Waterhemp and Soybean Herbicide Management

Nov 06, 2018

As I traveled across our trade territory this year, I took the time to make some mental notes on what I was seeing during those travels. Besides being amazed how great the crops came on after a bit of an early struggle getting the season started, I saw a noticeable increase of weed escapes across the countryside. Some of the observed weeds were; volunteer corn, lambsquarter, giant ragweed, and, of course, one that has been on our radar and seems to be at hand, Waterhemp. To help understand Waterhemp impact and control options, I have included parts of an article from a recent tech bulletin we received from Winfield United, prepared by their Wisconsin Agronomist Tryston Beyrer. 
Why should waterhemp be of concern?
Waterhemp was added to the noxious weed list for Wisconsin in 2017 partially due to its ability to adapt to cultural practices and as a result, is especially challenging to control. New fields are continuously being identified throughout the state as being infested with waterhemp, where introduction of seed may have occurred from multiple sources ranging from acquisition of ‘new’ used equipment, custom harvest operations, feed, manure, uncleaned seed, and wildlife. Yield loss from waterhemp has been documented to be up to nearly 50% for season-long competition from only 20 plants per square foot. One of the challenges with managing waterhemp is that earlier planting and tillage practices has selected for a majority of waterhemp to evolve and emerge much later (June-August) than many other weed species, resulting in emergence after many herbicide applications are completed.
Once emerged, plants have higher relative growth rates than many other weed species. Plant growth is approximately 1-1¼ inches per day during typical growing conditions, but can be greater during peak growth periods. Plants that emerge after herbicide applications can reproduce and set seed. Waterhemp is a prolific seed producer, where just once plant can produce ½ million seeds, with some reports of single plants possessing as many as 5 million seeds. This reiterates the importance of how just 1 missed plant can lead to an entire field being infested with seed in the following years.  Waterhemp also has separate male and female plants (dioecious), which allows for greater genetic diversification within offspring. Individual male waterhemp plants can spread pollen ½ mile when crossbreeding, leading to dispersal of traits (e.g. herbicide resistance) to their offspring.
With this prolific seed producer of a weed, buying the correct seed traits and herbicide selection come critical in keeping your fields clean and in top production. One of the best methods of controlling waterhemp is to use multiple modes of action (MOA’s). Weed management programs consisting of more than 2.5 MOA’s have been found to be 83 times less likely to develop herbicide resistance than those using 1.5 MOA. Switching MOA’s between crop years and using full rates of at least 3 effective MOA’s will greatly decrease the likelihood of developing herbicide resistance.  Preventing weeds from emerging by using residual pre-emergent herbicides will likely be a critical component of a successful waterhemp management program.
After the preemergent treatment you will need to have plans for a post application to control and manage Waterhemp. You likely will be choosing a herbicide based on your soybean variety trait(s).  Conventional, Roundup, LibertyLink or Xtend soybean seed. Please note, Xtend varieties and the application of the approved herbicides are great options for control of resistant broadleaf weeds, but restrictions per the label limit us on the number of days and even down to hours per day that one can get an application done. Do to that narrow window, we continue to work with our seed suppliers and CPP manufactures to sort out weed control options that offer the performance we will need to manage waterhemp and yet user friendly enough to get it applied timely per label guidelines.  For more information and follow up to what has been mentioned in this article, give your Premier Agronomist a call.

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