Taming the Two Giants

Christy Sprague

Christy Sprague
Assistant Professor

Phone: (217) 333-4424
E-mail: csprague@illinois.edu


Changes in weed management and production practices can cause significant shifts in the weed spectrum that corn and soybean producers encounter. Two good examples of weeds that are becoming more prevalent problems for Illinois growers are giant ragweed (Ambrosia trifida L.) and common pokeweed (Phytolacca americana L.). These two species are native to the Midwest; however, they have not been significant weed problems for many producers in Illinois until recently. These two weeds have risen to the top of management headaches for Illinois corn and soybean growers for a number of reasons.

Giant Ragweed

Giant ragweed is a native summer annual species in the Compositae family. It can be found throughout most of Illinois. In the 1960s and 1970s, giant ragweed was mainly found along drainage ditches, roadsides, and fields along flood plains. However, over the last decade, giant ragweed has spread throughout many production fields, particularly in northern parts of the state.

Giant Ragweed Biology

Research in the late 1960s showed that virtually all giant ragweed seeds germinated early in the season, usually before May 1. Management of giant ragweed at this time was not a major concern, because tillage and planting would control giant ragweed plants in most fields. However, recent research in Illinois has shown a shift in the emergence pattern of giant ragweed. It appears that giant ragweed seeds found in Illinois production fields can germinate well into June and early July. This shift in emergence period, along with the shift in production practices to less tillage and earlier planting, has made giant ragweed a tremendous weed management problem for many producers in Illinois.

Another attribute that makes giant ragweed a problem weed is its competitive ability with crops and other weeds. Giant ragweed is considered to be one of the most competitive weed species in Illinois. Season- long interference from a giant ragweed population of one plant per m2 can cause yield losses of up to 50 percent in soybean and 14 percent in corn with two plants per 10 m2. In addition, when giant ragweed is present at a site, it exerts strong dominance over other annual weed species. Giant ragweed exhibits a high degree of morphological and reproductive plasticity in response to encroachment by neighboring plants.

This summer, we monitored giant ragweed emergence, growth, and seed production in three cropping systems. In comparing these systems, giant ragweed got as tall as 8 feet in corn and 6 feet in soybean and when grown with no crop. However, the giant ragweed plants that produced the greatest biomass and seed were grown without a crop, followed by soybean. Seed production was as high as 11,000 seeds per plant grown without competition; 4,700 seeds per plant competing with soybeans; and 2,500 seeds per plant competing with corn. This is a relatively small amount of seed per plant in comparison with other weed species such as waterhemp, which can produce as many as 1,000,000 seeds per plant if grown without competition. Despite many of giant ragweed's competitive attributes, the success of giant ragweed as a weed of annual crops is perplexing in light of its low seed production and low seed survival. Giant ragweed seed mortality within a year after production can range from 20 to over 90 percent.

Giant Ragweed Control

There are a number of very effective herbicides available for controlling giant ragweed; however, this control can be rather inconsistent. Giant ragweed can escape control from soil-applied herbicides by germinating from considerable depths and emerging later in the season, when a soil-applied herbicide may have already dissipated. Inconsistencies with postemergence herbicide programs are usually attributed to giant ragweed's considerable growth rate. Often, giant ragweed may already be too large when the postemergence application is made, allowing for the plant to regrow after the treatment. In addition, with the shift in giant ragweed emergence, some of the plants may emerge after the postemergence application has already been made. Another caveat to add to the lack of giant ragweed control is the development of ALS-resistance that has limited the use of some very effective herbicides in some areas.

To overcome these different challenges, the most consistent giant ragweed control programs are those that combine a sequential management approach. The shift in emergence patterns of giant ragweed makes control increasingly difficult to achieve with only one herbicide application. Sequential programs may consist of a soil-applied herbicide followed by a postemergence herbicide or two postemergence herbicide applications. Including cultivation with herbicides can frequently increase the likelihood of successful management. It is also important to keep giant ragweed populations in undisturbed areas (such as fencerows, roadsides, and waterways) in check. These areas can often be the source of seeds to spread into fields.

Common Pokeweed

Common pokeweed is a warm-season perennial that is coma native to North America. It can be found throughout most of Illinois. Common pokeweed is another species that was found mostly along non-disturbed areas (such as fencerows, roadsides, and woodland borders) in the past. Because of its ability to propagate in undisturbed areas, it has become an increasing concern in corn and soybean fields that have been managed under continuous no-tillage crop production practices.

Common Pokeweed Biology

Since common pokeweed is a deep-rooted perennial, it reproduces from buds on the root or from seeds. Aboveground shoots of this plant arise from the taproot and consist of diffusely branched, fleshy stems (resembling a small tree) that can reach heights of 6 to 8 feet under fertile conditions. The fruit produced in late summer are clusters of green berries that turn purple to black at maturity and contain a profuse amount of red juice. The green leaves, fleshy stem, and purple berries of common pokeweed can inhibit the harvesting process and lead to discounts at the elevator for high moisture and stained seed. In addition, areas with heavy infestations of common pokeweed have been known to compete and reduce yield in both corn and soybeans. Numerous bird species are known to feed on the berries and are capable of randomly dispersing pokeweed seeds over sizeable areas. New plants from seed dispersed on undisturbed sites, such as no-till crop fields, are capable of becoming more entrenched as their taproots develop.

Many plants that form taproots generally do not tolerate intensive tillage practices. Therefore, common pokeweed is generally not a problem in crop fields that are exposed to tillage. In fact, one of the most effective ways to combat this weed species is the use of mechanical controls, such as moldboard plowing and disking. However, reducing tillage has become a desirable practice for corn and soybean producers to help reduce soil erosion, decrease energy and crop production costs, and save time during the planting season. Therefore, alternative or chemical control methods are more desirable.

Common Pokeweed Control

Since common pokeweed problems are relatively new in Illinois, we need to look to neighboring states that have dealt with this problem in the past to find effective control measures. In Kentucky, field studies have been conducted over a number of years to evaluate the effectiveness of postemergence herbicides on common pokeweed in no-tillage corn and soybean production systems.

In corn, herbicide products that have been evaluated include dicamba (Clarity, Distinct) and acetolactate synthase (ALS)-inhibiting herbicides that include active ingredients that are found in Accent, Basis Gold, Beacon, Permit, Exceed, Spirit, and Lightning. The best season-long suppression of common pokeweed growth each year (approximately 60 to 80 percent shoot inhibition) was observed with Clarity, Distinct, Lightning, and Exceed. Beacon and Permit provided some initial activity on pokeweed shoots, but significant regrowth was observed. Accent and Basis Gold provided very little control of shoot growth. Control of common pokeweed with Spirit provided control similar to Beacon but was less effective than Exceed.

In soybeans, herbicide options that have activity on common pokeweed are much more limited. Initial observations in field studies with two ALS-inhibiting herbicides, Synchrony STS and Raptor, indicated slight suppression of common pokeweed but provided poor results later in the season. In-season postemergence applications of glyphosate (i.e., Roundup UltraMAX, Touchdown) in Roundup Ready soybeans were much more effective (70 to 80 percent control). However, poor results on pokeweed have been observed when glyphosate was applied as a "burndown" treatment before planting soybean.

As of now, few herbicides are known to effectively suppress common pokeweed in continuous no-till corn and soybean production systems. Although Exceed, Clarity, and Distinct (i.e., dicamba) were effective in suppressing pokeweed growth in corn, herbicides designed for use on herbicide-tolerant crops offer other choices to consider. These herbicide options include glyphosate products (e.g., Roundup UltraMAX, Touchdown) labeled for use on Roundup Ready corn and soybean or Lightning on Clearfield corn hybrids. If pokeweed continues to be a management problem, intensive mechanical control methods may need to be used to get it under control.

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