AARON HAGER Extension Specialist Office: S-324 Turner Hall (UIUC) Phone: (217) 333-4424 E-mail: hager@illinois.edu

Changes in the weed spectrum frequently occur and when they do, new management challenges follow close behind. Three weed species that are becoming more common in Illinois include hophornbeam copperleaf (Acalypha ostryifolia), kochia (Kochia scoparia), and waterhemp (Amaranthus rudis).

Hophornbeam Copperleaf

Hophornbeam copperleaf is a summer annual species in the Euphorbiaceae family. It is indigenous to Illinois and most commonly found in the southern third of the state. Over the past five years, however, we have identified populations in corn and soybean fields progressively further north in the state, and in 2000 we identified a population in Tazewell county.

Hophornbeam Copperleaf Morphology and Biology

Hophornbeam copperleaf has pubescent cotyledons and true leaves with short hairs and finely toothed (serrated) margins. The leaves are simple and alternate and somewhat heart-shaped at the base. Additionally, a reddish coloration is often observed where the main leaf vein intersects the petiole. The plant is monoecious, with staminate (male) flowers produced on axillary spikes and pistillate (female) flowers produced on a long, terminal spike. Seed pods of hophornbeam copperleaf are dehiscent (pods split open at maturity to release seed), and seeds appear to require warm temperatures for germination. A warm soil temperature germination requirement may suggest that this species is able to germinate and emerge later during the growing season. A recently published experiment reported hophornbeam copperleaf average seed production by plants growing alone (without competition) was approximately 12,518 seeds per plant, much greater than the average seed production (980 seeds per plant) when grown with soybean.

Hophornbeam Copperleaf Control

In general, DNA herbicides do not control hophornbeam copperleaf, and response to ALS-inhibiting herbicides is variable. We initiated a field research experiment in 2000 to evaluate several soil-applied and postemergence soybean herbicides for hophornbeam copperleaf control. Results from the experiment appear in and. Six weeks after preemergence application, all rates of Authority, FirstRate, and Boundary provided good to excellent control, while most other soil-applied herbicides provided poor control. Postemergence control was good to excellent with all rates of glyphosate and the high rate of Cobra and Flexstar. Soybean injury can be a concern with Cobra and loss of soybean leaves, coupled with precipitation and the later emergence pattern of hophornbeam copperleaf, in some instances may allow additional hophornbeam copperleaf growth to occur.

Data on corn herbicides for hophornbeam copperleaf control is very limited. Atrazine, in previous work from Oklahoma State University in 1971, performed well, but present-day application rates may not provide sufficient residual control for a species that can emerge late in the growing season. Postemergence applications of atrazine and crop oil can also provide control, but again, application timing restrictions may reduce the effectiveness of this treatment.

Kochia

Kochia is an early emerging summer annual species commonly found in the western United States. It is a herbaceous dicot and member of the Chenopodiaceae family (the same family as common lambsquarters). Kochia was introduced into North America from Europe as an ornamental because of its red color in late summer and fall (hence the other common name ("fireweed").

In recent years, kochia has become more common in many areas of Illinois. It is commonly found along railroad rights-of-way and frequently spreads from these areas into neighboring corn and soybean fields. Kochia possesses several characteristics that make it well suited as a weed in agronomic production systems.

Kochia Morphology and Biology

Kochia leaves are alternate with simple blades that are highly pubescent. Stems are erect, highly branched, and vary in color from green to red, often with both colors present on an individual plant. Kochia has an imperfect flower that allows cross pollination to occur, which has important implications for the spread of certain herbicide resistance traits. Seed production is moderate to high, depending on environmental and competitive conditions. Seed dispersal occurs via a "tumbleweed" mechanism, by which the mature stem detaches from its base and is subsequently blown about by wind.

Kochia seed is short lived in the soil but possesses a high initial germination rate. Results from one published study indicate that up to 93 percent of kochia seeds produced the previous season germinates within one year. Seed germination is generally greater at shallow soil depths and progressively decreases with increasing soil depth, making no-till systems a good environment for kochia. Seedling emergence can occur very early in the spring, so kochia is typically one of the first summer annual weed species to emerge. Studies have reported kochia emergence when average minimum daily soil temperature ranged from 37 to 46 degrees Fahrenheit.

Herbicide Resistance

Kochia biotypes with resistance to triazine and acetolactate-synthase (ALS) inhibiting herbicides have been well documented. Triazine-resistant kochia first appeared in 1976 along railroads in Idaho and Iowa, where triazine herbicides had been used continuously for total vegetation control. In 1987, the first kochia biotype resistant to ALS-inhibiting herbicides was discovered. Since these initial reports, herbicide resistance in kochia has spread rapidly. Most Illinois kochia samples have demonstrated resistance to triazine or ALS-inhibiting herbicides. We have documented the existence of an Illinois kochia biotype resistant to both triazine and ALS-inhibiting herbicides. Widespread herbicide resistance in the Illinois kochia population should be considered when formulating a chemical control program for corn or soybeans.

Kochia Control

One of the most effective kochia control options is tillage. Since kochia germinates very early in the season, a tillage operation prior to corn or soybean planting can sometimes eliminate most kochia for the remainder of the season. If tillage is not an option, a burndown herbicide should be selected that has good activity on kochia. Some effective burndown herbicides include Gramoxone Extra, glyphosate, and dicamba. Glyphosate rates of 0.375 lb acid equivalent (1 pint Roundup Ultra) or less may not provide good burndown control, especially during cool temperature conditions. 2,4-D is generally less effective than dicamba. Triazine and several ALS-inhibiting herbicides have very good efficacy against kochia, but with widespread resistance to these herbicides in the Illinois kochia population, herbicides with these modes of action should not be relied upon exclusively for kochia control. Command is an effective soil-applied soybean herbicide, while glyphosate or diphenyl ether herbicides can be used for postemergence control in soybean. Balance and Epic are effective soil-applied corn herbicides, while postemergence kochia control in corn can be obtained with Tough, Buctril, or products containing dicamba.

Common Waterhemp

Common waterhemp is a summer annual species in the Amaranthaceae family. It is indigenous to Illinois, historically common in natural ecosystems. Examination of herbarium specimens from the Illinois Natural History Survey indicated waterhemp collections were made in Illinois as early as 1948, decades prior to the onset of rapid waterhemp expansion during the early 1990s. Changes in agronomic production practices, differential susceptibility to herbicides, and development of herbicide-resistant biotypes have contributed to the increased incidences and severity of waterhemp infestations.

Waterhemp Biology

One of the most important considerations concerning waterhemp management is the ability of the species to germinate and emerge later into the growing season than is common for most other annual weed species. Soil-applied herbicides may not have sufficient soil residual activity to control late-emerging flushes of waterhemp. Conversely, postemergence herbicides can afford control of waterhemp present at the time of application but may not provide sufficient control of plants that emerge following application.

Waterhemp is dioecious, meaning plants are either male or female. This biology leads to cross pollination, which increases the genetic diversity of a population. The genetic diversity can be expressed as morphological variations, differential response to herbicides, etc. Recent research has demonstrated that waterhemp can cross with other members of the Amaranth family, producing hybrid progeny with variable morphological characteristics. This research has also demonstrated that resistance to ALS-inhibiting herbicides can also be transferred across Amaranthus species via hybridization.

Waterhemp Control

The most consistent waterhemp control programs are those that combine a sequential management approach. The biology of waterhemp makes consistent control using only one herbicide application extremely difficult to achieve. 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.

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