By Bruce MacKellar, Field Crops Specialist 

The 2015 growing season was full of challenges. Finding timely opportunities to spray herbicides on corn and soybeans was one of them. Despite this, most fields remain remarkably clean as we head into harvest across southwest Michigan. 

However, a few fields are notably not weed-free this fall. Even more troubling is the fact that often a single weed species remains in many of these fields. Michigan State University Extension is promoting a research effort conducted by Erin Hill of the MSUWeed Control Program to help growers determine if the weeds that remain in fields are becoming resistant to the five most commonly used herbicide modes of action.

The top priority weeds we think growers might find this fall are pigweed species that refused to die despite herbicide applications that should control them. The most troublesome weed in this family is Palmer amaranth. Palmer amaranth tends to have long “flowing” type seedheads at the top of the plant that often “dance” in the wind. Keep in mind that Palmer amaranth has both male and female plants, so there may be quite a bit of variability in the look of the seedheads you find in fields. If you find a few of these plants, be sure to take a garbage bag out in the fields and clip off the seedheads before the seed sheds. This is important to do because each plant may produce as many as 450,000 seeds. If you find pockets of Palmer amaranth, be prepared to step up your vigilance in controlling the weed species over the next few years.

Palmer amaranth is not the only pigweed species we are finding in southwest Michigan that has a history of multiple herbicide resistance. Common waterhemp, which is resistant to both the ALS inhibitors and glyphosate, was confirmed by field crops weed control specialist Christy Sprague in samples submitted to the lab in Berrien and St. Joseph County in 2014. There is also an ALS inhibitor-resistant redroot pigweed that was found in Shiawassee County in 2014. Berrien County soybean fields have seen an increase in common waterhemp in 2015. If you find either common waterhemp or Palmer amaranth in small patches, be sure to try to remove all of the plants before the seed is dispersed.

If you find either Palmer amaranth or common waterhemp, recommendations for dealing with either of these commonly herbicide-resistant pigweed species will use the same weed control programs. In short, this will include a switch to a Liberty Link soybean variety and vigilant combinations of pre- and post-emergent herbicide treatments before Palmer amaranth reaches 3 inches in height. For complete recommendations from Sprague, see “Multiple herbicide-resistant Palmer amaranth in Michigan: Keys to management in soybean, corn and alfalfa.”

Some other commonly resistant weed species growers should be on the lookout for this fall include ALS- and glyphosate-resistant marestail, ALS- or glyphosate-resistant common ragweed and glyphosate-resistant giant ragweed. Resistant marestail has been with us for a few years now, especially in southeast Michigan. Sprague has not confirmed glyphosate-resistant giant ragweed in Michigan, but there is plenty of it just across our border in northern Indiana. There are also several corn and soybean fields in southwest Michigan with giant ragweed infestations this fall. Hill’s “2015 Status of herbicide resistant weeds in Michigan” article provides a complete list of resistant weeds confirmed in Michigan, and more on their identification and life cycles. 

If you think you have any of these (or other) weed species that survived full strength herbicide applications without injury, consider collecting seedheads from a minimum of five plants and submitting them to MSU for herbicide screening. This process can help you to better understand which herbicide programs can still be effective in controlling these weed species. The cost of the screening is being paid for by the Michigan Soybean Promotion Committee in 2015. Be sure to place the seed samples in folded-over paper grocery bags to allow for proper drying. 

 The screening process takes a considerable amount of time to conduct in the lab and greenhouse, so be sure to submit your samples early. You can drop off samples to your local MSU Extension office in southwest Michigan and we will make sure they get to campus. You can download the accompanying form at: FREE Screening for Herbicide-Resistant Weeds in Soybean Production Systems.