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, according to Hayes.
“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 suggestion 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 (horseweed) plagued many west Tennessee fields in 2002, noted Hayes. “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 preplant include applications of Valor after Nov. 15. “You have to have small weeds for it to do a good job,” he said.
“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 preplant options include 2,4-D and 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.”
Preplant or pre-emergence options include Gramoxone Max and Cotoran/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 postemergence, over-the-top is about the only 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.”
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