Herbicide resistant waterhemp has continued to spread and be major pest for Kentucky soybean growers. The introduction of multiple herbicide tolerant soybean varieties (i.e. RR2Xtend, Enlist E3, and LLGT27) that offer effective post-emergence herbicide options has decreased farmer incentive to use costly soil residual herbicides. The elimination of soil residuals or use of single active residual herbicides potentially increases herbicide resistance selection pressure on post-emergence herbicides, as well as allows for late season waterhemp escapes. University weed scientists recommendations for effective waterhemp control can increase crop production cost of upwards of $30 per acres for residual herbicides alone and often results in a perceived unfeasible added input cost for growers.

Research trials are to be conducted at two grower locations infested with herbicide resistant waterhemp. Experimental treatments will evaluate a range of soil residual herbicides with variable number of effective sites of action as well as variable timings of application and overall treatment cost. Treatments will be evaluated for end of season waterhemp densities as well as waterhemp seed production. Overall herbicide input cost will also be calculated considering the residual herbicide treatment plus total number of post-emergence applications applied on an as needed basis.

Conduct research at waterhemp infested locations to evaluate timing of herbicide application with multiple residual sites of action in comparison to single site of action residuals and their influence on late season waterhemp densities and seed production. Comparisons will include the following residual herbicide approaches:
- Multiple-SOA pre-emergence followed by a single SOA residual applied post-emergence
- Multiple-SOA applied pre-emergence only
- Single-SOA pre-emergence followed by a single SOA residual applied post-emergence
- Single-SOA applied pre-emergence only

This research can help improve our knowledge and understanding on how to integrate residual herbicides into soybean weed control programs while balancing herbicide input cost and resistance management. By having this information, growers and crop advisors will be in a better position to make informed decisions for developing and implementing weed control programs to mitigate and manage herbicide resistant weeds. Information will be extended to soybean producers, crop consultants, extension agents, agricultural retailers, and industry representatives through various mechanisms including newsletters, meetings and field days.

University of Kentucky research supports the historical message of weed scientists that the use of a soil residual herbicide with multiple effective sites of action is the best tool for waterhemp control, regardless of soybean herbicide tolerance and post-emergence herbicide packages. Despite overwhelming evidence behind this message, growers tend to cut back use of residual herbicides especially with the recent influx of new herbicide-tolerant soybean traits that include Roundup Ready 2 Xtend, Enlist E3, Liberty Link GT27, and XtendFlex, all of which offer effective post-emergence control of waterhemp. The decision of farmers to cut back on soil residuals or use only a single effective SOA (Single-SOA) residual herbicide versus a multiple-SOA residual often comes down to added crop production cost. Multiple-SOA residuals typically cost about $20 to $30 per acre and are significant investments, as compared to a single-SOA residual which can be half the cost at $10 to $18 per acre. University of Kentucky Weed Scientists have observed late-season waterhemp escapes on multiple farmer fields where single-SOA residuals were followed by one to two applications of a post-emergence herbicide. While these late season escapes are usually sporadic throughout the field, they do produce seed that continues to build back the soil seed bank for future generations and are likely spread to other areas by harvesting equipment.

The objective of this research was to evaluate timing of herbicide applications with multiple residual sites of action in comparison to single site of action residuals and their influence on late season waterhemp control. Experiments were established at two sites in KY with known infestations of waterhemp in grower fields located in Taylor and Caldwell counties.

In summary, the short-term benefits of the results when using multiple residual herbicide sites of action were not obvious within this research as two of the single site of action residuals resulted in similar waterhemp densities at the end of the season as compared to residual treatments with multiple sites of action. However, when looking at an end of season waterhemp density and seed production from a program approach, it was evident that residuals with multiple sites of action were beneficial at the Caldwell County site. At both sites, the use of metribuzin alone as a residual herbicide did not result in acceptable season-long waterhemp control, and thus the use of additional residual sites of action were needed.

These results were collected over a single year and thus do not represent the long-term outcomes relative to the use of residual herbicides with multiple sites of action. The short-term benefits of multiple residual site of action programs are not always obvious in a single year, but weed management, and more specifically waterhemp management, must be approached from a long-term goal perspective rather than short-term.

This research benefits the soybean farmer by helping improve our knowledge and understanding on how to integrate residual herbicides into soybean weed control programs while balancing herbicide input cost and resistance management. By having this information, growers and crop advisors will be in a better position to make informed decisions for developing and implementing weed control programs to mitigate and manage herbicide resistant weeds, while reducing or balancing input cost for soybean production. Information will be extended to soybean producers, crop consultants, extension agents, agricultural retailers, and industry representatives through various mechanisms including newsletters, meetings and field days.