Source: Syngenta Crop Protection

With production costs for growers, the bottom line is dollars and cents—or dollars and sense. Past experience has proven that herbicide over-use can lead to weed resistance and higher costs.  And now weeds are becoming resistant or showing higher tolerance to glyphosate, the foundation of many weed control programs, raising the question, “How will glyphosate resistance impact profits?”

Since the introduction of glyphosate-tolerant (GT) cropping systems in 1996, growers saved money and expanded their operations.  But today, glyphosate is losing some of its economic benefits.  Fields with resistant or difficult-to-control weeds cost growers additional herbicide applications, machinery expenses, time and more.

“The reality is programs that relied solely on glyphosate quit working on a few specific weeds,” says Bill Johnson, Purdue University weed scientist. “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.”

Today, glyphosate-resistant horseweed (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.  Forty-three percent of those growers reported an increase of $2 to $7 per acre, while 20 percent reported an $8- to $15-per-acre increase.

Southern Illinois producer Trent Funk farms 5,500 acres with his father, John, in Elkville.  He has dealt with uncontrolled marestail in their fields the past two years.  His marestail infestation ranged from spotty in some fields to completely solid in others, including one 60-acre-soybean field.    

“Last year we thought our marestail problem was application error,” says Trent Funk.  “So this year we started with a burndown application and sprayed again in April shortly after emergence. We did field trials with different rates of glyphosate and added AMS or FirstRate®, to see if we could get better control.  But, nothing was acceptable and our fields were not clean at harvest.

“When you include herbicide costs and additional tillage, uncontrolled marestail will cost us an additional $20 per acre.”

Herbicide Applications Add Up

When weeds escape control, extra herbicides are the first noticeable factor to hit growers’ pocketbooks. Additional glyphosate applications, higher rates or alternate modes of action can increase input costs.

“For growers who use GT systems, what they use in addition to glyphosate is going to make the difference when it comes to resistance,” says Bryan Young, Southern Illinois weed scientist.  “Depending on what herbicide controls the weeds present, growers could add $5 to $15 per acre to their programs.”

“Adding a different mode of action, plus an alternate burndown of 2, 4-D and Gramoxone®, added about $12.50 per acre to our input costs,” says Funk. 

However, hidden costs of machinery use are often 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 these costs vary for each tractor, operating a 150 horsepower diesel tractor costs more than $77 per hour based on the study’s calculations.  Covering 150 to 200 acres per day, these costs add $5 to $7 per acre.

Weeds Steal Profit

Uncontrolled weeds take yield, as well. Funk’s most severely infested soybean field averaged 32 bushels per acre less than others because of the uncontrolled marestail.  Marestail steals yield at very high densities, but four other, more competitive weeds have been confirmed resistant to glyphosate in the U.S including common ragweed and Palmer pigweed.

A common ragweed population of just 2.4 uncontrolled weeds per square yard (roughly the size of a hula-hoop), cost about $37 per acre in soybeans, a 14 percent yield loss, according to a 2003 study by the Ministry of Agriculture, Food and Rural Affairs, Ontario, Canada.

Palmer pigweed, the most recent weed to be confirmed glyphosate-resistant, “is the king of the pigweeds when it comes to competition,” says Larry Steckel, University of Tennessee weed scientist.  “Palmer pigweed easily could cut yield by 30 percent. I’ve seen it completely wipe out corn yields, and it is even more competitive in cotton.”

Hidden Costs Count

Aside from the accounting books, glyphosate resistance 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 weed scientist.  “You can’t just pour glyphosate on anymore; growers are going to have to return to more complicated management programs.”

The convenience of herbicide-tolerant technology includes less management time, simplicity in herbicide selection and more flexibility for herbicide rates, time of application and placement.  In addition, GT systems have encouraged the adoption of no-till or conservation-till programs.

However, with the advent of glyphosate-resistant weeds, tillage becomes an important control option.  For example, Funk plans to chisel plow an additional 500 acres to help control marestail instead of leaving the fields completely no-till.

“We haven’t seen a problem in conventional-till fields, so we expect the chisel plow will take care of some of the marestail problem,” says Funk.  “But, our farm estimates the additional plowing will cost $5 to $7 per acre.”

Unfortunately, in land with highly erodeable soil, tillage is not an option for growers if they plan to continue farming. “In west Tennesee, pigweed needs light to germinate and can be better controlled with tillage,” Steckel says. “However, our soils are so fragile that we discourage tillage.  Roughly 80 percent of our cotton is no-till and we need to stay that way to be sustainable.”

Options Protect Pocketbook

Growers and researchers agree, GT systems need to be preserved and there are options that still control problem weeds. 

“A pre-emergence herbicide up-front is an option for growers to pick back up,” says Young.  “Planned sequential herbicide applications offer low risks and allow growers to use different modes of action in each field.”

Purdue University also advocates resistance management and solid weed control options.

“Growers are still very dependent on glyphosate because it is simple,” says Johnson.  “By adding a soil-residual product to their program, growers can take a proactive step.  Residuals control weeds longer into the season and early-season weed control is important for maintaining top yields in corn and soybeans.”

In Illinois, Funk is following these recommendations and plans to integrate a different burndown into their herbicide program next spring.

Pre-emergence herbicides, alternative burndowns, rotating modes of action and tankmixes are options growers can incorporate into their management programs to preserve GT technology and control problem weeds.   

 “We have a system based on glyphosate, but using one mode of action is a high risk choice,” Young says.    We 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.”