Despite the fact that its effects could be devastating to those farmers affected by off-target herbicide movement, Arkansas wheat specialists say the problem probably could have been predicted.
“Considering the large number of acres that receive a burndown application of glyphosate, some off-target drift could be expected and the number of occurrences in wheat may not be abnormally high,” says Jason Kelley, wheat and feed grains Extension specialist for the University of Arkansas Cooperative Extension Service in Little Rock, Ark.
In Arkansas, Kelley says, the number of wheat fields with glyphosate drift symptoms is far from an epidemic, but enough acres are affected to get noticed.
Increasing the likelihood of drift is the sheer number of burndown treatments being made, which Kelley attributes to reduced prices for glyphosate-containing products over the past couple of years and the increase in Roundup Ready crop acres.
Many of the drift incidents reported in Arkansas most likely occurred as long as four or five weeks ago. In many of these situations, glyphosate drift went unnoticed until the flag leaf and seedheads started to emerge.
Bob Scott, Extension weed specialist for Arkansas, says the injury symptoms that have been observed in wheat are very similar to those reported earlier in rice.
“Though not documented for certain, we believe most of the drift we're seeing in wheat happened after the wheat entered the reproductive stage of development,” Scott says. “One characteristic of glyphosate herbicide is that once it is applied to a plant it moves or is translocated to the most active growing point in that plant. In the case of wheat at this time of year, that active point of cell division is where the flag leaf and seedhead are being formed.”
That means that very low rates of glyphosate can cause malformed flag leaves and seedheads in developing wheat. “The wheat is far from being killed, but grain production can be severely reduced,” he says.
When glyphosate is used in the spring for burndown treatments high rates of 1.5 quarts per acre or more of a 4-pound per gallon formulation are often required to kill the vegetation present. As a result, Scott says, when a drift event does occur, the results can often be very destructive to an off-target species such as wheat.
“Unfortunately, we currently have little data available to estimate yields with a given amount of damage. The full extent of yield loss will not be known until harvest, and will depend on the extent of damage to the flag leaf and the amount of sterility in the seed heads.”