Season-long weed control in soybean crop production begins with the use of a soil herbicide applied pre-emergence (PRE). The most consistent control of weeds is achieved when weeds are germinating and most vulnerable, making a PRE most effective. The most inconsistent control of weeds is achieved with post-emergence (POST) applications of herbicides. However, regardless of the application timing, herbicides with effective sites of action are required to control the weeds that may be in soybean producers’ fields and reduce the selection pressure for resistant weed biotypes.
One needs to understand the history of weed control in Illinois to prevent the spread and selection of resistant biotypes with the new soybean technologies, i.e., dicamba-tolerant (Xtend) and 2,4-D tolerant (Enlist™). In the 1970s the primary broadleaf weeds in soybean production were large-seeded broadleaf weeds such as common cocklebur, velvetleaf, morning glory, giant ragweed, etc. The available soil herbicides to control these weeds were inadequate, requiring a POST application.
New POST herbicides such as Basagran® and Blazer® were developed to control these weeds. These herbicides controlled these weeds, but also injured soybeans and weed control was inconsistent. Eventually, new soil herbicides were discovered to control these weeds in the mid to late 1980s.
As new herbicides came to market in the 1980s conservation tillage also increased in Illinois, i.e., no-till and reduced-till (no moldboard plow), which further enhanced the proliferation of small-seeded broadleaf weeds such as common waterhemp.
The new class of herbicides developed to provide broad-spectrum broadleaf weed control were the acetolactate synthase (ALS) herbicides. These herbicides proved to be very effective on both large- and small-seeded broadleaf weeds in Illinois. In addition to providing control when applied to the soil, these herbicides could also control weeds POST and, in most cases, didn’t injure soybeans.
By the early 1990s, soybean producers began eliminating the use of a soil herbicide and relied on controlling weeds with POST-application herbicides. Eventually, selection pressure for resistant biotypes to these herbicides increased and now ALS-resistant common waterhemp is widespread throughout Illinois. Therefore, tank-mix partners such as Cobra®, Flexstar® and Blazer were added with the ALS herbicides to control common waterhemp and producers were again injuring the soybeans by the mid 1990s.
In 1996 the commercialization of Roundup Ready® soybeans occurred, allowing producers to again use a single herbicide and single pass to control grass and broadleaf weeds (annual and perennial) POST and not injure the soybeans. However, its great efficacy discouraged the use of a soil herbicide to control weeds in soybeans. Unfortunately, in Illinois glyphosate-resistant (GR) common waterhemp is now commonly found throughout the state.
Common waterhemp is dioecious plant (male and females are separate plants), a shallow soil germinator, big seed producer (100,000 or more seeds per female plant), and germinates over an extended period (May – August). Palmer amaranth, a close relative to common waterhemp, also has some of the same characteristics as common waterhemp, but can flourish in hot dry conditions as well. It is more competitive than common waterhemp and will germinate late May through September. GR-common waterhemp and GR-Palmer amaranth have been discovered in Illinois, but GR-resistant common waterhemp is the much more prevalent of the two.
It is imperative that soybean producers realize and accept that weed resistance to the new soybean technologies will happen if these weeds aren’t managed correctly. Weed scientists strongly encourage soybean producers to use soil herbicides to control weeds prior to a POST application of herbicides, especially for common waterhemp and Palmer amaranth control. Overlapping residuals are suggested for season-long control of these weeds given their extended period of germination.
Overlapping residuals refers to applying a soil herbicide in a POST herbicide application. These herbicides may include soil herbicides such as Anthem® Maxx, Dual Magnum®, Outlook®, Warrant® and Zidua®. POST applications should be applied when weeds are small, less than 4 inches in height. The overlapping herbicides have residual activity and very little POST activity, so soybean producers should not wait until all possible weeds have emerged before applying a POST herbicide. If they wait too long, the weeds may be off label for the POST herbicides in the tank-mix.
Regardless of the tillage regime, it is important to start clean. Therefore, in no-till, weeds should be burnt down when they are small to achieve successful weed control and plant into a clean seedbed. If weeds are not fully controlled by planting, they will not likely be controlled by herbicides once the crop emerges. Likewise, in reduced-till, if the weeds are not destroyed by the tillage and allowed to reestablish, they will not likely be controlled by herbicides once the crop emerges. In other words, weeds injured due to herbicide or tillage are more difficult to control the next time.
Soil herbicides that are effective on problematic weeds should be applied at full label rates. With common waterhemp and Palmer amaranth control, soil herbicides containing a protoporphyrinogen oxidase (PPO) inhibitor should be used which includes herbicides such as Authority® products, Valor® products, Rowel® products, Envive® and Prefix®. These products should also be applied as close to planting as possible.
If possible soybean injury from some of these products is a concern, applying those herbicides no more than two weeks prior to planting and using overlapping residuals is a possible option to achieve season-long weed control. In addition to herbicides, soybean producers should consider cultural practices such narrow row spacing to promote early crop canopy closure to prevent further weed germination.
The two-pass (PRE followed by POST) herbicide system has proven to provide the most consistent weed control across crops, tillage systems and weed spectrums. Using herbicides with effective sites of action, at full label rates and at the right time, combined with crop rotation, will provide the best and most consistent weed control. These principles should also be used with the new soybean herbicide technologies as well. Total POST programs are risky and provide inconsistent weed control and, above all, they have proven to hasten the selection for resistant weed biotypes.