PART VI. WEED MANAGEMENT AND SUSTAINABILITY
Weeds in soybean fields arguably pose the greatest threat to sustainable soybean production in Mississippi and the Midsouth. The battle to control/manage weeds has been at the forefront of soybean production issues since soybeans became a major US crop.
US soybean farming based on weed management tools can be divided into four distinct periods.
Period before herbicides
Tillage, both pre- and post-plant, was the only available tool for weed control in soybeans. Wide-row spacings were used so that post-plant cultivation could be effectively conducted. Weed escapes were common, resulting in reduced yield and difficult harvest. Late in this period, over-the-top applications of 2,4-DB were made at or soon after layby to provide late-season control or suppression of problematic broadleaf weeds. Johnsongrass became a major weed problem in many fields.
Period before glyphosate-resistant (GR) soybean varieties (pre mid-1990's)
Before transgenic GR soybean varieties appeared in the mid-1990's, weeds in soybeans were controlled/managed by a combination of tillage and “conventional” or non-glyphosate herbicides, followed later in the period by sole reliance on pre- and post-plant applications of conventional grass and broadleaf herbicides.
- Early in this period, preplant herbicides such as Treflan were applied either in the fall or before planting in the spring. This operation required tillage but did aid in early-season weed management, especially of johnsongrass and pigweed.
- Glyphosate (Roundup) entered the weed management scene as an in-season herbicide applied through recirculating sprayers and rope-wick applicators. This methodology was directed at weeds that were taller than conventional soybeans; thus, it was only effective later in the season after early-season competition between weeds and soybeans had already occurred.
- In the late 1970's/early 1980's, Roundup and paraquat came into use as preplant, foliar-applied (burndown) herbicides that were effective without tillage. This use of herbicides in place of preplant tillage fostered the currently-used stale seedbed planting system.
- During the latter part of this period, pre- and post-emergence selective herbicides became available to control most grass and broadleaf weeds.
- Narrow row culture of soybeans and limited and no-tillage systems became manageable.
Period of GR soybean varieties (mid-1990's to late 2000's)
Development of transgenic GR soybean varieties was arguably the most significant step toward a sustainable weed management system for soybean production. Glyphosate used on GR soybeans became the primary weed control system because this one herbicide killed both grass and broadleaf weeds when applied either pre- or post-plant. Limited- and no-till systems became widely used, weed control costs were lowered, total poundage of applied herbicides declined, labor inputs declined, and narrow row culture became the norm. GR soybean varieties were planted on nearly all of the US soybean acreage toward the end of this period.
The period of GR weeds (present and future)
The selection for resistance to glyphosate by numerous weed species has tempered the effectiveness of relying solely on applying glyphosate to GR soybeans. In fact, weed resistance to glyphosate has generated possibly the most intense discussions and activity in the history of soybean weed management. It is widely agreed that this problem will change the soybean farming tools available for weed management. Suggested solutions have included:
Reverting to a conventional weed management system (non-glyphosate herbicides and non-GR soybean varieties). Problems with the sustainability of this system are:
- Producers will have to consider tillage as a component in a conventional weed management system. This would necessarily require reverting to wider row spacings for post-emergence cultivation. Increased pre- and post-plant tillage are not feasible or practical in today’s agricultural environment because of erosion concerns, labor constraints, and farm size.
- Available non-glyphosate herbicide chemistries are disappearing.
- Few new non-glyphosate herbicide chemistries are forthcoming.
- The number of conventional varieties has decreased to a level that will not sustain a large acreage of their production.
- Weed management solely with conventional varieties and conventional herbicides was not/will not be all that successful.
- Weed management solely with conventional varieties and herbicides can be prohibitively expensive, even with the lower cost for seed of conventional varieties.
- Future soybean breeding efforts likely will use transgenic, herbicide-resistant parents to develop new varieties.
Soybean varieties with transgenic traits will be the soybean farming tools that will provide the foundation for future sustainable weed management systems in the US and Midsouth. Factors supporting this statement are:
- Transgenic varieties with resistance to nonselective, non-glyphosate herbicides have been and will continue to be developed. These include Liberty Link, Optimum GAT, and varieties with 2,4-D and dicamba resistance.
- Forthcoming varieties will have transgenic traits that will impart resistance to more than one herbicide or class of herbicides. An example is the Enlist™ weed management system forthcoming from Dow AgroSciences.
- Forthcoming transgenic varieties with multiple transgenic weed management traits will be necessary to slow or stop the development of weeds that are resistant to particular herbicides.
- All of the tools for weed management in conventional soybeans can be used with transgenic herbicide-resistant varieties. Using conventional herbicides with current and forthcoming new herbicide-resistant varieties in an integrated approach will ensure that glyphosate and GR soybean will continue to be a major component of a sustainable, environmentally acceptable system for weed management in soybeans.
Using soybean varieties with transgenic traits as the primary weed management tool in production systems that include soybeans presents the following challenges.
- The use of varieties and hybrids, all with transgenic herbicide resistance traits, in a soybean:grain crop rotation will require that producers and consultants acquire a thorough knowledge of the transgenic traits in each crop and herbicide mode-of-action to ensure support of resistance management.
- Weed scientists, specialists, and consultants must be trained and skilled in cross-crop weed management since most crops in a rotation system will be transgenic, with genetic traits for resistance to multiple herbicides.
- Controlling volunteer plants of the off-season rotation crop will be more complex when all crops in a rotation system are transgenic for the same herbicides or classes of herbicides.
The dynamics of weed populations and species composition in response to soybean production practices and weed management systems creates a moving target for management that nullifies attempts to create a static sustainable weed management system in soybeans. However, the above points can be used to ensure that soybean farming tools used for future weed management are as sustainable as possible.