Incidents of off-target herbicide movement appear to be on the slide downward. With that said, however, dozens of farmers each year are faced with distinguishing whether or not the sick-looking crops in their fields have fallen victim to pesticide injury or something else entirely.
Charles Snipes, Extension cotton specialist at Delta Research and Extension Center in Stoneville, Miss., has been studying the effects typical rates of off-target glyphosate can have on conventional cotton varieties.
“There is quite a difference between conventional cotton and Roundup Ready cotton when it comes to glyphosate tolerance,” Snipes says. “The conventional cotton sprayed with Roundup Ultra Max yellowed, stunted, and exhibited symptoms similar to that of thrips-damaged cotton.”
The good news, he says, is that in most cases the cotton can recover from the rates of glyphosate most commonly found in cases of off-target herbicide injury.
In Snipes' three years of research, he tested the response conventional cotton at the two- to three-leaf stage and the six- to seven-leaf stage to sub-lethal concentrations of glyphosate, using rates from 0.03 to 0.48 pound active ingredient per acre.
In both 2002 and 2003, cotton injury hit 70 percent at the 0.48 rate of Roundup Ultra Max 30 days after treatment. The treatment to two- to three-leaf cotton also increased the height of first fruiting node, indicating a possible delay in fruiting, Snipes says.
“In 2003, we saw the same delay in fruiting even at herbicide rates on the lower end of the rate spectrum,” he says. “In the last three years, 2003 was the only year that we saw a reduction in lint yields below the untreated check. That year, we saw the most injury and the greatest response to the Roundup Ultra Max applications, possibly due to environmental conditions.”
When conventional cotton was treated with glyphosate at the six- to seven-leaf stage, Snipes reported 70 percent visual injury to the cotton plants but no significance in first fruiting node location as compared to the untreated check.
“At this point in the season, the first fruiting node is typically already set,” Snipes explains. “Lint yields did drop significantly all three years when 0.48 pound of glyphosate was applied to cotton at the six- to seven-leaf stage. Again, the response was stronger to all rates of glyphosate in 2003, due to both the weather and the stage of cotton growth at the time of application.”
While visible injury to the cotton plants ranged from 10 to 70 percent when more than 0.12 pound active ingredient of glyphosate was applied to the cotton, no injury was reported to herbicide rates below that level.
Based on three years of work, Snipes believes the drift rate of glyphosate must exceed 0.12 pound active ingredient per acre to cause a yield reduction. “The injury caused by off-target glyphosate on conventional cotton seems more significant at the higher rates, but the cotton seems to recover in most cases. While mid- to late-season varieties seem more affected by glyphosate, all of the conventional cotton varieties we tested seem remarkably resilient to glyphosate,” he says.
“Anything from a pint on down will not be as significantly damaged as it looks to be at first glance. It's not necessarily the time to give up, and go on to another crop because most of the time the cotton will recover.”
Corn and grain sorghum aren't nearly as resilient to glyphosate drift as cotton, primarily because they are both grass species, according to Erick Larson, Extension corn specialist at Mississippi State University in Starkville, Miss.
Determining where the drift originated is often more difficult because in many cases there won't be a drift pattern in the field, and the drift may be coming from a substantial distance from the affected field.
Off-target applications of glyphosate can be identified in corn or sorghum by looking for chlorosis, stunting, red or necrotic leaves, although no root injury will result. New tillers may also develop from the base of sorghum plants in response to the injury caused to the primary stem. You can also look for injury to plants or weeds adjacent to the field. Glyphosate injury will often cause chlorosis on johnsongrass leaves.
Drift injury can also be distinguished from soil-active herbicides by the presence of a distinctive banding pattern on leaves initially protected within the whorl. “You're likely to see contact injury on those leaves that had emerged from the whorl at the time the glyphosate drift injury occurred. But, it may take 10 to 14 days from the date the drift occurred before you see stunting and symptomology on newly emerging leaves in the whorl,” Larson says.
If your corn field is the victim of off-target herbicide movement, how do you decide whether or not replanting is necessary?
In many cases, Larson says, the answer depends on whether or not you can replant, based on the calendar date and the history of chemicals applied in that field. “Because of the time period before symptoms appear, growers are often already behind the eight-ball. Growers should look at it as a stand loss type of situation when deciding whether or not to replant,” he says.
Larson suggests taking a close look at the health of the affected plants to determine how likely it is that each of those plants will recover from the herbicide injury. Are they fully recovered, moderately injured or severely injured?
Fully recovered plants have recovered or are likely to recover quickly. Moderately injured plants will likely suffer some reduction in yield, depending on the size and color the plant's leaves. It's not uncommon to see more than a 50 percent yield loss in fields with moderately injured corn plants due to the shorter plants and smaller ears of corn, Larson says. In cases of severe injury, severe yield reduction is likely.
“In most cases, growers will see variable rates of injury throughout the field,” Larson says. “If either at least 10 percent of the field exhibits severe injury symptoms or at least 25 percent of the field shows moderate injury, you're probably justified in replanting the field, likely to another crop.”
The use of Roundup Ready technology will not solve all drift problems, cautions Larson. “Corn is more intolerant of Staple injury than glyphosate injury,” he says. “Be aware there are other chemicals out there that corn is as, or more, susceptible to, and that can cause severe injury to corn.”