Weed resistance to herbicides a growing threat with annual crops

Documented resistance by annual weeds to herbicides is still at less than a handful of confirmed cases in Wisconsin but that is no reason to dismiss the possibility, University of Wisconsin Extension Service annual cropping systems weed specialist Vince Davis reminded the more than 60 attendees at the crop pest management update.

Davis pointed out that resistance by giant ragweed to glyphosate herbicide was confirmed in 2012 and then to FirstRate (an ALS inhibitor) herbicide this year and by horseweed to glyphosate in a Jefferson County field this year.

He noted that horseweed resistance to one or more herbicides has now been confirmed in 22 states.

But waterhemp was the weed about which he received the most calls in 2013 from Wisconsin farmers and crop consultants, Davis indicated.

Waterhemp is part of a weed group, along with Palmer and Powell amaranth and redroot and smooth pigweed, for which there has confirmed herbicide resistance in Illinois, Indiana, and Michigan but none so far in Wisconsin.

Amaranth concern

There's no certainty yet on the identification of a probable amaranth plant that was showing resistance in a Rock County field in 2011, Davis reported. He said attempts at definite identification by specialists in both Wisconsin and Illinois have not been conclusive and suggested the plant in question might have been a hybrid.

But with confirmation of herbicide by waterhemp and giant ragweed in many counties in Indiana and positive identification of Palmer amaranth in nearly two dozen counties there, Davis is calling for heightened awareness in Wisconsin about both of those trends.

For identification of waterhemp and pigweed species, Davis recommends a table created at Iowa State University. It and a sketch for identifying amaranth and closely related plants are available on the new http://wcws.cals.wisc.edu website, which is also uploading the latest research reports, he stated.

Weed escape survey

Davis updated attendees on the results of the research conducted in the state for the past two crop years on the late-season weeds that escaped a herbicide treatment. This is a project handled mainly by graduate student Ross Recker, who will give a detailed presentation at the Wisconsin Crop Manager Conference on Jan. 14-16 in Madison.

Based on cooperation by landowners and crop consultants, 90 corn and 63 soybean fields were visited during 2012 in the much of the major agricultural area of the state.

Among the 43 broadleaf weed species and 21 grassy weed species documented, those most frequently found in the escape settings were common lambsquarters (58.3 percent), dandelion (57.6 percent), velvetleaf (32.5 percent), giant foxtail (24.5 percent), and yellow foxtail (22.5 percent). Davis was surprised that waterhemp wasn't in the top five.

For the 2013 surveys in 110 corn and 80 soybean fields, there were again 21 grassy type weeds but the broadleaf total jumped to 61. Dandelions moved to the top with a 62.5 percent frequency, followed by lambsquarters (54.7 percent), velvetleaf (34.7 percent), fall panicum and volunteer corn (both at 22.1 percent).

Suspicious specimens

During the two years, Recker and Davis relied on field management history and other criteria to collect weed plants for which there was suspicion of resistance to glyphosate. What caught their attention was the increase in the number of those specimens from five for both giant ragweed and waterhemp in 2012 to 10 and nine respectively in 2013.

Giant ragweed was most prominent in the western and south central areas of Wisconsin while waterhemp was also most prominent in the south central and also prevalent in the east central and western parts of the state, Davis reported. For giant ragweed, he recommends a two-pass herbicide application, one of them being a pre-emergent treatment, and the inclusion of atrazine where it is legal.

One result of the project was the collection of the glyphosate-resistant horseweed in Jefferson County along with a suspect specimen in Grant County, Davis indicated. He said horseweed is most typically found in no-till cropping, noted its small seed has about two years of viability in a seedbank, and recommended treatment before the weed reaches a height of four inches with 2,4-D and residual products in the herbicide mix.

Other suggestions for dealing with potentially herbicide resistance weeds and for controlling weeds in general are to know what weeds are present, establish crops in clean fields, use residual herbicides, rotate herbicide modes of action, conduct in-crop tillage, and introduce a rotation of forage into annual weed populations that have proven to be difficult to control, Davis advised.

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