Growers may have to rely more on residual herbicides and/or make timely applications of glyphosate to control problem weeds in Roundup Ready cotton, according to a survey of weed scientists around the Cotton Belt.
The results of the survey, conducted by University of Tennessee weed scientist Bob Hayes, were presented at the 2003 Beltwide Cotton Conferences, in Nashville.
The most common problem weed cited by those surveyed was morningglory, according to Hayes. Poor results in morningglory control were attributed to weeds “that were too large at the time that an over-the-top application of glyphosate was made,” Hayes said.
“At other times, rates were too low, plants were drought-stressed or there was poor coverage during the application.”
Suggestions for improving morningglory control include making more timely over-the-top applications. This can be accomplished more easily by making two labeled over-the-top applications of glyphosate, Hayes said.
“This lets you treat small weeds at the one-true-leaf stage, then come back at the four-leaf stage with the second over the top. That way, you're going to be treating smaller weeds.”
Another suggestions is to use full-labeled rates of glyphosate.
Hayes also suggested adding Staple to the over-the-top application. “It has some residual activity that can help.”
Other residual herbicides may also help control the weed, noted Hayes. “We've gotten away from that as we've gone to a Roundup Ready system. Certainly, the DNAs provide a lot of activity.”
When post directing, “do an early post-directed spray rather than waiting and trying to control morningglories with a layby, Hayes said.
Glyphosate-resistant marestail, or horseweed, plagued many west Tennessee fields in 2002, he said. “We've used glyphosate only for many years on our soybean and cotton fields. Resistance has occurred because in west Tennessee we put a lot of selection pressure on horseweed. We've also cut the rates.”
Solutions at pre-plant include applications of Valor after Nov. 15. “You have to have small weeds for it to do a good job. We did not have nearly as good results with spring applications as with fall applications. One of the good points about Valor in our area is that it will leave a lot of the poa annua in the field which will protect the soil from erosion.”
Other pre-plant options include 2,4-D or Clarity. Hayes noted that Valor, 2,4-D, and Clarity have time interval restrictions prior to planting. Between those intervals and planting, MSMA plus Karmex/Direx “has done a good job on resistant marestail, but this year, we had a lot of cool temperatures which diminished the performance of that material.”
Pre-plant or pre-emergence options include Gramoxone Max and Coto-ran/Meturon or Gramoxone Max with Karmex/Direx.
“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.
“If you still have horseweed escapes early in the season, MSMA post-emergence over the top is about the opportunity we have,” Hayes said.
The application prior to pinhead squaring, “will suppress horseweed, but control is usually incomplete and cotton injury may occur, although yield is seldom reduced.
A post-directed application of Karmex/Direx and MSMA is also an option. “We may have to use Cotoran/Meturon and MSMA until the cotton gets up to 6 inches tall. We encountered a lot of late-season emergence this year.
“In fact, we had horseweed emerge all year long in Tennessee this year.”
Other problem Beltwide weeds and suggestions for improved control in glyphosate-resistant cotton include:
Dayflower complex. It's a problem (in the Southeast) because it has “a lot of natural tolerance to glyphosate and it emerges throughout the season,” Hayes said. “Solutions include getting residual herbicides back in the program.
“Dual as a pre-emergence or as an early over-the-top application with a glyphosate product is one of the best ways of managing this. Caporal or Valor post-directed is also an option.”
Florida pusley “has a lot of tolerance to glyphosate,” according to Hayes. “It's become a problem (in the Southeast) because we've dropped the DNAs out of the programs we've been using and gone to strictly glyphosate over the top. One solution is the get the DNAs back in the program.
Nutsedges are tolerant to many herbicides included glyphosate, noted Hayes. “Growers are making headway where they apply two applications of glyphosate before the fifth true leaf followed by a layby with glyphosate or MSMA.”
Russian thistle, or tumbleweed, is a problem cited by weed scientists in High Plains of Texas, Hayes said. “Often, it's too large when applications are made. A suggested solution is to treat when the weed is smaller. Make sure you get good coverage of the plant.”
Other problem weeds reported by those surveyed include cutleaf evening primrose, docks, fleabanes, palmer amaranth (and other pigweeds) hophornbeam copperleaf, hemp sesbania, prickly sida and velvet leaf.
The take-home message from Hayes? “When we take a two-step approach (in glyphosate-resistant cotton) we're treating small weeds and we don't allow weeds to get large enough to begin to compete.
“A timely application doesn't cost anymore. In fact, it pays. Finally, think proactive, and not reactive to these situations. The growers in our area who have been proactive on horseweed resistance to glyphosate have had good success.”