The effectiveness of glyphosate on a wide spectrum of weeds, and its simplicity of use and application, its broad range of application timing, its lower cost, and the freedom to rotate crops following its use all led to the rapid adoption of a glyphosate-only weed management system on glyphosate-resistant (GR) crops. Plus, GR crops and the subsequent glyphosate-only weed management system meshed well with conservation tillage systems, narrow rows, and greater reliance on post-emergence (POST) weed control that was being adopted on a wider scale in the mid-1990's. Simply stated, glyphosate changed the paradigm for weed management in soybeans.

The over-reliance on a glyphosate-only weed control program resulted in heavy selection pressure for the development of GR weed biotypes. The management of these weeds has become the new frontier in weed science.

A 5-year “Benchmark Study” (2006-2010) was conducted in grower fields in Illinois, Iowa, and Indiana (Midwest), Nebraska (West), and North Carolina and Mississippi (South) to compare crop yields and net returns from standard weed management programs (SP) used by growers to those from programs using best management practices (BMP) recommended by university weed scientists. The BMP programs were designed to prevent or mitigate herbicide-resistant (HR) weeds on an individual field basis. The results from that study are reported in a 2014 issue of Pest Management Science (Vol. 70:1924-1929) authored by C. Blake Edwards et al.

First, a few details about the study and its components.

A representative group of growers from 6 states was included in the study.

• Fields were 40 acres or more in size, and were divided into two similar-sized halves.


• One half of each field had weeds managed according to a BMP system prescribed by a state’s weed scientist according to field history and in-season weed scouting. The system for each field was designed specifically to manage and/or mitigate the development of HR weeds.


• One half of each field had weeds managed according to typical SP’s, which generally consisted of glyphosate only.


• BMP weed control systems used alternative residual herbicides and postemergence herbicides, all representing multiple modes of action, whereas the SP system in many cases used only glyphosate.


• Growers applied all treatments and provided data for all production input variables and crop yields. Each state’s weed scientist provided cost data for input into their state’s budget generator. Weed control costs consisted of herbicide costs, technology fees for GR seed, and application and tillage costs related to weed control.


• Seven crop production systems involving corn, cotton, soybeans, and rice were used by producers, but only results from the soybean component are presented here.


• Three tillage systems–conventional tillage (CT), minimum tillage (MT), and no tillage (NT)–were used.

 

 

• Weed control costs were 31% higher ($36 vs. $27/acre) for the BMP system than for the SP system across all cropping systems in the survey.


• Weed control costs ranged from 26% to 36% greater for the BMP vs. the SP system in the three individual regions of the study. The Midwest and South had nearly identical percentage cost differences.


• Yield differences between the BMP and SP systems were very small or nonexistent in all of the above cases. The biggest differences occurred in the South (5.9% greater yield from BMP) and Mississippi (5.3% greater yield from BMP). SP did not outyield BMP in any of the above tabled cases.


• Net returns to weed management were similar for the BMP and SP systems in all of the above tabled cases. In the South and Mississippi, returns to the BMP system were 4.2% and 3.6% greater, respectively, than net returns to the SP system.

The above tabled values and statements are based on averages across the variables in the study. There were, of course, cases where differences were greater than those shown above. For example, in one case a soybean grower in Mississippi used the SP system of glyphosate as a burndown, followed by a POST application of glyphosate. The BMP weed management used on the other half of the field included herbicides in addition to glyphosate in both the burndown and and POST applications. This BMP program for that field cost $53/acre vs. only $27/acre for the SP system, or a near doubling of weed control costs.

The above data and results indicate that a more intensive BMP weed management program that is designed to manage or mitigate HR weeds by including multiple applications of diverse herbicides with multiple modes of action does not result in economic loss in soybean production compared to an SP management system that relies on only glyphosate.

Therefore, weed management systems in soybeans with BMP’s that are designed to manage and mitigate HR weed populations are both agronomically and economically sustainable, and can be used without concern that net returns will be lowered with their use. This is especially important for producers to know so that they can institute weed management BMP’s before HR weeds develop without concern that profits will be lowered.

Composed by Larry G. Heatherly, Sept. 2015, larryheatherly@bellsouth.net