Volume 57 Number 12 Date 06/28/2012
GLYPHOSATE RESISTANCE - Extension Weed Scientists at the UW-Madison report that a population of glyphosate-resistant giant ragweed has been found in Rock County, representing the first confirmed case of weed resistance to glyphosate in Wisconsin. The widespread utilization of glyphosate and glyphosate-resistant crops has substantially increased selection for weed resistance to glyphosate, and more cases of resistance will inevitably develop if farmers are not proactive about resistance management.
Signs of herbicide resistance can be difficult to recognize in a field setting, but growers can isolate probable causes by process of elimination. Common reasons for ineffective weed control include: late herbicide application, untimely activation, excessive rainfall too soon after application, sprayer skips, reduced herbicide rates and herbicides that are applied too early.
Indicators that weed "escapes" are the result of human or environmental causes include the presence of multiple weed species, a spatial arrangement that follows the path of herbicide application, or a uniform lack of response to herbicides within a field. By contrast, characteristics of herbicide resistance include moderate to high densities of a single weed species, random patterns or patchy distribution of the same species field-wide, variable response to herbicide, and vigorous plants next to dead plants of the same species.
If herbicides were applied on time, at an effective rate, yet fields contain many escapes of one weed species, growers should consult their county agent or a crop advisor.
WEEDS IN SOYBEANS - Herbicide applications in soybeans should not be delayed, especially for drought stressed fields. Yield reductions due to weeds are greater during dry periods as compared to when moisture levels are adequate. Early morning applications, at slower-than-normal speeds, are most effective since leaf surface exposure is greatest at this time and dust interference is minimal.
--Clarissa Hammond, DATCP Weed Scientist