This coming season, west Tennessee growers should be better prepared for outbreaks of glyphosate-resistant marestail.

Weed scientists estimate that resistant marestail infested about 400,000 total acres in west Tennessee in 2002, divided evenly between cotton and soybeans. Infestations occurred in fields with and without a history of resistance.

“We've used glyphosate only for many years on our soybean and cotton fields,” notes Bob Hayes, professor of weed science at the University of Tennessee. “Resistance has occurred because in west Tennessee we put a lot of selection pressure on horseweed. We've also cut the herbicide rates.”

There were a number of reasons for the severity of the outbreak in 2002, including no-till systems, marestail's prolific seed production, the ease at which the seed migrates and weather.

The first incidence of the glyphosate-resistant marestail, which is actually horseweed, occurred in 1999 with reports of non-performance in Lauderdale County.

In 2001, Hayes officially confirmed glyphosate-resistant marestail in west Tennessee. Since then, it's spread rapidly. Following are west Tennessee recommendations for control of glyphosate-resistant marestail in cotton:

Pre-plant or pre-emergence: Gramoxone Max plus Cotoran/Meturon or Karmex/Direx — Effective in suppressing glyphosate-resistant marestail during the interval within 21 days before planting cotton. Terminal regrowth can occur. A second application may be required.

“Before we got into Roundup Ready, these were standard programs in west Tennessee,” Hayes said. “We would go with a glyphosate burndown followed by Gramoxone and one of the residual pre-emergence herbicides.

MSMA plus diuron — One of the best options to suppress glyphosate-resistant marestail during the interval within 21 days before planting of cotton. Weed control is most effective when temperatures are above 70.

Postemergence: MSMA — Postemergence over-the-top of two to five leaf cotton will suppress marestail, but control is usually incomplete and cotton injury may occur. Crop maturity may be delayed, but yield is seldom reduced.

Post-directed: Cotoran/Meturon plus MSMA — May be applied post-directed to 3-inch cotton and will suppress marestail with minimal risk provided contact with cotton is minimal. “We may have to use Cotoran/Meturon and MSMA until the cotton gets up to 6 inches tall,” Hayes said. “We encountered a lot of late-season emergence this year. In fact, we had horseweed emerge all year long in Tennessee this year.”

Karmex/Direx — May be applied to cotton at least 6 inches tall.

Layby: Karmex/Direx plus MSMA — May be applied after cotton reaches 12-inches tall. May injure fall-seeded cover crops.

Pre-plant: Valor — Use after Nov. 15 in combination with burndown herbicides to control emerged weeds and provide residual weed control the following spring. A minimum of 30 days must pass, and 1 inch of rainfall/irrigation must occur between Valor application and cotton planting.

Clarity — Best results are obtained when weeds are small and actively growing and during warm weather. Following application, a minimum of 1 inch of rainfall or overhead irrigation and a waiting period of 21 days is required before planting cotton. May be mixed with Caparol, Gramoxone Max and glyphosate for control of additional grasses and broadleaf weeds.

2,4-D — Best results when applied during warm weather and to actively growing weeds. Cotton can be planted 30 or more days following application without concern for illegal residues. However, under certain conditions, there may be a risk of injury to susceptible crops, including cotton. Unless prohibited, 2,4-D can be tank-mixed with other burndown herbicides.