Two weeds which farmers need to pay particular attention to are cleavers and chickweed. Both are becoming increasingly common on the prairies. What is even worse is both are rapidly developing resistance to popular herbicides — especially Group 2 herbicides.
Group 2 broadleaf weed resistance has become a big problem in areas of Western Canada. In the 2007 Alberta weed survey, Group 2 resistant chickweed was found in 40 per cent of the surveyed fields that had chickweed populations. The previous 2001 weed survey had documented Group 2 resistant chickweed in only 17 per cent of fields.
In fields with cleavers, 17 per cent had Group 2 resistant biotypes in 2007. In 2001 none of the surveyed fields had resistant cleavers.
The author of the 2007 Weed Survey Report, Dr. Hugh Beckie wrote: “resistance is steadily increasing in chickweed and spiny annual sow-thistle, and most recently, cleavers.”
Furthermore, a study by Leeson in 2005 found the abundance of cleavers is increasing faster than any other weed.
What can farmers do about it
It is important that farmers rotate herbicide groups to slow the spread of Group 2 resistant chickweed and cleavers or we will end up in the same situation we now have with kochia and spiny annual sow thistle. In the 2007 weed survey, every field with spiny annual sow thistle and 85 per cent of fields with kochia were Group 2 resistant. We have basically lost the use of Group 2 herbicides to control these two weeds already. We definitely do not want to add chickweed and cleavers to this list.
Fortunately, growers have quite a few herbicide options for controlling chickweed and cleavers. In fact, the online Alberta Herbicide Selector lists over 30 products and over 150 tank mix combinations that will control cleavers at the 3-4 leaf stage in wheat. However, over half of the products which control cleavers are Group 2 actives. So it is very important that you take the time to study which actives will control cleavers and chickweed in your cereals because you have the most herbicide options in those crops. Make sure you rotate the group 2 broadleaf products with other chemistries that will also control these weeds. The accompanying table categorizes herbicides which will control or suppress chickweed and cleavers in wheat and barley into the group and active ingredients.
Famers have fewer options for controlling these two weeds in other crops. Growers of Clearfield canola can control both cleavers and chickweed with Absolute, Odyssey and Odyssey DLX, but these products are all Group 2s. These products will provide season long control of later flushes. Solo and Tensile, also group 2s, do not control chickweed but will supress cleavers. Again, both are Group 2s.
Liberty and all brands of glyphosates will control both cleavers and chickweed in Liberty Link and Roundup Ready canola systems. However these products only control the emerged weeds at time of spraying. They provide no control of second flushes.
Pulse growers have even fewer options. Most pulse growers rely on Group 2 products like Odyssey, Pursuit, or Solo in pulse crops. The only other option they have is Group 6 bentazon products like Basagran or using a pre-plant incorporated product like Edge.
Herbicide resistance is a growing problem. Extrapolation of the 2007 Alberta Weed Survey results indicates there could be up to 3.1 million acres in Alberta alone where one or more weeds are resistant to one or more products. This is 40 per cent of the cropland in the province. Make sure you are doing everything possible to slow the increase of resistant weeds, including rotating your herbicides through all groups when controlling weeds like chickweed and cleavers. †