As The Weeds Turn
As glyphosate-resistant weeds spread, retailers and growers are being faced with increasingly difficult choices when it comes to control measures.
September 9, 2008
To completely understand just how deeply the problem of weed resistance now runs, maps don't lie. During his presentation at the Total Weed Management Answer Forum in Wisconsin Dells, WI, Dr. Chris Boerboom, Extension weed scientist at the University of Wisconsin, showed attendees maps of the continental U.S., Ohio, and Indiana. In each case, the various counties and states were shaded in multiple hues of yellow and red — illustrating just how widespread the problem of resistant weeds is.
"One field in Montgomery County in Indiana has glyphosate-resistant marestail, glyphosate-resistant giant ragweed, and glyphosate-tolerant common lambsquarters," said Boerboom. "This has increased the proposed herbicide program cost for this grower from $18 per acre to $37 per acre, which doesn't include the cost of application."
Continuing to address the problem, CropLife® magazine — with sponsorship assistance from Monsanto Co. — hosted a series of seminars in a half-dozen cities across the Midwest and South during the winter months. Regardless of location, however, the goal remained the same — to help growers figure out ways to combat this growing threat to crop yield in their areas.
If nothing else, said Boerboom, growers should get used to the idea that weed control will take more active ingredients — and money — than ever before. "Multiple-resistant giant ragweed brings us closer to the point where it is impossible to obtain control in soybeans," he said. "Control requires money, and $50 per acre for weed control, exclusive of the hidden cost of weed control in seed price, is not out of the question."
Boerboom also reviewed some of the new herbicides for corn and soybeans scheduled to hit the marketplace during 2008. These included Halex, a POST product for glyphosate-resistant corn; Laudis, a POST broadleaf and grass herbicide for corn; and Evive and Enlite, a pair of burndown/PRE broadleaf herbicides for soybeans.
At the Total Weed Management Answer Forum in Lafayette, IN, Dr. Bill Johnson, associate professor, weed science for Purdue University, told attendees that there were times when applying more herbicide was useful, depending upon the weed. "Shattercane is a different story," said Johnson. "With this weed, we still recommend using a residual and glyphosate late to prevent seed production."
At the Answer Forum in Columbia, MO, Extension Weed Specialist Kevin Bradley noted that enemy No. 1 for growers and retailers has become common waterhemp. The main practice that led to waterhemp developing resistance in the state has been growers that plant Roundup Ready soybeans year after year, in some cases for nearly a decade.
Waterhemp is also a growing problem in east Texas, with some glyphosate-tolerant and potentially resistant biotypes emerging, says Paul Baumann, professor and Extension weed specialist,Texas A&M University. Baumann spoke at a Total Weed Management event in El Campo, TX. Among the more widespread issues is sulfonylurea resistant ryegrass, reported in several counties in the state.
To address the problem, Wisconsin's Boerboom recommended growers take the following steps:
• Apply integrated weed management practices, including multiple modes of actions and crop rotation.
• Use the full recommended herbicide rate and proper application timing.
• Scout fields after herbicide application to ensure control has been achieved.
• Use good agronomic principles that enhance crop competitiveness as well as scouting, monitoring, and cleaning equipment between fields.
"Good weed management is more than good weed control," concluded Boerboom.
While agronomy is key, taking the discussion right down to the grower's bank account is also important. Bradley says that even with the threat of resistance, in his experience, growers have been most responsive to dollars and cents. "I think growers are more willing to utilize a preemerge if we talk about how it maximizes crop yield and prevents yield loss," said Bradley.