The number of cotton varieties carrying genes for resistance to Roundup has increased significantly over the last few years. “Consequently, the problem of volunteer seedlings having Roundup resistance is also increasing,” said Wayne Keeling, professor of weed science at the AgriLife Research Center at Lubbock.
“When a Roundup-resistant cotton variety, or any other Roundup-resistant crop, is planted in a field that grew Roundup-resistant cotton the prior year, a problem arises because volunteer seedlings from that crop will be Roundup resistant.”
Adverse weather conditions can cause preharvest losses, and harvest operations always result in a certain amount of seedcotton loss. “If that seedcotton is buried deeply enough by tillage there will be no volunteer problem,” Keeling said. “However, that is usually not feasible, and applying herbicides with a hooded sprayer can be an effective option.”
To this end, Keeling and associates tested the following 10 herbicides and two herbicide mixtures: Gramoxone Inteon, Ignite, ET, Aim, Buctril, Direx, Direx + MSMA, Layby Pro, Layby Pro + MSMA, Caparol, Reflex, and Valor. These were applied at various rates at the 5 to 8 and 10 to12 true-leaf stages. The percentage control was determined at 3, 7, 14, and 28 days after treatment.
“Because significant amounts of regrowth occurred with some treatments, we found percent control at 28 days after application was the most reliable indicator of chemical efficacy,” Keeling said.
“When applied at the 5 to 8 stage, only Buctril, Ignite, ET, and Aim (resulted in) a 90 percent or more seedling control after 28 days,” Keeling said.
The range among the other eight treatments in percent control 28 days after treatment was 16 percent to 78 percent.
“If applications were delayed until the 10 to12-leaf stage, none of the treatments achieved more than 30 percent seedling control after 28 days,” Keeling said.
In a separate study Gramoxone Inteon was applied at the 10 to 12 leaf stage at rates of 16, 24, and 32 ounces per acre. At the 28-day evaluation, control was 8 percent, 61 percent, and 67 percent for the respective treatments. “Failure to get an acceptable level of control, even at the highest rate, emphasizes the necessity for applying herbicides no later than the 6 to 8 leaf stage,” Keeling said.
“If the producer is reasonably sure volunteer seedling emergence is complete at the 4 to 7 leaf stage, he might consider a herbicide application at that time as it will increase the odds of getting satisfactory control.
“When the producer elects to control the seedlings chemically he should use a properly-adjusted hooded sprayer to ensure the cotton-crop plants will be adequately protected.
“If the producer is using a no-till program, his only viable option is chemical control. However, if he is not in a no-till program, he can control the seedlings with tillage but we suggest he furrow-dike his field as he cultivates out the seedlings. Subsequently he can control the weeds with an acceptable over-the-top chemical,” Keeling said.