published in


November 2011

Thistles Scout Early, Scout Often

For questions or comments regarding this publication contact: Reyna Hallworth

If you’re looking for a proactive approach to mitigate the negative impacts associated with enhanced, holiday-related supplemental nutrient intake, consider taking some nice long walks through your pastures.  While you’re out there walking around – now is an excellent time to start scouting for thistles to determine control needs.  The NOAA has declared that “La Nina is back” (Climate Prediction Center, September 8, 2011).  These warm, dry conditions are favorable for after-supper strolls, as well as thistle growth, and it is likely that we will see dense thistle populations emerge in the coming months.

Thistles are (with the exception of Leconte’s thistle) biennial.  This means they grow from seed during year one, and produce seed during year two (Figure 1).  During the first ‘non-seed-producing’ year thistles grow as a rosette, primarily during the winter months.  The following year, a stalk bolts from the rosette between late January and July; the plant then flowers, reproduces, and dies during the spring and summer.  A single thistle plant can produce 4,000 or more seeds, quickly resulting in large-scale infestations if left uncontrolled.  Thick stands compete with desirable species for space and nutrients, decreasing the quantity graze-able forage, and can ultimately result in lower calf weaning weights.

No matter the means, timing control activities early in the thistle life cycle to prevent seed production is paramount.  Control options after thistles flower are limited; those that exist are less effective and more expensive.

Mechanical.  Though time consuming, removing rosettes by hand can provide adequate control for very small infestations.  This option may be attractive if you have children who require creative discipline during the winter months.  If your children are well behaved and you have no interest in performing this manual task yourself, the alternative mechanical option is mowing or clipping.  This must be performed, however, after thistles have bolted, but before flowers form.  Rosettes of all thistles and thistles in the early bolting stage will grow back if mowed, and mowing flowering thistles will distribute seeds.  Because of the variation in growth among thistles in a given pasture, timing a mowing treatment may prove difficult.  As a result, multiple mowing treatments may be required to clip all pre-flowered, bolted thistles.  The requirement of multiple treatments at high fuel prices may cause mowing to be cost-ineffective. 

Chemical.  Fortunately, we have a few herbicide options for controlling thistle.  While a number chemicals will provide control, IFAS recommendations include 2,4-D, dicamba+2,4-D (Weedmaster, others), or aminopyralid+2,4-D (GrazonNext HL) (Table 1).  Timing is critical for treatment with 2,4-D; It is highly effective, but only during the rosette stage.  If thistles have bolted, 2,4-D will not provide acceptable control.  During cool temperatures, the ester formula is most effective.  Dicamba + 2,4-D can be applied during the late fall or early spring when daytime temperatures are above 50⁰F.  Applications are most effective when made prior to bolting, and can be enhanced with the addition of crop oil.  Though slightly more expensive, aminopyralid + 2,4-D provides superior control at all stages of growth, and is an excellent option if thistles have bolted.  While aminopyralid + 2,4-D will control flowering thistle, herbicide applications at this stage are generally not recommended.  If seeds have already been produced by the flowering thistle, the plant is beginning to die; at this stage herbicide may enhance short term control, but will not be effective in long term management.  In this situation, mowing may be the best option for the short term.

The aminopyralid+2,4-D trade product may look a little different to you.  That is because it is-  marginally.  The old GrazonNext formulation contained 3.0 lbs of active ingredient per gallon (0.33 lb aminopyralid + 2.67 lb 2,4-D amine).  The new GrazonNext HL contains 0.41 lb aminopyralid and 3.33 lb 2,4-D amine, for a total of 3.74 lb active ingredient per gallon.  As a result, the old rate structure of 1.5 to 2.6 pt/acre converts to 1.2 to 2.1 pt/acre.

The current drought and climate conditions will likely limit forage growth as we move through the fall and winter.  The warm, dry conditions associated with La Nina favor thistle infestation, as we saw during 2007 and 2008.  If left uncontrolled, dense infestations can render pastures virtually ungrazeable- further limiting the forage available to our cowherds and calves.  Early scouting and control –via mechanical or chemical means during the appropriate life stages- are key for long term management.

Adapted from previous works published by Brent Sellers, et al. 
For more information visit http://edis.ifas.ufl.edu or http://rcrec-ona.ifas.ufl.edu, or contact your local county extension office.
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