Weeds resistant
to a widely used broad spectrum herbicide can cost you in both increased
weed-control costs and reduced yields.
By adding different modes of action, tankmixing products and using mechanical
controls, growers can prolong the life of glyphosate, the generic name
for the active ingredient in a host of herbicides, including Roundup
and Touchdown.
“The reality is programs that relied soley on glyphosate quit
working on a few specific weeds,” says Bill Johnson, Purdue University
weed scientist in West Lafayette, Ind. “From the standpoint of
the grower, a weed is resistant if they can’t control it with
the same practice they used to use. But now, growers can recognize the
warning signs and change their management practices before they see
resistance in their fields.” Glyphosate-resistant horseweed, sometimes
called marestail, has spread to more than 3.3 million acres in 13 states.
A 2004 University of Delaware survey reported that 85 percent of responding
growers with glyphosate-resistant marestail experienced increased production
costs. Of those growers, 43 percent reported an increase of $2 to $7
per acre, while 20 percent reported an $8- to $15-per-acre increase.
When weeds escape control, extra herbicides are the first noticeable
factor to hit the grower’s pocketbooks. Additional glyphosate
applications, higher rates or alternate modes of action can increase
input costs.
But hidden costs of machinery used to control resistant weeds often
are overlooked. An Iowa State University study updated in 2005 details
the operating expenses for tractors and implements, including fuel,
oil, maintenance, labor and depreciation. Although the costs vary for
each tractor, operating a 150-horsepower diesel tractor costs more than
$77 per hour, based on the study’s calculations.
Marestail steals yield at only very high densities. But four other,
more competitive weeds have been confirmed resistant to glyphosate in
the United States, including common ragweed and Palmer pigweed.
Palmer pigweed, the most recent weed to be confirmed glyphosate resistant,
“is the king of the pigweeds when it comes to competition,”
says Larry Steckl, University of Tennessee weed scientist in Jackson.
Besides the accounting books, glyphosate resistance also adds hidden
costs in extra labor and management.
“It’s the headache factor – we have lost the simplicity
of glyphosate technology,” says Tom Mueller, University of Tennessee
Extension Weed Scientist in Knoxville. “You can’t just pour
glyphosate on any more; growers are going to have to return to more
complicated management programs.”
The program includes rotating herbicides with different modes of action.
For example, Syngenta Crop Protection, the Greensboro, N.C., company
that markets Touchdown herbicide, recommends growers make no more than
two applications of glyphosate to any one field in a two-year period.
Where possible, growers should also use mechanical controls, such as
disking, to remove weeds.
“We have a system based on glyphosate, but using one mode of action
is a high-risk choice,” says Bryan Young, a Southern Illinois
associate weed science professor in Carbondale, Ill. “You need
to stay on top of weeds by rotating modes of action and controlling
them throughout the season.
“By incorporating alternative modes of action now, growers will
preserve glyphosate and delay the onset of the additional production
costs associated with resistance.”
