Our newsletter topics for this week will be a general update of some of the issues that have crossed our desk over the last week. As we move into this time of the year, weed control operations in corn are mostly complete. Weed control operations in soybeans are still ongoing with many folks putting on the first and second postemergence treatments. In wheat, we are harvesting and deciding whether or not to put out a preharvest treatment to help with harvest operations, and deciding whether or not to do a burndown in fields that will be double cropped to soybean. Hay field and pasture weed control operations during this time of year consist primarily of treating for thistles if they’re in the bud stage, or thinning out broadleaf weeds right after a cutting for hay, or if labor is available, doing stump treatments for tree or multiflora rose control.
HPPD carryover to soybean
The herbicide carryover situation is driven mostly by the weather we had in 2019. Last year we had a wet spring and a very late average planting date for the main agronomic crops grown in the state. In addition, July and August rainfall was sparse well below 30 year averages. Late planting dates and sparse rainfall late in the growing season is the perfect setup for carryover of herbicides that typically have nine month or longer rotational restrictions. We are seeing carryover with a couple of group #27 herbicides which include isoxaflutole (Balance), mesotrione (Callisto and others), and topramezone (Impact/Armezon). However, most of the cases involve mesotrione. The symptoms of this type of injury include bleaching (chlorosis) of newer leaves and growing point, narrower soybean leaves that are also smaller than normal, and overall stunted growth (Figure 1).
Figure 1. Mesotrione carryover to soybeans (Photo Credit: Marcelo Zimmer).
Isolated instances of poor waterhemp control with dicamba in soybean.
In 2019 we had a few instances of poor waterhemp control with the dicamba herbicides labeled for use in soybean. We’re currently testing a few of these populations to see how they respond to different rates and application timings. We have a few new populations that are not well controlled that have been brought to our attention again this year and we are in the process of collecting seed or soil from these areas. We do not have the capability of doing commercial screening for herbicide resistance this year since we do not have Indiana Soybean Alliance funding for this service like we have had in the past. However, if you have a field that seems particularly problematic, feel free to contact Marcelo (zimmer6@purdue.edu) or myself (wgj@purdue.edu) and we might be interested in collecting some seed or soil from the field.
Video on marestail and giant ragweed burndown.
In this new covid-19 world in which we are operating, we are not able to do face-to-face training and field days. So we are experimenting with the filming of more videos to use for training and information exchange. Last week, Marcelo and I put one video together on marestail and giant ragweed control with burndown herbicides. Here is a video on Burndown Strategies for Horseweed and Giant Ragweed.
Next week we’re going to work on a video about volunteer corn control in soybean.
Enlist soybeans cupped after 2,4-D application?
These questions and claims started rolling in about two weeks ago. We have worked with the Enlist soybean system in research plots for over 10 years. The only injury symptoms we have observed were leaf drooping within a few hours after application, and some necrotic spots on the leaves when we applied high rates of 2,4-D choline. We have never observed cupping that is commonly associated with dicamba exposure on Enlist soybean. If any soybeans other than Xtend varieties are showing leaf cupping, it’s most likely due to dicamba. 2,4-D does not cause leaf cupping. But it does cause leaf strapping, stem twisting and callous tissue formation on soybean stems. See this publication for more details on differentiating dicamba versus 2,4-D exposure on soybeans: https://ag.purdue.edu/btny/weedscience/Documents/WS-56.pdf
