During the past couple of years, concern has grown that a population of herbicide-resistant Johnsongrass existed within the county.  We were contacted by a farm to investigate whether this was in fact the case and what the implications might be for them going forward.  After a consultation and a field inspection, it was determined that a replicated variable rate trial with different classes of herbicides would be in order.  

Three herbicides were applied over the top of the marked and gridded plot at the recommended rate and at double that rate for each herbicide.  Each replication also contained an unsprayed control.  This application took place in the last week of September and the first observation of the results took place on October the 12th.  Initial indications are that there appears to be herbicide resistance in the field.  Following the observation and consultation with the landowner, it was determined that in order to make sure what was being observed was in fact true the study should be repeated in the late Spring of 2022.  

In the spring of 2022, the plot was once again installed and 9 different treatments were prepared.  The attachment shows the products and treatments used in the Spring.  This would be the same treatment that occurred in the fall of 2021 but with the addition of the Clethodim Product.  As the data indicates we have Nicosulfuron-resistant Johnsongrass and possibly Quizalofop-P-ethyl as well.  To undeniably verify this finding we will continue with a greenhouse trial. 

The continuation of this trial in the fall of 2023,  includes the greenhouse element of the evaluation. A recently hired position will allow us to investigate, at a molecular, why we have clear resistance to one class of chemical and some level of resistance to a different class of chemical while not having any resistance to others.  For this phase of the evaluation, multiple samples were collected from two locations.  The original location is in Nelson County and at another location in the neighboring county of Larue.  

22JG-Nelson_ MeansTable.pdf