Information about weeds and herbicides for the agricultural community
A few observations from windshield scouting
We spent some time windshield scouting fields northeast of Columbus this week after being called out to determine whether a mystery weed was Palmer amaranth. There were only a few plants of this mystery weed in the field, and it tuned out to be common mullein. The inflorescence stalk on mullein can resemble a long Palmer seed head from a distance, but upon closer examination it’s difficult to confuse the two weeds. Mullein has a single stalk inflorescence while Palmer can have many, and the rest of the mullein plant doesn’t resemble an amaranth at all. Mullein rarely shows up in large numbers in corn and soybean fields, being primarily a weed of more undisturbed places. Use of the “images” tool in a Google search will allow you to pull up photos of both and see for yourself. Also take a look at the video on amaranth identification in the Palmer amaranth section of this website under the “weeds” tab. A couple photos below of mullein (left) and Palmer (right) to prove the point – the mullein photo was from the field in question:
In the soybeans behind the mullein, symptoms of PPO herbicide application were evident (fomesafen), which was intended to control giant ragweed and I suppose possibly marestail. The giant ragweed was pretty well controlled and the marestail unaffected. We have received more calls than usual asking for any possible solutions to control marestail in Roundup Ready soybeans, and there still aren’t any. Money spent on PPO inhibitors (fomesafen, Cobra, Cadet, etc) or 2,4-DB in an attempt to control marestail is just wasted in our opinion. Reports of glyphosate resistance in giant ragweed and common ragweed have also been on the rise. There has been much use of fomesafen remediate this, although expectations of effectiveness are often too high. Fomesafen is about an 85% herbicide on small giant ragweed plants, and this number decreases with plant size. Keep in mind also that when fomesafen is the primary herbicide in the mix for control of glyphosate-resistant ragweed, the rate, adjuvants, and application volume need to be optimized for fomesafen. This means use of the highest rate possible per geographic area, inclusion of COC or MSO, and upwards of 15 gpa.
We also found an infestation of waterhemp on this trip, in a field of LibertyLink soybeans. The LibertyLink system can be effective for management of waterhemp, but the plants in this field were already up to 20 inches tall. The field was supposed to be treated with glufosinate “soon”, which still was way too LATE. Glufosinate won’t control waterhemp plants this tall, and we suggested the addition of fomesafen, which still might not be enough. And finally, one of our educators sent us the following photo of waterhemp in soybeans (I think there are soybeans in there):
These plants are of course well beyond any control measures that involve herbicides. The grower has decided at this point that continuous soybeans and use of only glyphosate may not be the best course, and is thinking about residual herbicides and corn and cover crops for next year. Some people will change when the sky actually falls on them I guess.