by the livestock or is unable to compete with the weeds.

For additional information and resources on plants that are poisonous to livestock, see these sites: Cornell University, Colorado State University, and the Weed Science Society of America. In addition, there are numerous other websites that contain information on this subject. Simply conduct a web search for poisonous plants and livestock.

Information in the table below is adapted from Fishel 2000; Hardin 1973; and Hill and Folland 1986; and D. Wolfgang, (retired, Penn State)

Is now the best time to spray perennial and biennial weeds in pastures?

In general, the answer is NO. For example, poison hemlock is a biennial, so it forms a rosette in the fall and bolts and is now flowering and setting seed. For biennials, herbicide applications are most effective when they are sprayed in the fall when poison hemlock and others are in the rosette stage or before they bolt in the spring. Effective herbicides include 2,4-D + dicamba, Crossbow (2,4-D+triclopyr), NovaGraz, or glyphosate as a spot treatment. When the weed is flowering and 3-7 feet tall, herbicide applications are not effective. Similarly, now is not the most optimal time to control perennial weeds such as Canada thistle, milkweed, hemp dogbane, and others in a pasture. During the late spring and summer, we typically suggest mowing pastures a couple of times or so to "set up" the weeds for a more effective herbicide application in the fall. Time the late-season mowing to occur early enough to allow perennial weeds to grow about 12-15 inches or so (usually about a month after cutting) before a herbicide application in the fall. Warm-season perennials such as horsenettle, hemp dogbane, common milkweed, smooth groundcherry, pokeweed, bindweed, poison ivy, and brambles should be sprayed with a systemic herbicide (such as 2,4-D, dicamba, triclopyr, etc.) from September 1 through October 15 or before a hard frost. In general, applications by October 1 may be more effective. In northern areas of Pennsylvania, consider making the application before October 1. An additional 2 to 4 weeks or more application window can exist for Canada thistle and quackgrass, because of their cool-season habit of growth. In the 2025 Penn State Agronomy Guide (Table 2.6-17), there is a graph that shows the optimum time of year to spray systemic herbicides on various weeds.

Multiflora Rose Management Suggestions

As spring progresses, multiflora rose grows aggressively and eventually blooms in late May/early June. Several tactics can be used to control this problem weed, and these methods will be briefly discussed.

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 multiflora rose management.

Management techniques that 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 that 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.

Although the above control practices help, several herbicides provide good control of multiflora rose, especially when applied during the bud-to-bloom growth stages (which usually occur about this time of year). Three foliar-applied herbicides suggested for late spring/summer are: metsulfuron (Cimarron Plus), Crossbow, and glyphosate. Glyphosate has been more effective in Penn State research at fall application time. GrazonNext HL is generally not as effective on multiflora rose, while 2,4-D and dicamba (Banvel/Clarity) and NovaGraz are not effective.

Metsulfuron (sold as a generic or as Cimarron Plus) provides good control of multiflora rose and can be used as a broadcast or spot treatment. For best control, apply either product at a rate of 1 oz/A plus a surfactant for broadcast treatments or 1.0 oz/100 gallons of water plus surfactant for spot treatments. Applications should be made in the spring, soon after plants are fully leafed out. Rose plants must be less than 3 feet tall for treatment to be effective. There is no application to grazing interval for metsulfuron.

Crossbow applied to the foliage and stems can be effective on multiflora rose. For spot treatments, use 4 to 6 fl oz/3 gallons of water and spray until foliage is uniformly wet. For broadcast applications, use 1.5 to 4 gallons of Crossbow in enough water to deliver 10 to 30 gallons of spray per acre. Early to mid-June is an excellent time to make these applications. Follow-up treatments may be necessary. An interval of 14 days is required for lactating dairy if using 2 gallons/A or less.

Glyphosate can be used as spot treatments on isolated patches of multiflora rose. Apply a 1 percent solution (about 1 qt/25 gallons water) of glyphosate with a hand-held sprayer. Uniformly wet leaves and green stems, but avoid runoff. Application should be made in late summer or early fall when plants are actively growing (after fruit formation). A 7-day interval is required for grazing animals.

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 multiflora rose and, in turn, will encourage pasture growth.