Much has been and is being written about glyphosate-tolerant weeds in the southern United States. Articles about the subject are appearing regularly in numerous agricultural publications. There is general agreement among weed scientists that glyphosate tolerance in weeds will become a major factor in weed management, and that currently it is more critical for cotton than for soybean production.
The following content is presented in concert with Trey Koger, USDA-ARS research agronomist at Stoneville, Miss.; Dan Poston, MAFES weed scientist/soybean specialist at Stoneville; and Eric Walker, USDA-ARS research agronomist at Jackson, Tenn.
These young colleagues, trained as weed scientists, provide valuable insights into the problem and how it can be addressed in soybean production systems. They are involved in soybean research and are working toward the development of production systems that sustain and improve profitability of soybean culture in the Mid-South. Thus, glyphosate-tolerant weeds are of paramount importance to them.
For the last decade, soybean producers have spent a decreasing percentage of total production costs on weed control because of Roundup Ready technology and the advent of generic glyphosate. This phenomenal accomplishment has offset cost escalations for other inputs, allowing soybean producers to continue to make a reasonable profit.
The flexibility and simplicity of using glyphosate, plus the normally excellent weed control from its use, have allowed growers to give more attention to other factors such as disease, insect, and irrigation management.
Presently, glyphosate-tolerance concerns are associated with the broadleaf weeds horseweed (marestail), several pigweed species, and common ragweed. There is documented ryegrass resistance/tolerance as well. The advent of glyphosate-tolerant horseweed has occurred in conjunction with reduced-tillage cropping systems.
Soybean production practices will have to change if glyphosate-tolerant weeds begin to affect yield. These changes likely will involve increased preplant tillage and the inclusion of non-glyphosate herbicides in weed management systems; both will increase production costs.
Due to erosiveness of soils in some areas of the Mid-South and escalating fuel costs, increased tillage for preplant weed control may not be environmentally and/or economically acceptable.
The most sensible approach to control and prevent glyphosate-tolerant weeds in soybean production systems is to change herbicide weed management and adopt a crop rotation program. A rotation of soybeans followed by corn, with weed management in corn using atrazine or herbicides other than glyphosate, could help reduce the development of glyphosate-tolerant weeds. Atrazine is still commonly used in glyphosate-tolerant and conventional corn due to its broad-spectrum weed control and low price.
The accompanying table presents representative weed management systems for soybeans, along with their costs. Costs for herbicides are a composite from several sources, and thus may not represent the actual cost for an individual producer. However, we believe they are reasonable representations for the purpose of this article.
The cheapest system, of course, uses glyphosate for both pre- and post-plant weed control. Where weed tolerance does not occur and is not anticipated, this system can and should be used for obvious economic reasons.
Presently, the efficacy, safety, and economics of a glyphosate-only weed management system for soybeans make it the best option, and it should be used to the fullest extent possible. However, to preserve this technology, producers must continually monitor fields for changes in control by glyphosate.
If weeds that were previously controlled by glyphosate survive an application, producers should make note of the timing, rate, sprayer settings, and spraying conditions at the time of application. If warranted, this system should be supplemented or replaced with alternative weed management strategies.
One alternative system involves replacing or supplementing glyphosate in the preplant or burndown portion of weed management. Examples in the table use Ignite or Gramoxone to replace glyphosate, 2,4-D or Clarity (dicamba) to supplement glyphosate, and the residual herbicide Valor added to glyphosate and 2,4-D. All of these non-glyphosate herbicides have good activity on broadleaf weeds, including those with proven tolerance to glyphosate.
However, environmental conditions can influence efficacy of Ignite and Gramoxone, and excellent spray coverage with both products is required to achieve optimal performance. With each of these burndown changes, post-plant glyphosate is used to control in-season weeds.
Costs for this system with alternative preplant herbicides range from about $2.50 to $8 per acre greater than the cost for a total glyphosate weed management system.
The increased cost of the three-way mixture of glyphosate, 2,4-D, and Valor is somewhat offset by the elimination of a possible additional burndown at planting and an in-season glyphosate application.
According to Poston, residual herbicides prevent flushes of weeds like glyphosate-tolerant horseweed that may emerge between burndown and planting. He also states that net returns for early-planted soybeans in Mississippi are typically higher when residual herbicides are used in preplant burndown programs.
Koger proposes that other residual herbicides such as Prowl can be used to prevent horseweed from germinating between burndown and planting.
A second alternative system uses glyphosate and non-glyphosate herbicides applied preplant, non-glyphosate herbicides (FirstRate, Canopy, and Canopy XL) applied pre-emergence, and glyphosate applied postemergence. The FirstRate label indicates control of all weeds presently exhibiting tolerance to glyphosate, while the Canopy and Canopy XL labels do not show control of horseweed. However, either 2,4-D or Clarity included in the burndown application will control horseweed.
Costs for this system range from about $14 to $22 per acre greater than the cost for the glyphosate-only system, and about $9 to $17 per acre more than the system where non-glyphosate herbicides replace or supplement glyphosate in the burndown application. (It is important to note that some varieties may be sensitive to the metribuzin in Canopy and the sulfentrazone in Canopy XL). Because of the much greater cost of this system, it should only be used on those acres that have significant infestations of glyphosate-tolerant weeds.
FirstRate is a viable postemergence (0.4 oz. and about $10.50 per acre) control alternative for glyphosate-tolerant horseweed in soybean. However, it is an ALS-inhibiting herbicide with a single site of action and weed tolerance is likely to occur if it is relied on too heavily. Therefore, Poston cautions against its overuse.
Other pre-emergent herbicides can be used in this system to offset this concern and possibly lower cost.
On the alluvial soils of the Delta that are not subject to erosion, fall or spring tillage should be considered for control of fall-emerging weeds such as horseweed that are developing tolerance to glyphosate. However, with ultra-early planting now being practiced, fall tillage may be the only tillage input that can realistically be used to supplement control of glyphosate-tolerant species that emerge in the fall. This will not necessarily represent an additional cost because many users of the stale seedbed planting system already use secondary fall tillage such as disking and/or spring-tooth cultivation.
According to research conducted by Koger, Poston, and Tom Eubank at the Delta Research and Extension Center at Stoneville, a production system of fall tillage followed by Ignite applied in February provides excellent control of horseweed and results in a clean seedbed for early spring planting of soybeans.
Fall tillage followed by glyphosate plus 2,4-D applied in the spring provides the same level of horseweed control, and is the common stale seedbed system on the alluvial soils in the Delta. Valor can be applied after Nov. 15 with or without tillage to prevent weed emergence during the winter and early spring.
Weed shifts that result from using weed management based solely on glyphosate may be just as important as the development of glyphosate-tolerant weeds. The exclusion of residual herbicides in such a system has resulted in the presence of late-season grasses in soybeans across the Mid-South. Glyphosate does an excellent job of controlling annual and perennial grasses, but has no residual activity. Thus, grasses that germinate throughout the growing season and under a crop canopy have become problematic in soybeans late in the season after the intended final glyphosate application.
Late-season grasses can reduce yield, interfere with and reduce harvest efficiency, and increase the occurrence of late-season soybean diseases such as Phomopsis seed decay, according to Koger and Poston. The inclusion of late-season residual grass herbicides in a glyphosate-based weed management system is a viable option for their management, but adds expense and requires rainfall or irrigation for their activation.
However, this should reduce the weed seed bank for following crops and reduce the selection pressure placed on glyphosate alone in the development of glyphosate-tolerant weeds.
According to Walker, if late-season grasses are present and increased selection pressure for glyphosate-tolerant weeds is not a concern, a late-season glyphosate application should control these grasses and also reduce seed production of many other weed species that may be present.
The important message in this discussion is that glyphosate-tolerant weeds in soybeans can be managed with present technology. Thus, the concern about glyphosate-tolerant weeds becoming unmanageable in soybeans is not justified at this time.
Glyphosate tolerance may increase with its continued use. If so, producers should be prepared to pay more for weed management than they presently spend with glyphosate-only systems. Unfortunately, this increased expense will not translate to increased yield, but rather to the sustainability of soybean production if glyphosate tolerance becomes significant in problem weed species.
Larry G. Heatherly is a retired USDA-ARS research agronomist and current crop consultant. e-mail firstname.lastname@example.org