Revisit Residuals

Reliance on postemergence herbicides has lead to resistance, calling for the return of residual herbicides to
weed management programs.

By Dr. Jason Ferrell
Extension Weed Scientist, University of Florida

The 1990s proved to be an era when postemergence herbicides with unparalleled efficacy and crop safety entered the market. These new postemergence herbicides controlled a wide variety of weeds, and in some cases, even provided lengthy residual control. No longer were producers required to apply herbicides at planting, perform in-season tillage or spray postemergence herbicides on extremely small weeds.

But just as the 1990s will be remembered as the decade of new postemergence herbicides, the 2000s will be known as the decade of resistance.

Time and experience have taught us that opposition to change is a recipe for resistant weeds. A diverse herbicide strategy that utilizes multiple mechanisms of action is now required, regardless of whether resistance is currently present.

The best way to develop a more diverse herbicide strategy is to resume use of the highly effective preemergence herbicides that are available (see table). Not only will these herbicides provide effective weed control, but many have a low probability of resistance. Although these herbicides may require a greater investment “up front,” they will pay great dividends by preventing resistance and providing excellent weed control.

Unlike the postemergence herbicides, most soil-applied herbicides must be activated to work. A rainfall or irrigation of .5 to 1 inch is needed within seven to 10 days after application. Dryland peanut fields should not be treated with a soil-applied herbicide if there is no possibility for rain within seven days of application.
As with any chemical product, always read the label, and apply as directed.
Soil-applied herbicides available, their mode of action (MOA), control issues
and the likelihood of development of resistance.
Herbicide/MOA Control Issues Likelihood Of Resistance
Prowl or Sonalan
MOA - root inhibitor
Advantage: controls a wide variety of grass and small-seeded broadleaf weeds.
Disadvantage: must be incorporated.
Low to moderate
Advantage: excellent control of Florida
beggarweed, pigweed and other broadleaf species.
Disadvantage: sprayer cleanout, potential
for peanut injury.
Metolachlor (Dual Magnum,
Cinch, Stalwart and others
MOA - very long-chain fatty
acid inhibitor
Advantage: excellent control of several grass and small-seeded broadleaf weeds. Suppression of yellow nutsedge.
Disadvantage: misses Texas millet and must be incorporated with rainfall or irrigation.
MOA - very long-chain fatty acid inhibitor
Similar to metolachlor. Low
Solicam (previously Zorial) MOA - pigment synthesis inhibitor Advantage: season-long persistence, control of several grass and broadleaf weeds, such as morningglory and cocklebur.
Disadvantage: weak on pigweeds, potential carryover to corn and small grains.
MOA- ALS inhibitor
Advantage: good on nutsedges, pigweeds and wild radish.
Disadvantage: weak on coffeeweed and Florida beggarweed. Does not control ALS-resistant weeds.
MOS - ALS inhibitor
Advantage: excellent on a wide variety of broadleaf weeds, common ragweed and bristly starbur in particular. Little potential for crop injury.
Disadvantage: does not control ALS-resistant weeds.