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Funded Project

Funding Program: Regional IPM Competitive Grants - Northeastern

Project Title: Impact of Weed Management Approaches on Population Shifts

Project Directors (PDs): Mark VanGessel [1] Sujatha Sankula [2]

Lead State: DE Lead Organization: University of Delaware

Research Funding: $95,928

Start Date: Sep-01-1999 End Date: Aug-31-2002

Pests Involved: weeds

Site/Commodity: field crops, corn, soybeans

Area of Emphasis: herbicide resistance

Summary: Weed community composition and density are often in flux. Shifts in weed species composition are likely to occur as a result of selective forces of weed management. The development of herbicide resistant crops (HRCs) is a recent technological innovation. Most herbicide-resistant crops provide the in-crop use of broad-spectrum postemergence herbicides. HRCs facilitate the implementation of IPM approaches to weed management with attributes that minimize environmental risks. There is concern that repeated use of any weed management approach, including HRCs, can select for weed species that survive or adapt to those weed management treatments, leading to population shifts. HRCs and their respective herbicide programs can result in a range of weed species that are not adequately controlled. Changes in weed population often intensifies the perception of risk to cropping systems and can lead to inappropriate management decisions. We propose to investigate the potential weed species shifts that may result from long-term rotation of herbicide-resistant corn and soybeans as well as conventional herbicide programs. Treatments will provide a range of weed management scenarios with an emphasis on glyphosate-resistant soybeans and corn. Our first objective is to study impacts of various weed management programs on weed population dynamics with emphasis on glyphosate-resistant crops. Selected treatments will result in various approaches to weed management and differing levels of weed management. The second objective is to determine the temporal changes of weed seedbank of a long-term cropping system. Experimental sites consist of a diverse flora representing monocots and large and small-seeded dicotyledonous species. The proposed research will improve understanding of weed population shifts by identifying species that are favored as weed management practices are changed. As a result, weed management systems that target the appropriate weed species can be developed. This research will also identify if species shift is due to inadequate control or selection of more adoptive species. Data obtained from this research will serve as a guideline to develop environmentally and economically sound, long-term sustainable integrated weed and pest management systems. This research is timely, and especially important given the rapid commercialization and increasing popularity of HRCs. The proposed project is in the research category and conforms to the goals of the IPM Grants Program by providing information to implement a profitable and environmentally sound IPM system over the long-term, and to understand and conserve the quality and diversity of ecosystems. This study was initiated in 1996 with limited support from Delaware Soybean Board (DSB). Delaware Soybean Board agreed to provide initial support for three years. Objectives: 1. Study impacts of various weed management programs on weed population dynamics with emphasis on glyphosate-resistant crops. Selected treatments will result in various approaches to weed management and differing levels of weed management. 2. Evaluate the temporal changes of weed seedbank of a long-term cropping system in order to better manage weed communities. Problem, Background and Justification Corn and soybeans are major food crops of the northeastern region of United States. Corn is grown in all states of the region, accounting for 4 million acres while soybeans are grown in four states (Delaware, Maryland, New York, and Pennsylvania) on 1.27 million acres. Delaware planted 150,000 and 225,000 acres of corn and soybeans in 1997, respectively3. Weeds cause an annual loss of 700 million dollars in corn and 500 million dollars in soybeans in the United States. In the United States, 95% of the corn and soybean acreage is treated with herbicides each year (Burnside 1993). A survey by National Agricultural Statistical Service showed that 55% of the total farm expenditure is due to agricultural chemicals (excluding fertilizers). Herbicide use has often been the critical component for increased acreage of crop production. Herbicide use has a number of consequences that must be considered and prevented. Movement to surface and ground waters, off-target movement to injure other plants, or development of herbicide-resistant plants are to name a few. The evolving resistance of weeds to herbicides used repeatedly is a growing problem (LeBaron 1991). Experimental evidence suggests that herbicides affect the relative abundance of species, yet they will seldom lead to the disappearance of a species altogether. Continued use of triazine herbicides in corn fields in Ontario altered weed species composition and resulted in large increases in triazine-resistant weeds (Cavers and Benoit 1989). The emergence of highly adapted weeds is enhanced under repeated herbicide applications (Holt and LeBaron 1990). Weber et al. (1974) reported that three annual applications of fluometuron, prometryne, and trifluralin shifted the large crabgrass [Digitaria sanguinalis (L.) Scop.] populations to yellow nutsedge and crowfootgrass (Dactyloctenium aegypticum L.). A four-year study by Manley (1996) showed greatest increase in common lambsquarters when fomesafen was used alone continuously or in rotation with other herbicides. Most weed shift studies to date dealt with small-seeded species (Cardina et al. 1991; Menges 1987). There is a lack of research on large-seeded annuals. Management practices that influence the competitive ability of a crop also can select against less competitive weed species. Such changes in weed community composition may signal the effectiveness of control measures or the emergence of potential problem species that may require alternative control measures. Holzner (1978) suggested that herbicide selection pressures which eliminate susceptible species will select for weed communities poor in species diversity but high in density of individuals. Furthermore, such individuals may be larger in density than occur in more competitive situations (compensation development). A number of studies have demonstrated the impact of agricultural practices or weed management techniques on the composition of weed flora (Chauvel et al. 1989; Cussans 1976; Derksen et al. 1993; Derksen et al. 1995; Froud-Williams et al. 1993; Hammerton, 1968; Liebman and Dyck 1993; Pollard et al. 1982). Crop production practices exert selection pressure on weed communities and create niches that influence various species. As a result, weeds in a multi-species community are in constant change with shifting composition, dominance, density, and distinct patchiness in space and time (Cousens 1987). Species which are better adapted to the new conditions will survive whereas those which are less fit tend to be eliminated. Past improvements in agriculture have had profound effects upon weeds, in some instances completely eliminating particular species. The classic example is the disappearance of Agrostemma githago L. attributed to improvements in seed cleaning process (Godwin 1956). Improved soil fertility by the increased use of fertilizers has also modified weed flora composition by improving crop competitiveness (Borowiec and Kutyna 1974; Borowiec et al. 1974; Jones 1966; Landi 1975; Smirnov et al. 1975; Thurston 1976). At a regional level, there appear to be changes in weed species over time. A survey since 1971 conducted by Southern Weed Science Society of America in corn, soybeans, cotton (Gossypium hirsutum L.), and peanuts (Arachis hypogea L.) showed largest increases in rank for sicklepod and bermudagrass [Cynodon dactylon (L.) Pers.], and largest decreases for johnsongrass [Sorghum halapense (L.) Pers.], crabgrasses (Digitaria spp.), and common cocklebur (Webster and Coble 1997). Shifts in relative frequency of species within weed populations may not necessarily have occurred in response to permanent selection pressures. Rather, a short term or immediate response to transient changes in agriculture such as tillage, cropping sequence, or herbicide application may be responsible for species shifts. Tillage systems affect weed emergence, management, and seed production. Therefore, changing tillage systems change the composition, vertical distribution, and density of weed seedbanks in agricultural soils (Buhler and Oplinger 1990; Buhler 1992; Cardina et al 1991; Forcella 1992; Koskinen and McWhorter 1986; Robinson et al. 1984; Teasdale et al. 1991; Wilson et al. 1985; Wrucke and Arnold 1985; Yenish et al. 1992). Seed depth in soil and corresponding emergence differences contribute to weed species shifts under different tillage systems (Buhler 1988). At the present time, there is no consensus on the value of weed community diversity (Clements et al. 1994). Some argue that low weed diversity will result in a less stable agroecosystem which provides optimal conditions for unhampered growth of weeds, insects, and diseases as many ecological niches are not filled by other organisms (Radosevich et al. 1997a). Other school of thought is that increased vegetation complexity may lead to increased problems with phytophagous insects (Crepps and Ehler 1983; Murdoch 1975; Van Emden and Williams 1974; Van Emden 1981). Development of HRCs is a new approach for weed control systems which expand the utility of proven, previously non-selective, broad-spectrum herbicides such as glyphosate and glufosinate. Herbicide-resistant crops allow an in-crop application of these non-selective POST herbicides in major crops such as corn and soybeans, and provide farmers with new weed control options. More importantly, HRCs can reduce the amount and number of herbicides applied during a growing season. The large number of weed species often present in growers' fields, representing diverse botanical families, requires the application of two or more "standard" herbicides in different chemical families for effective weed management in corn and soybeans. Non-selective herbicides such as glufosinate and glyphosate are active at varying levels on a very broad range of weed species thus reducing number of herbicides and number of spray applications. A management system utilizing glyphosate or glufosinate and a transgenic crop, allows for a single application before the canopy closure. This postemergence (POST) approach foregoes the prophylactic applications of soil-applied herbicides. Weed control by glyphosate applied to glyphosate-resistant soybeans can be equal to or greater than control by standard herbicide programs (Baldwin 1995; Clay et al 1995; York 1995). Herbicides such as glyphosate (Anonymous 1994c; Malik et al. 1989) and glufosinate (Anonymous 1994b) are known for their environmentally favorable and safety characteristics including low toxicity, little or no leaching, and no soil activity. Other benefits of these non-selective herbicides and HRCs include cost-effective weed control, increased flexibility of crop rotations, and compatibility with minimum tillage systems (Wilcut et al. 1997). Herbicide-resistant crops can be used to manage resistance, in that they will give the farmer the option of a herbicide with different mode of action with which to rotate. However, HRCs could increase the selection pressure for herbicide resistance if used unwisely (eg. extended use, year after year). The recent discovery of glyphosate-resistant ryegrass (Lolium rigidum L.) in Australia is a reminder that sound herbicide resistance management strategies remain important after the widespread adoption of glyphosate-resistant crops. It bears mentioning that non-selective does not imply the respective herbicide controls all species. Differential response has been observed with non-selective herbicides, even in sensitive species. Tardif and Leroux (1993) observed differential sensitivity of five quackgrass biotypes to glyphosate which was attributed to mechanisms other than translocation such as variable metabolic rate and site of action susceptibility. Ridley and McNally (1985) found greater than 70-fold difference in susceptibility of seven plant species to glufosinate, possibly due to different isoenzymes of glutamine synthetase that can occur in plants. Among giant foxtail (Setaria faberi Herrm.), common lambsquarters (Chenopodium album L.), common cocklebur (Xanthium strumarium L.), and Pennsylvania smartweed (Polygonum pensylvanicum L.), common lambsquarters was the most tolerant species to glufosinate and control was inconsistent (Steckel et al. 1997). As with conventional herbicide programs, weed species and size influence glyphosate efficacy (Baldwin 1995; Clay et al. 1995; Stapleton et al. 1994). Annual morningglories (Ipomea spp.) and hemp sesbania [Sesbania exaltata (raf.) Rybd. Ex A. W. Hill] are among the more tolerant weeds to glyphosate (Baldwin 1995; Jordan et al. 1997; York 1995). Since glufosinate or glyphosate does not have residual activity (Anonymous 1994b; Anonymous 1994c), weeds that emerge after the herbicide application will not be controlled. This could possibly favor those weed species that emerge later in the growing season. Studies by VanGessel and Majek (1997) showed that weed control due to glyphosate applied at cracking or unifoliate stages of soybeans was not adequate due to weeds that emerged later, necessitating a second application or use of residual herbicide program. Even if these later emerging weeds are fewer in number, the impact of the seeds produced by these few weeds on the seedbank needs to be considered. In a three year study, permitting subcompetitive densities of sicklepod (Senna obtusifolia L.) in soybeans to reach maturity each year resulted in dramatic increase in seed numbers in the soil of conventionally planted soybeans (Bridges and Walker 1985). The use of non-selective herbicides in HRCs may lead to the emergence of weeds that are more adapted and posing new weed problems. RATIONALE AND SIGNIFICANCE Since the 1970's, weed population shifts, the increasing frequency of herbicide-resistant (or -tolerant) weed biotypes, and the need to decrease pesticide residues in food, water, and soil have pinpointed the limitations of past approaches in weed science. Long-term strategies are required based on an understanding of biological, ecological, and economical processes that drive the cropping system (Roush et al. 1990). Harper (1957) noted that weed populations tend to be very stable. Stability was attributed to weed seed dormancy, reinfestation, and prolific reproductive capabilities of weeds. However, the lasting changes that will result due to "relentless" pressure from herbicide applications have been demonstrated. Recent ecological shifts in the weed spectrum which resulted in the emergence of new problem weeds have emphasized the ability of weeds to respond quickly to changes in man-made selection pressures (Gasquez 1986). Weed species shifts that may result from HRCs must be understood. Understanding weed population shifts will identify critical periods in demographic processes that can be exploited in management systems. The value of a diversified weed management approach over time will be evaluated. This study will utilize chemical weed management, but principles will be applicable to all types of weed management. The proposed project helps in better understanding weed species shifts which in turn can be managed through integrated weed management approaches. This contributes to long range improvements in and sustainability of U.S. agriculture by increasing the efficiency of non-renewable and on-farm resources and stabilizing the viability of agricultural operations. HYPOTHESIS Use of same weed management repeatedly over time often leads to higher density of a particular species. For example, continuous use of a broad-spectrum herbicide such as glyphosate may lead to a build up of weed species which glyphosate is moderately effective on such as common ragweed (Ambrosia artimesiifolia L.) and morningglory. Use of non-residual herbicides will not effectively control late emerging weed species. As a result, there will be an increase in community evenness due to greater relative abundance of the above species which may be harbingers of future weed problems. These temporal changes in weed communities will impact weed-seed rain and crop-weed competition thereby necessitating adjustment in weed management strategies based on weed species. Outcomes and Impacts Summary from 2001 IPM Center report Corn and soybeans are major food crops of the Northeast, grown on more than 5 million acres in the region. Nationwide, weeds cause an annual combined loss of $1.2 billion in these crops. Herbicideresistant crops are a recent and increasingly popular weed-control tool. About 75 percent of soybeans and 15 percent of corn hybrids planted in and around Delaware are resistant to herbicides, so maintaining the long-term viability of these crops is essential. These crops may allow growers to use a type of herbicide that can kill a broad range of weeds but has a very low toxicity, does not leach into groundwater, and does not persist in the soil. There is concern, however, that repeated use of any weed management approach, including this one, might select for weed species that resist treatment. Mark VanGessel and colleagues are investigating potential weed species shifts that might result from long-term rotation of herbicide-resistant corn and soybeans. Their work will identify species that are favored by changes in weed control practices, enabling us to develop new, better-targeted weed management systems. At completion, this research will be incorporated into recommendations on the most effective ways to use herbicide-resistant crops over multiple years. Publication Sankula, S., M.J. VanGessel, W.E. Kee, and J.L. Glancey. 1999. Impact of row spacing and herbicide rate and application method on weed control and harvest efficiency of lima bean. HortTech 9(4):633-637.

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