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Waterhemp--Biology, Identification, and Management Considerations

April 27, 2001
Waterhemp will likely continue to be a problematic weed species during the 2001 growing season. In recent memory, few weed species have caused so many headaches for Illinois corn and soybean producers as waterhemp. This weed represents a prime example of why an understanding of weed biology/ecology is needed to implement effective management strategies. Simply stated, waterhemp doesn't always "behave" like other, more familiar summer annual weed species.

Waterhemp Biology

Waterhemp belongs to the botanical Amaranth family, which also includes the other pigweed species found in Illinois. The Latin, or scientific name, of each pigweed includes the genus name Amaranthus; each respective species name differentiates among the genus members. Many taxonomic references recognize common (Amaranthus rudis) and tall (Amaranthus tuberculatus) waterhemp as discrete waterhemp species, although differentiation between the two species is based on minute floral characteristics. Specifically, the only way to accurately differentiate between tall and common waterhemp is to examine how the thin membrane surrounding the seed (utricle) fractures when separated. Common and tall waterhemp can be found in Illinois, but from a management standpoint, there is little reason to differentiate between these two species. We are not aware of any data that suggest these two species respond differently to any herbicide.

Tall and common waterhemp (hereafter referred to collectively as waterhemp) are two of nine pigweed species that can be found in Illinois. Prior to the rapid expansion of waterhemp, smooth pigweed (Amaranthus hybridus) was probably the most prevalent pigweed across much of Illinois. During early vegetative stages, smooth pigweed is nearly impossible to distinguish from redroot pigweed (Amaranthus retroflexus), another commonly encountered pigweed species. Palmer amaranth (Amaranthus palmeri) may be the most aggressive pigweed species with respect to growth rate and competitive ability. Palmer amaranth can be found in the southern one-quarter of Illinois and from personal observations, appears to be moving northward in Illinois. Powell amaranth (Amaranthus powellii) is usually found in the northern portions of Illinois but can also be found in central regions of the state. Spiny amaranth (Amaranthus spinosus) is rapidly identifiable by grabbing the stem with bare hands. As the name accurately implies, sharp spines are present where leaves attach to the stem. Although not very common in agronomic cropping systems, spiny amaranth can be found in pastures and around cattle feedlots. The two other pigweeds, tumble (Amaranthus albus) and prostrate (Amaranthus blitoides), are generally regarded not to be as troublesome as other Amaranthus species.

Waterhemp plants are either male or female (dioecious). Thus, male plants produce only pollen, while female plants produce only seed. This type of biology leads to cross-pollination, or the fertilization of female plants with pollen from one or more male plants. Cross-pollination can greatly increase the genetic diversity of a population, and with genetic diversity comes a wide range of morphological and biological characteristics. Seeds produced by female waterhemp plants are small and usually germinate from very shallow depths in the soil (1/2 inch or less). The number of seeds produced by female waterhemp plants can vary depending on numerous factors, but waterhemp is generally considered to be a prolific seed producer.

It has been known for many years that certain Amaranthus species are able to cross-pollinate and produce fertile hybrids. It is more likely that two dioecious species will cross, but crosses between monoecious and dioecious species can also occur. Hybrid plants produced from monoecious by dioecious crosses are less fertile than their parents, but they may produce some seed. Recently, research at the University of Illinois has demonstrated that not only can waterhemp and smooth pigweed hybridize but also herbicide-resistance characteristics can be transferred to hybrid progeny. For example, if a male waterhemp plant that is resistant to ALS-inhibiting herbicides crosses with a smooth pigweed that is susceptible to ALS-inhibiting herbicides, some of the resulting progeny can carry the ALS-resistance trait. While waterhemp in Illinois is generally not effectively controlled by ALS-inhibiting herbicides any longer, smooth pigweed (for the most part) remains susceptible to this herbicide family. If cross-pollination between waterhemp and smooth pigweed occurs substantially under field conditions, additional difficulty controlling Amaranthus species might result due to increased rates of herbicide resistance evolution.

One of the most important factors to effectively managing waterhemp is to understand its germination and emergence characteristics. The germination and emergence patterns of waterhemp are characteristics that contribute significantly to management problems. While the peak emergence of other, more familiar summer annual weed species generally occurs during the early portion of the growing season, waterhemp emergence can easily occur during the middle to late portions of the growing season. Research at Iowa State University has indicated that while velvetleaf emergence is nearly complete by early June, a significant number of waterhemp plants can emerge well into July. Soil-applied herbicides may not have sufficient soil-residual activity to control late-emerging flushes of waterhemp. Conversely, certain postemergence herbicides can control waterhemp present at the time of application but may not provide sufficient residual control of plants that emerge following application.

Identification

Accurate identification of the various Amaranthus species can be very challenging, especially when the plants are in early vegetative stages. While each of the pigweeds previously described is recognized as a distinct species and has unique identification characteristics, hybridization among some of these species may produce offspring possessing characteristics of each parent, further complicating identification. The best time to accurately identify the various Amaranthus species is when the plants are at the reproductive stage with flowering structures present.

Waterhemp plants typically have no hairs (pubescence) on their stem and leaf surfaces. In contrast, smooth and redroot pigweed have small, fine hairs on stem and leaf surfaces that make the plant feel rough to the touch. The leaves of waterhemp plants are often glossy and more elongated (lanceolate) compared to redroot or smooth pigweed. Stem color of waterhemp can vary from light green to dark red, with multiple shades sometimes on the same plant. There does not appear to be a strong correlation between stem color and sex of the plant. Female plants may be completely red, completely green, or some combination of red and green. Male plants may exhibit a similar color pattern. Table 2 contains information for identification of the various Amaranthus species.

Management Considerations

What is the best way to manage waterhemp in corn or soybean production systems? While there may not be any one "best" way, there are some methods that may be much more consistent than others. Whereas waterhemp may, in some instances, be adequately controlled by a single soil-applied or postemergence herbicide, this is generally not considered the most consistent method to manage this weed. The most consistent waterhemp management programs in either corn or soybean production systems consist of a sequential management approach. By sequential, we are referring to utilization of multiple control options, including tillage, cultivation, soil-applied herbicides, and postemergence herbicides. While a single postemergence herbicide application may sometimes provide acceptable waterhemp control, this is the exception rather than the rule. Waterhemp may well be the "poster weed" for an integrated weed-management program.

Considerations with Soil-Applied Herbicide Programs

There are numerous soil-applied herbicides that possess good activity on waterhemp and other small-seeded species. Time of application can have a significant impact on the success of soil-applied herbicides for waterhemp control. A common practice in no-till systems is to apply a herbicide several weeks prior to planting in order to receive sufficient precipitation to incorporate the herbicide. Keep in mind, however, that the earlier a herbicide is applied, the earlier within the growing season the level of weed control begins to decline. Waterhemp can emerge much later in the growing season than is common for other summer annual species. If the herbicide was applied several weeks prior to planting, it may not have sufficient residual activity remaining to control a late-emerging species such as waterhemp.

What can be done to extend the length of control afforded by soil-applied herbicides? Three possible options include:

1. If allowed by label, increase the rate when applications are to be made several weeks prior to planting.

2. Apply the herbicide in a split application (generally two-thirds early with the remaining one-third at planting).

3. Apply the herbicide closer to planting time.

In our research, we have had better and more consistent results with soil-applied herbicides that were applied within 1 to 2 weeks of planting or at planting compared to the same herbicides applied several weeks (up to 5 weeks) prior to planting. It's not reasonable to assume that all soil-applied herbicides can be applied immediately before planting due to time and equipment constraints, but fields with a significant waterhemp problem would be excellent candidates for soil- applied herbicide applications immediately before planting.

Considerations with Postemergence Herbicides

Similar to soil-applied programs, there are several postemergence herbicides that are very effective on waterhemp. The factors governing the effectiveness of postemergence herbicides are critically important when dealing with waterhemp. Herbicide rate, application timing, and spray additive influence how well postemergence herbicides perform against waterhemp.

Often, producers like to wait as long as possible to apply postemergence herbicides, especially those that lack any significant soil-residual activity, to have as many weeds emerged as possible. Because waterhemp can germinate and emerge for an extended time, there typically exists a wide range of plant sizes by the time postemergence herbicides are applied. This can present problems with spray interception by smaller plants under the protective canopy of larger plants. Adjustments in spray volume and pressure can help to overcome some of the problem with coverage. Spray volumes of 20 gallons per acre with application pressures of 40 to 50 pounds per square inch generally provide a very uniform coverage of the target vegetation.

The next issue of the Bulletin will contain more information about waterhemp management in corn and soybean production systems.--Aaron Hager and Christy Sprague

Author: Aaron Hager Christy Sprague


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