It's not enough to treat glyphosate-resistant horseweed in the fall. You'll need spring treatments, too.
That's one recommendation coming out of an extensive research program at the Mississippi State University Delta Research and Extension Center, which is developing control strategies for glyphosate-resistant marestail, also known as horseweed.
For years, Mississippi producers planted through marestail when mostly minor infestations of the weed emerged in crop fields. But that's often not possible now.
While the bulk of marestail emerges in October and November, the spring flushes of this weed cause the most havoc in production agriculture. On-farm and research plot studies have underscored the importance of controlling this fall flush of weeds when population numbers are high.
But controlling fall-emerging marestail may not be enough. While only about 10% of the marestail weeds in a field emerge in the spring, the seed bank is sufficient enough in the period between spring burndown and planting for new marestail plants to emerge as quickly as others die off.
AFTER THE FALL flush, the next major flush usually begins in late February and continues through March. If growers do not burn down during this period, the weed will germinate into June, and it could create major production problems.
It's essential to use a residual burndown in the spring to prevent the spring flush of marestail. Fall burndown treatments are very convenient, and some growers believe they will be protected until planting by using a residual herbicide like Valor.
But research at the Delta Research and Extension Center (DREC) clearly shows that Valor will work until about late February. After that, residual activity plays out and emergence is again possible. Other herbicides, such as ALS inhibitors, are effective from fall burndown until planting, but lock growers into soybeans.
Growers should identify and treat marestail from late February through early March, but the effectiveness of herbicide applications to control this weed depends on weather and field conditions.
In a spring burndown, 2,4-D or Dicamba are essential additives to glyphosate. Glyphosate is unacceptable as a stand-alone burndown on marestail because of widespread resistance. Growers also will need to include a residual herbicide. The end result is a three-way mix: glyphosate, plus 2,4-D or Dicamba, plus Valor. Valor can be used 30 days preplant in nearly every crop planted in the Delta.
This late-February to March application of a three-way mix should be followed by the first postemergence application of glyphosate to ensure a marestail-free field. In a soybean system, Valor XLT, Envive, Gangster, FirstRate or Canopy EX likely will provide adequate control of marestail.
When treating for this weed, good coverage is essential. Using very low-volume aerial applications often are not very effective.
Glyphosate-resistant weed management research in the Mississippi Delta began in 2007 and is continuing under the direction of DREC researchers Vijay Nandula, Jason Bond, Tom Eubank and Robin Bond.