Yellow Starthistle Information
Mech control
-hand
-tillage
-mowing
Cultural control
-grazing
-burning
-re-vegetation
Biocontrol
-insects
-biocontrol table
-plant pathogens
Chem control
- risks
--spray
--water
--toxicology
--herbicide
resist
--effects
-herbicides
--preemergence
--postemergence
--late season
--pre- & post-
--imazapic
--clopyralid
--picloram
Management
Clopyralid
General characteristics. In the mid-1990's few herbicides were available in California for season-long control of yellow starthistle in pasture, rangeland or wildland areas. With the registration of clopyralid in California in 1998, ranchers and land managers had a highly selective herbicide available for starthistle management. It is a growth regulator with similar activity to 2,4-D, dicamba, triclopyr, and picloram. Unlike 2,4-D, dicamba, and triclopyr, clopyralid has excellent soil (preemergence) and foliar (postemergence) activity. It is important to remember, however, that it is a much slower acting compound than the other postemergence growth regulators and often requires two months to control starthistle. Injury symptoms are typical of other growth regulators and include bending and twisting of the stems and petioles, stem swelling and elongation, and leaf cupping and curling. This is followed by chlorosis (yellowing), growth inhibition, wilting and eventually death. At low concentrations, the young leaf tips may develop narrow feather-like extensions of the midrib.
Clopyralid is very effective for the control of yellow starthistle at very low rates (1.5 to 4 oz ae/acre; 4 to 10 oz product/acre). It also has a broad timing window, it does not appear to negatively impact insect biological control agents (Pitcairn and DiTomaso 2000), and has a very low toxicology profile (Caution Signal Word) with no grazing restrictions. The registration of this compound in California has generated great interest by ranchers and landowners threatened by large yellow starthistle infestation. Click here for a detailed evaluation of the toxicology of clopyralid (pdf publication, 32 pages 93 kb).
Soil properties. Clopyralid is weakly adsorbed to soil, does not volatilize, and is not photodecomposed to any degree. It is degraded by microbial activity with an average half-life in soil of between 12 and 70 days, depending on the soil type and climate. The mobility of clopyralid in soil is considered moderate, so some leaching can occur. For example, following a 4 oz ae/acre treatment (highest labeled rate) of clopyralid to bare ground in Fresno, California, 99% of the residual herbicide was found in the top 18 inches of the soil 4 months (80% normal rainfall) after treatment. In the same study, clopyralid degraded to 1% of the total applied herbicide after 3 months. Thus, under normal rainfall conditions in yellow starthistle infested western grassland, clopyralid leaching into groundwater is unlikely to present a problem.
Selectivity. Clopyralid is a very selective herbicide and does not injury grasses or most broadleaf species. However, depending on the timing of application, it does damage or kill most species in the legume family (Fabaceae), as well as the sunflower family (Asteraceae), and this may not be a desired outcome in a control program with the goal of increasing native plant diversity or enhancing a threatened native plant population susceptible to the herbicide. It can also cause some injury in members of the nightshade (Solanaceae), knotweed (Polygonaceae), carrot (Apiaceae), and violet (Violaceae) families. In contrast, many other broadleaf species, including species in the mustard (Brassicaceae) and poppy (Papaveraceae) famlies and filarees (Erodium spp.) are very tolerant to the herbicide.
Rate and timing. Clopyralid provides excellent control of yellow starthistle seedlings and rosettes between its registered use rates (in California) of 1.5 and 4 oz ae/acre (Carrithers et al. 1997, DiTomaso et al. 1999b, Gaiser et al. 1997, Johnson 2000, Lass and Callihan 1995b, Northam and Callihan 1991, Wrysinski et al. 1999) [Table A, B]. In other western states the highest use rate of clopyralid is 8 oz ae/acre. Season-long control can be obtained with one application made from December through April, but maximum grass forage was obtained with mid- to late winter treatments (DiTomaso et al. 1999b). The most effective timing for application is from January to February, when yellow starthistle is in the early rosette stage. Applications earlier than December may not provide full season control and treatments after May usually require higher rates. Clopyralid is also effective on plants in the bolting and bud stage, but higher rates (4 oz ae/acre) are required. Applications made after the bud stage will not prevent the development of viable seed (Carrithers et al. 1997, Gaiser et al. 1997).
Combinations and adjuvants. When clopyralid is used to control seedlings a surfactant is not necessary (DiTomaso et al. 1999b). However, when treating older plants or plants exposed to moderate levels of drought stress, surfactants can enhance the activity of the herbicide. A combination of clopyralid and 2,4-D amine is sold as Curtail® in western states other than California. It can be used at 0.25 to 1 pint/acre after the majority of starthistle rosettes have emerged but before bud formation. Clopyalid can also be treated using a liquid fertilizer as a carrier (Evans 1998). This can provide effective control of starthistle and enhance the growth of desirable grasses and broadleaf species.
Effects of forage. Late season treatments with clopyralid (April to June) can have a negative impact on forage quantity. Reduced forage at later treatment times is likely due to the competitive effects of yellow starthistle on grass development during the early spring months. Desirable forage biomass is maximized with an early season treatment (December to March). At this timing, yellow starthistle is in the early rosette stage. Early season treatments also give better control of starthistle.
Coverage with previous years skeletons. It is often questioned whether clopyralid treatment over the top of skeletons from the previous years' infestation will the activity of the herbicide. However, DiTomaso et al. (1999c) demonstrated that control of yellow starthistle was better in the presence of skeletons compared to the areas where skeletons were removed. This difference was attributed to the reduced number of seedlings present in the area shaded by the previous year's skeletons. Consequently, fewer seedlings were available to escape injury in the shaded plots.
Fit into strategic management plan. In most circumstances clopyralid can be an important component in a yellow starthistle management program. For example, clopyralid is often a very effective first year option in a multi-year program. This is particularly true in heavily infested areas. The herbicide can substantially reduce the starthistle population, thus depleting much of the seedbank. Because clopyralid is typically used in late winter to very early spring when the competitive interactions for soil moisture are minimal, the control of yellow starthistle will result in high grass forage production during that growing season (DiTomaso et al. 1999b). If yellow starthistle seedling numbers in the second winter are also very high, a second year of treatment may be needed. However, in subsequent years it may be more advantageous to delay the use of clopyralid or other preemergence herbicide until the extent of the problem can be evaluated. In these situations, a prescribed burn, mowing, physical removal, or spot application can be sufficient. In some instances one or two years of control can dramatically reduce the starthistle infestation to very low or even nearly insignificant levels. When this occurs, an additional application of clopyralid or another postemergence herbicide would be unnecessary.
