By Travis Legleiter, Weed Science Program Specialist; Bill Johnson, Weed Scientist

The occurrences of failure to control waterhemp and Palmer amaranth with post-emergence herbicides has continued to increase during the 2016 field season in Indiana. The Purdue Weed Science Group (with funding assistance from the Indiana Soybean Alliance) has been accepting samples for resistance screening from farmers and industry reps who have experienced poor control of waterhemp and Palmer. The samples received have been tested for PPO-inhibitor (group 14), glyphosate (group 9), and ALS resistance (group 2). So far samples from 22 fields from 10 counties within Indiana this year have been tested for resistance, with more samples waiting to be run through the lab.

The occurrence of glyphosate resistance in waterhemp continues to increase with a large majority of populations submitted having one of the resistance mechanisms we can screen for in our lab. We have currently confirmed 31 counties with glyphosate-resistant waterhemp and 19 of 22 samples submitted to the lab contained at least 1 plant with resistance and a majority of samples contained predominantly resistant plants. 

With widespread glyphosate resistance in waterhemp, growers are forced to rely heavily on PPO-inhibiting herbicides to control the problematic weed in soybean. Other than Liberty in a LibertyLink soybean system, post-emergence PPO-inhibiting herbicides (Flexstar, Cobra, Ultra Blazer, etc.) are the only products available to control waterhemp post-emergence. These post applications of PPOs often follow a pre-emergence herbicide with a PPO-inhibiting active ingredient (Valor and Authority products). The repeated use of PPOs in these systems in conjunction with often late post applications on large plants has quickly led to the selection of PPO-resistant waterhemp populations.

To date Purdue Weed Science has confirmed PPO-resistant waterhemp in 14 Indiana counties with the majority of occurrences occurring in the southwestern portion of the state.  The laboratory has screened samples from 20 fields for PPO-resistance so far in 2016, with 10 of those samples returning at least one plant that was resistant. As PPOs continue to be heavily relied upon for management of glyphosate-resistant waterhemp, it is expected that the occurrence of PPO-resistance will continue to increase.

Purdue researchers have also confirmed one population of PPO-resistant Palmer amaranth that has occurred in Daviess County. While this is the only population confirmed in Indiana, it is worth noting that Palmer amaranth also has the potential for selecting for PPO-resistance.

Growers who are currently dealing with glyphosate-resistant populations of Palmer amaranth and waterhemp are encouraged to diversify their modes of action as much as possible in their soybean acres. This would include using a group 15 (Warrant, Dual II Magnum, Outlook, Zidua, etc.) or 5 (metribuzin) in conjunction with the group 14’s (PPO-inhibitors) in their pre-emergence programs. Growers should also consider the use of LibertyLink soybean to add another mode of action option to their post-emergence programs. 

Post-emergence applications of PPO-inhibitors as well as Liberty should be maximized to reduce the risk of selecting for resistant plants. Maximizing these applications includes making timely applications to plants that are no larger than 6 inches tall, as well as ensuring proper spray coverage of these contact products by using higher spray volumes (15-20 GPA) and smaller droplet producing nozzles.