- Postemergence Management of Glyphosate-Resistant Horseweed in Soybeans
- Palmer Amaranth Control in Soybeans
- Harvest Aids for Small Grain
Postemergence Management of Glyphosate-Resistant Horseweed in Soybeans
Options for controlling horseweed (marestail) resistant to glyphosate after the soybeans have emerged are very limited. Liberty Link soybeans are an exception, because Liberty 280 is fairly effective on horseweed (be sure to keep your rates up and keep spray volume up to ensure good coverage).
For non-Liberty Link soybeans the options are very limited. I have had very inconsistent results trying to control horseweed with ALS products (such as FirstRate, Classic or Synchrony). Part of this is due to ALS resistant horseweed populations scattered around the region; and partly due to application to larger weeds or weeds “burnt off” and starting to recover. If using you want to try either FirstRate or Synchrony on STS soybeans, use the highest rate allowed and full-adjuvant systems (refer to the labels). Horseweed plants are generally not very tolerant of shade and most soybeans will begin to canopy over the horseweed and out-compete them. Additional glyphosate applications will provide some suppression of horseweed and sometimes the soybeans have a chance to outcompete them. It is always best to treat the horseweed plants soon after they start regrowing from the burndown application.
Palmer Amaranth Control in Soybeans
Since a lot of the Palmer amaranth in Delaware and Maryland are resistant to glyphosate, a second herbicide needs to be included for POST applications. We sampled a number of fields last fall and found 5 out of 19 fields had plants resistant to ALS-inhibiting herbicides (Group 2 herbicides such as Pursuit, Raptor, Classic, Harmony, and Synchrony). This leaves fomesafen or Liberty 280 (if you have Liberty Link soybeans). Fomesafen is available as Reflex or in combination with glyphosate as Flexstar GT. Controlling Palmer with Reflex or Flexstar GT requires application to weeds 3 inches or less, which means earlier than most people often spray. Maximum effectiveness requires a full rate of fomesafen, which means 1.5 pts of Reflex. In a 2013 trial, we sprayed Palmer amaranth when it was 5 inches tall, with 1 and 1.5 pts of Reflex. Palmer amaranth control improved from 72 to 84% control when Reflex rate went from 1 pt to 1.5 pts/A.
Harvest Aids for Small Grain
A number of glyphosate products such as Roundup and Touchdown are labeled as harvest aids for winter wheat and barley. Check the label of other formulations of glyphosate to be sure the product you use is labeled as a harvest aid. Applications must be made after the hard-dough stage and at least 7 days prior to harvest. Aim is labeled as well, but the spectrum of control is limited to velvetleaf, morningglory, pigweeds, and few other weeds. Apply at least 3 days before harvest. Use of 2,4-D (or products containing 2,4-D) is generally not recommended as a harvest aid due to its volatility, and potential damage to the crop during application.
I am not aware of any paraquat formulation labeled as a harvest aid for small grains.
