On-Farm Network® Studies: Help Identify Solutions to Manage Glyphosate Resistant Weeds

There’s no doubt that the introduction of glyphosate resistance into soybean varieties and corn hybrids greatly simplified weed management in Iowa’s major row crops. Since the herbicide was introduced in the late 1970s, glyphosate has become recognized as an economical non-selective preplant/burndown herbicide, paving the way for many growers to switch to conservation- or no-till for soybean production.

When Round-Up Ready® soybeans were commercialized in 1996 (nine years after a gene for glyphosate resistance was successfully transferred into a soybean variety), growers were able to switch to an all-glyphosate weed management program. Throughout the commercialization process, there were those who warned growers and seed companies of the need to rotate herbicide chemistries in order to avoid build-up of resistant weeds in the general weed population. While most growers have tried to be responsible in their use of glyphosate, the two-pass program has been used widely because it is economical, simple and has been very effective.

In response to grower observations that waterhemp, in particular, was becoming increasingly resistant to glyphosate, the ISA On-Farm Network in 2008 began a series of replicated strip trial studies comparing a glyphosate-only weed control program to one that uses both a pre-emergence residual herbicide and a post-emergence application of glyphosate. Each trial had at least three sets of paired strips for comparison.

“The focus for these studies was not on which weeds were better controlled, but rather on whether yield was affected by the herbicide system,” says Dr. Tracy Blackmer, Iowa Soybean Association director of research. “We looked at what was involved in the systems we compared – how many passes, product costs, and then at yield differences.

“We also scouted fields to assess weed densities for each treatment in 2008 and 2009, and we’ve used aerial imagery to look for weed density differences between the all-glyphosate and the residual/glyphosate strips.”

Based on observations from trials, densities of glyphosate resistant waterhemp do appear to be increasing. Remember that growers selected the fields for these trials because they were concerned about the density resistant weeds in those fields. Incidental observations this past summer suggest that adjacent fields may also have high densities of resistant waterhemp and marestail. “However, without additional information on weed management practices in those fields, we can’t say the weeds we observed there were glyphosate resistant,” says Blackmer.

Growers have seen small yield differences between the two treatments over the first three years of these studies. “We have been able to verify that weed control – particularly waterhemp control – has been better in the strips that received the residual herbicide application. But when the weed management programs are compared in the way the growers execute them, we’ve not seen a yield response that we can attribute to better weed control,” says Matt Sweeney, On-Farm Network project manager. “Rate and timing of herbicide application and weather, especially rainfall, continue to be major factors in influencing the effectiveness of any weed management program.”

Summarized results from the 2008, 2009, and 2010 trials are shown in Table 1. Complete results can be found at in annual strip trial summaries posted at http://www.isafarmnet.com/striptrial.html.

Results from our 2011 trials are just now coming in. Aerial imagery of one trial shows definite differences between the two treatments in the trials. Photos shot at field level, particularly early in the season, show heavier weed pressure in the glyphosate only strips. “We’ll have a summary of the 2011 season studies, as well as work to date at our annual conference in February,” Blackmer says. “We’ll also post trial data as soon as it’s available following harvest. Watch our website (www.isafarmnet.com) for updates as the season progresses.”