Pasture Weed Management

In 1997, grazing lands occupied about 588 million acres, or 39% of the total land area of the United States. The invasion of weeds onto these lands represents a significant threat to livestock producers as well as the environment. Losses from weed and brush infestations on grazing lands have been estimated conservatively at $2 billion per year. Weeds can reduce pasture production directly by interfering with grazing or indirectly by lowering the yield and/or quality of the forage. In addition, certain weed species are poisonous to livestock and can become a problem when pastures are overgrazed. Although many producers eliminate weeds with broadcast herbicide applications once their pastures reach some predetermined “unacceptable” infestation level, some studies indicate that certain weed species do not have deleterious impacts on either forage yield or quality. Other research indicates that some weed species can reduce both forage yield and/or quality considerably. This is the focus of much of the research being conducted in our pasture weed research program.

Cultural Control of Weeds:
The use of herbicides without good cultural practices will generally give poor results. Cultural practices that encourage a vigorous, thick stand of pasture grass are important in achieving acceptable levels of weed control. Maintaining optimum soil fertility and pH favors the pasture crop. Rotational grazing and periodic mowing of grass pastures enhances the ability of the pasture grass to compete with most annual weeds and can also prevent weed seed production. In general, allowing established pastures a recovery period after grazing by excluding cattle for 3 to 4 weeks will do much to reduce weed populations. Lastly, be sure to reseed thin areas before weeds become established.

Chemical Control of Weeds:
Herbicides are one of the most effective ways of eliminating weeds from pastures. However, herbicides should only be used where the pasture is thick and vigorous enough to fill in areas where weeds are killed. If this is not the case, renovation of the entire pasture stand may be required. Recommended rates of pasture herbicides will not injure established grasses, but will kill desirable forage legumes such as white clover, red clover, and bird’s-foot trefoil. This loss should be considered before deciding to make any herbicide application.

Another important factor that should be considered before deciding to apply a herbicide is the interval between herbicide application and grazing, or the interval between application and haying. This information is clearly listed on the specific herbicide label and can also be found in the Weed and Brush Control Guide for Forages, Pastures, and Noncropland (MP581). For pastures where poisonous weeds are a problem, herbicide applications often make these weeds more palatable to livestock that might normally avoid these plants. For this reason, livestock should be excluded from areas where poisonous plants are present for 7 to 10 days after treatment.

Herbicide Application and Timing:
Herbicide application timing varies with the specific weed species. As a general rule, annual weeds are controlled most effectively with herbicides when they are small and actively growing. For winter annuals, this corresponds to a late fall to early spring timeframe while for summer annuals, this usually corresponds to late spring to early summer application timing. For biennial weeds (two year life-cycles), the optimum time for herbicide application is when these weeds are in the rosette stage of growth. This corresponds to fall or early spring for most biennial weeds that occur in pastures and rangelands in Missouri. Some of the most common biennial weeds that we have in pastures are musk and bull thistle.

Established perennials are most susceptible to herbicides in the bud to bloom stage, which often occurs in the fall when food reserves are moving into the roots of these species. Woody brush species should be sprayed when they are fully leafed out and actively growing. Optimum control will usually be achieved by clipping brush species in the fall and treating young species that are fully leafed out in the spring. However, it is important to keep in mind that multiple applications may be needed over several years in order to obtain complete control of perennial and woody brush species.

New Herbicides Registered for Use in Pastures:
PastureGard and Surmount are two of the newest herbicides from Dow AgroSciences that are now registered for weed control in grass pastures and rangelands in Missouri. PastureGard is a pre-packaged mix of triclopyr (also sold under the trade names of Remedy and Garlon) plus fluroxypyr (Starane) while Surmount is a pre-packaged mix of picloram (also sold under the trade name Tordon) plus fluroxypyr. Both products contain one of DowAgroSciences ‘newest’ active ingredients, fluroxypyr.

Fluroxypyr is a member of the pyridine family of herbicides and has a ‘growth regulator’ type of herbicidal action. As with 2, 4-D, dicamba, triclopyr, and almost all of the herbicides that are registered for use on grass pastures and rangelands, the growth regulator herbicides prematurely stimulate cell elongation and disrupt normal cell growth. This leads to twisted (epinastic) plant growth, curled and/or cupped leaves, and eventually death of susceptible broadleaf species.

PastureGard contains 1.5 lbs of triclopyr and ½ lb of fluroxypyr per gallon of product and will primarily be marketed for the control of troublesome brush and perennial weeds like blackberry, multiflora rose, Osage orange, Sericea lespedeza, and dock species in permanent grass pastures and rangelands.

Surmount contains 2/3 lb of picloram and 2/3 lb of fluroxypyr per gallon of product and provides control of a number of annual and perennial broadleaf weed species. Some of the more notable of these are ironweed, horsenettle, ragweed, and a variety of thistles. Like Grazon P+D, Surmount is a restricted use pesticide.

Influence of Herbicide Treatment and Application Timing on Musk Thistle Control:
As illustrated in Table 1 and discussed previously, the timing of herbicide application is a key component in the control of musk thistle as well as other thistle species like bull thistle. When comparing the same treatments applied at different timings, the more favorable application timing for acceptable levels of control is when these plants are in the rosette stage of growth. All of the herbicide treatments evaluated in our trials, including a new experimental herbicide expected to be released in the near future (XDE-750), provided excellent control of musk thistle plants when applied in the rosette stage of growth. As illustrated in Table 1, many of these same treatments were much less effective when applied in the bolting stage of growth.