Illinois
Fertilizer Conference Proceedings
January 26-28, 1998
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A.G. Hager1
Few weed species have garnered as much attention over the past few years as waterhemp. Waterhemp has gone from obscurity just a few years ago to widespread distinction at the present time. How has this problem developed? What factors have contributed to waterhemp's rise to prevalence? In this paper we cover a few basics on waterhemp identification and biology, and describe some of the factors responsible for the increased severity of waterhemp infestations across much of Illinois. Some control considerations and options are also included.
There are actually two species of waterhemp in Illinois: common (Amaranthus rudis) and tall (Amaranthus tuberculatus). These are distinctly separate species, yet it is not possible to differentiate between them while in the vegetative stage. Only when the plants have produced seeds can tall and common waterhemp be differentiated. Is there a need to be able to identify tall from common? To our knowledge, no data exists to suggest that these two similar, yet distinct, species respond any differently to soil-applied or postemergence herbicides.
Similar to the other pigweed species in Illinois (there are nine pigweed species commonly encountered in Illinois), waterhemps have notched leaf tips and reddish colored roots. When pigweeds are small, it can be very difficult to accurately identify which species you actually have. Waterhemps generally have no hairs on the stem or leaf surface and so feel fairly smooth to the touch. Redroot and smooth pigweed, the more familiar pigweed species, have small, fine hairs that cover the stem and leaf surfaces. These species will feel rough when you run your finger along the stem or leaf surface. The leaves of waterhemp are typically glossy and often more elongated or lanceolate compared to redroot or smooth pigweed. Stem color of more mature waterhemp plants varies greatly; most are a shade of green but others can be some shade of red. Plants with green stems and red petioles or red stems with green petioles can be found.
For further information on pigweed identification, consult an excellent guide developed jointly by Kansas State University, USDA/ARS, and the University of Illinois: Pigweed Identification: A Pictorial Guide to the Common Pigweeds of the Great Plains is available from the Kansas State University Cooperative Extension Service Distribution Center, 16 Umberger Hall, Kansas State University, Manhattan, Kansas 66506-3406. This guide contains color photographs of the various pigweed species as well as brief descriptions of vegetative and reproductive characteristics.
Both common and tall waterhemp are species native to Illinois. In years past, these species have been prevalent primarily south of Interstate 70, but during recent seasons waterhemp could be identified growing about as far north in Illinois as Interstate 80.
Waterhemp plants are either male or female (dioecious). Male plants produce only pollen while female plants produce only seed. In contrast, redroot and smooth pigweeds have both male and female reproductive structures on the same plant (monoecious). What is the significance of this? Dioecious species are cross-pollinated, meaning that pollen from more than one source can fertilize a female plant. Cross-pollination can greatly increase the genetic diversity of a population, and with genetic diversity comes a great range in physical appearance and other biological characteristics.
Like other pigweed species, waterhemps are annuals that produce an abundance of very small seeds. These small seeds are ideally suited for shallow germination (likely less than ¼"). High seed production and small seeds make waterhemp well-adapted to reduced tillage systems where the soil is not disturbed and the seeds remain near the soil surface.
Seed germination and plant emergence of waterhemp often extends later into the growing season than for most other annual weed species encountered in agricultural systems. The extended germination and emergence present the potential for several flushes of waterhemp during the course of a growing season. Soil-applied herbicides can provide effective control of earlier-season flushes but may not persist long enough to control later-emerging waterhemp. Conversely, postemergence herbicides are most effective when waterhemp are small, but few provide sufficient residual activity to prevent future emergence.
Many producers who have been confronted with waterhemp have asked why this weed has increased so rapidly and extensively. From our perspective, there does not appear to be one single reason for this spread. Rather, several factors appear to be responsible and many of these likely work in concert.
Why would no-till systems tend to favor species such as waterhemp? Three features of no-till systems that likely contribute are:
Many older, soil-applied herbicides possess substantial activity on small-seeded broadleaf species such as lambsquarters and pigweeds. When broadcast applications of soil-applied herbicides were used in conjunction with one or more cultivations, pigweeds of any species only occasionally escaped control and developed into substantial problems. However, with the reduced use of soil-applied and incorporated herbicides that control grasses and small-seeded broadleaves, pigweeds have escaped control more frequently in recent years. Many of today's postemergence herbicides control a wide spectrum of weeds, but few have adequate residual activity to control weeds, such as waterhemp, that may emerge several days or weeks following application.
Biotypes of waterhemp have been identified in Illinois that are resistant to particular mode of action herbicides. In particular, biotypes of waterhemp that are resistant to ALS-inhibiting herbicides in addition to biotypes resistant to triazine herbicides have been identified. Which type of resistant biotype is more prevalent? It appears that ALS resistant biotypes are more widespread than biotypes resistant to triazine herbicides. Is every waterhemp plant encountered resistant to some herbicide? Although there are confirmed instances of waterhemp biotypes that are resistant to ALS or triazine herbicides, by no stretch of the imagination is every waterhemp plant resistant to some herbicide. Waterhemps vary widely in appearance and also appear to be quite variable in their response to herbicides. This variation in response can range from highly susceptible to very resistant. The instance of resistant waterhemp does, however, appear to be widespread enough to warrant good herbicide management practices that may slow the further development of resistant weeds, such as rotating herbicide modes of action and tankmixing herbicides with different modes of action.
There are several soil-applied and postemergence herbicide programs that can provide good control of waterhemp, but each type of application timing has basic considerations that can influence the degree of success achieved.
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 successfulness of soil-applied herbicides for waterhemp control. A common practice in no-till systems is to apply the 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 that the level of weed control begins to decline. Waterhemp does not appear to have much difficulty emerging well after the time that most other summer annual species have emerged. If the herbicide has been on the ground for several weeks, there may not be enough herbicide remaining to control emerging waterhemp.
What can be done to extend the length of control afforded by soil-applied herbicides? Three possible options include:
In our research, we have had better and more consistent results with soil-applied herbicides that were applied within one to two weeks of planting or at planting compared to the same herbicides applied several weeks (up to 5 weeks) prior to planting. It's not likely that all soil-applied herbicides can be applied immediately before planting due to constraints of time and equipment, but fields with a significant waterhemp problem would be excellent candidates for applications just prior to planting.
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 all influence how well postemergence herbicides perform.
Often, producers like to wait as long as possible to apply postemergence herbicides, especially those that lack any significant soil residual activity. Because waterhemp can germinate and emerge for an extended period of 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 60 pounds per square inch generally provide a very uniform coverage of the target vegetation.
The following outline lists chemical families and representative members that we have evaluated for waterhemp control. Rates should be based upon label recommendations; when attempting to control waterhemp, reducing rates under those on the respective label is generally not advisable.
Whatever program you decide to use, keep in mind that the most consistent program to control waterhemp includes an integrated approach utilizing soil-applied herbicides, postemergence herbicides, and mechanical cultivation.
The most consistent control programs for field corn are those that combine a soil-applied herbicide with either one or more cultivations or the application of a postemergence herbicide.
Soil-applied
Triazines - atrazine and metribuzin applied no earlier than two weeks prior to planting.
Chloroacetamides - alachlor, metolachlor, acetochlor, and dimethenamid. Even though these herbicides are primarily for grass control they do afford some control of waterhemp. By themselves, however, they generally will not provide sufficient residual control. Selection of these herbicides should likely be based upon the need for grass control - all have performed similar with respect to controlling waterhemp.
Growth regulators - dicamba or dicamba + atrazine can be used as soil-applied treatments, but these products usually perform more consistently on waterhemp when applied as a postemergence treatment.
ALS-inhibiting herbicides - these herbicides should be used as premixed or tankmixed treatments because of the presence of waterhemp biotypes resistant to this class of herbicides.
Postemergence
Growth regulators - dicamba, dicamba + atrazine, 2,4-D. These herbicides usually provide the most consistent level of waterhemp control with respect to postemergence corn herbicides. Many ALS-inhibiting herbicide labels recommend tankmixing reduced rates of dicamba to enhance control of waterhemp. The rate of dicamba used in the tankmix should be high enough to control waterhemp as if it were being applied alone.
Triazines - atrazine and metribuzin are the two available options. Both of these are contact herbicides and will provide better control when applied to waterhemp under 4 inches in height. Atrazine must be applied before corn reaches 12 inches in height and metribuzin must be applied with a tankmix partner.
The most consistent control programs for soybeans are those that combine a soil-applied herbicide with either one or more cultivations or the application of a postemergence herbicide.
Soil-applied
Dinitroanilines - pendimethalin and trifluralin. Pendimethalin may be surface applied without subsequent incorporation whereas trifluralin requires mechanical incorporation. If pendimethalin is surface applied (no earlier than 2 weeks prior to planting in waterhemp fields) and no precipitation is received between application and planting, a shallow incorporation may prove beneficial.
Chloroacetamides - alachlor, metolachlor, dimethenamid. These herbicides can provide some control of waterhemp in soybeans, but use rates are sometimes lower than those used in corn.
Triazines - metribuzin applied within 2 weeks of planting.
Phenylureas - linuron applied preemergence (after planting).
ALS-inhibiting herbicides - these herbicides should be used as premixed or tankmixed treatments because of the presence of waterhemp biotypes resistant to this class of herbicides.
Authority Broadleaf and Canopy XL have attracted a great deal of attention as new soil-applied options for waterhemp control. Both of these premixes contains Authority and Classic at equivalent ratios. The Authority component will contribute the most toward control of waterhemp, especially in fields containing ALS-resistant biotypes. Field results with these premixes from the 1997 growing season suggested good residual control of waterhemp, however we continue to recommend these products be applied no earlier than 2 weeks prior to planting.
Authority STS is a product marketed by DuPont for use in STS soybeans. This prepacked product contains Authority alone and Synchrony STS. The Authority component is soil-applied for control of nightshade and waterhemp.
Postemergence
Diphenyl ethers - lactofen, fomesafen, acifluorfen. These herbicides have provided consistent control of waterhemp. Best control is achieved when applications are to waterhemp less than 4 inches in height. With all these herbicides, crop injury should be expected and is generally more severe under conditions of high temperature and relative humidity and when crop oil or 28% UAN solution is included.
Glyphosate - Roundup Ultra only on soybeans designated as Roundup Ready. Rates of Roundup Ultra should begin at 1.5 to 2 pints per acre and increase when large waterhemp is present. We would caution producers not to wait too long after crop and weed emergence to make postemergence applications of Roundup Ultra.
ALS-inhibiting herbicides - these herbicides should be used as premixed or tankmixed treatments because of the presence of waterhemp biotypes resistant to this class of herbicides.
The key points to remember to effectively manage waterhemp are:
For those interested in obtaining further information on waterhemp, a color brochure has been developed through a collaborative effort between the University of Illinois and the USDA/ARS. Waterhemp Management in Agronomic Crops (publication number X855) is available for purchase ($2 per copy) from Vocational Agricultural Service, Information Services, College of Agricultural, Consumer, and Environmental Sciences, University of Illinois, 1401 S. Maryland Dr., Urbana, IL 61801, FAX (217) 333-0005, Phone (217) 333-3871.
1A.G. Hager is an Extension Specialist, Dept. of Crop Sciences, University of Illinois.
