The advent of herbicide-resistant (HR) crops and weeds has led to the reevaluation of weed control strategies and systems. This is especially so for soybeans that have varieties with traits that are classified as conventional (CONV), glyphosate-resistant (GR), and glufosinate-resistant (GLR) with respect to herbicide tolerance, and that are plagued by problem weeds such as GR Palmer pigweed.

A just-published research report entitled “Comparison of weed control, yield, and net income in conventional, glyphosate-resistant, and glufosinate-resistant soybean” from research conducted in 2009 and 2010 sheds some light on the returns associated with using varieties in these three classes and their associated weed control systems.

The objective of this research conducted in central (waterhemp site) and southeast (Palmer pigweed site) Missouri was to evaluate the effect of preemergent (PRE) and postemergent (POST) herbicide programs on weed control, soybean yield, and net income when using CONV, GR, and GLR soybean systems. The Palmer pigweed at the southeast Missouri site was susceptible to both glyphosate and ALS herbicides.

The summary of these results presented here will concentrate on those from the southeast Missouri site since Palmer pigweed is a major problem weed in Midsouth soybeans.

The herbicides used in each treatment of the above three systems are those commonly used to control both susceptible and HR populations of Palmer pigweed, and are shown in the below chart. For each weed control system, a nontreated control was included for comparison.

Net return to each herbicide program within each soybean system was calculated by subtracting the estimated costs of each treatment from gross income.

Major Findings From The Study

Across all herbicide programs and soybean systems, the greatest level of soybean injury was measured in the CONV system where at least one POST herbicide application was made.

In the CONV soybean system, PRE herbicides were required to achieve the highest level of Palmer pigweed control (86%). POST treatments resulted in <30% control.

The 2-pass POST treatment resulted in Palmer pigweed control in the GR system (90%) and the GLR system (86%) that was equal to that resulting from the treatments that included a PRE herbicide application. Obviously, the presence of GR-resistant Palmer pigweed at the site would have influenced the results from these POST-only treatments.

In the CONV system, the PRE-only and PRE fb POST treatments resulted in the highest yields of 30 and 25 bu/acre, respectively. POST-only treatments in the CONV system resulted in yields that were similar to that from the nontreated control, and all were lower than the above yields.

In the GR and GLR systems, yields from the 2-pass POST treatments were among the highest (37 and 35 bu/acre, respectively).

The 2-pass POST treatment in the GR system at the Palmer pigweed site resulted in a net return of $333/acre above the treatment cost. All other treatments in this system resulted in net returns that were at least $61/acre lower than this.

All treatments in the CONV system resulted in net returns that were at least $61/acre lower than the standard 2-pass POST treatment in the GR system.

In the GLR system, the 2-pass POST treatment resulted in a net return that was only $14/acre lower than that from the 2-pass POST treatment in the GR system.

Take Home Message

The greatest soybean injury, poorest control of Palmer pigweed, and lowest soybean yields were measured in the CONV system with the POST-only herbicide program.

In GR and GLR systems, a 2-pass POST herbicide program resulted in the highest net returns above herbicide costs. Using programs with PRE herbicides in these two systems resulted in yields that were similar to or lower than those from the 2-pass POST programs, and resulted in lower net returns.

In the absence of GR Palmer pigweed, a POST-only herbicide program in both GR and GLR soybean systems resulted in highest net returns.

Even though these results support the use of POST-only herbicide programs in both GR and GLR soybean systems, it has become evident that multiple modes of action must be incorporated into soybean herbicide programs to both control GR Palmer pigweed and prevent selection for it. Residual herbicides such as those used in this study meet that objective.

Rotating GR and GLR soybean systems with their accompanying POST-only herbicide programs can and should be considered as a tool for managing the occurrence of HR Palmer pigweed.

Results from other studies not cited here support: 1) use of GLR soybeans with glufosinate is an excellent tool for controlling GR Palmer pigweed; 2) glyphosate should still be considered as a viable weed control option when used in conjunction with both PRE and POST herbicides with a different mode of action; and 3) POST glyphosate alone is no longer a viable stand- alone weed control program for GR soybeans even if no GR weeds are present.

 

HERBICIDE PROGRAMS FOR AND HERBICIDES APPLIED TO CONVENTIONAL (CONV), GLYPHOSATE-RESISTANT (GR), AND GLUFOSINATE-RESISTANT (GLR) SOYBEAN SYSTEMS (PRE = preemergent; POST = postemergent; fb = followed by).

PRE-only

CONV–Authority + FirstRate + Dual + Reflex

GR–Authority + FirstRate + Dual + Reflex

GLR–Authority + FirstRate + Dual + Reflex

PRE fb POST

CONV–Dual + Reflex fb Cobra + Select

GR–Dual + Reflex fb Glyphosate

GLR–Dual + Reflex fb Glufosinate

2-pass POST

CONV–Cobra + Select

GR–Glyphosate fb Glyphosate

GLR–Glufosinate fb Glufosinate

1-pass POST with Residual

CONV–Cobra + Select + Dual + Reflex

GR–Glyphosate + Dual + Reflex

GLR–Glufosinate + Dual + Reflex

1-pass POST

CONV–Cobra + Select + Resource

GR–Glyphosate

GLR–Glufosinate

Composed by Larry G. Heatherly, Feb. 2014, larryheatherly@bellsouth.net