Herbicide-Tolerant Crops & Integrated Weed Management

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Use of Herbicide-Tolerant Crops as a Component of an Integrated Weed Management Program

Stevan Z. Knezevic, Assistant Professor, Haskell Agricultural Laboratory, University of Nebraska, 57905 866 Road, Concord 68728-2828; Kenneth G. Cassman, Professor and Head, Agronomy and Horticulture Department, University of Nebraska, Lincoln 68583-0915

Corresponding author: Stevan Knezevic. sknezevic2@unl.edu

Abstract

Integrated weed management (IWM) advocates the use of a combination of preventive, cultural, mechanical, and chemical tools to keep weed pressure below threshold levels that reduce yields and profits. Herbicide-tolerant crops (HTCs) represent a relatively new weed control technology that can be used in an IWM program and have been readily adopted by farmers in the US and Canada. HTCs enhance weed control options and greatly expand market demand for certain herbicides. HTCs provide many benefits to the producers and to the companies that own the intellectual property rights to this technology. However, HTCs should be considered only as one component of an IWM approach that also utilizes other management tools to ensure the long-term benefits of a profitable and environmentally sound weed management program. Widespread use and over-reliance on HTCs without the benefit of an integrated weed management program can result in the development of herbicide-tolerant weeds or a shift in weed populations dominated by species that are more tolerant of the herbicide in question. Therefore, our objective is to provide a brief overview of advantages and disadvantages regarding the use of HTCs in order to help those involved in weed management at the field level be aware of both benefits and risks associated with this technology.

Introduction

Integrated weed management has been commonly described as “a combination of mutually supportive technologies in order to control weeds” (26). Some have also called it “a multi-disciplinary approach to weed control utilizing the application of numerous alternative control measures” (28). In practical terms, it means developing a weed management program using a combination of preventive, cultural, mechanical, and chemical practices. It does not mean abandoning chemical weed control. Instead, chemical control is considered to be one of many mutually-supportive weed management options, although a reduction in herbicide use can result from implementing an IWM approach. An IWM approach advocates the use of all available weed control options such as: selection of a well-adapted crop variety or hybrid with good early season vigor and appropriate disease and pest resistance; appropriate planting patterns and optimal plant density; improved timing, placement, and amount of nutrient application; crop rotation; tillage; cover crops; mechanical cultivation; and biological and chemical control methods (27). A single weed control measure is not feasible due to the number of different weed species, their highly variable life cycles, and survival mechanisms (26). In addition, controlling weeds with only one or two methods gives weeds a chance to adapt to those practices. Therefore, instead of relying on only one or two management tools, the IWM toolbox includes a large number of options.

Herbicide-tolerant crops are a powerful new tool in this toolbox (15,19,24). Their use has grown steadily since they became commercially available less than a decade ago. Growers have readily integrated HTCs into their crop production practices. For example, currently more than 80% of the 65 million acres of soybeans grown in the US annually are cultivars genetically engineered to be tolerant to glyphosate, a broad-spectrum herbicide. In some regions, 90% of the soybeans planted are glyphosate-tolerant varieties. Herbicide-tolerant crops have become a common weed control tool in our cropping systems, and their usage is steadily growing, especially for soybean (29). For example, use of glyphosate-tolerant soybean has grown from 41% in 1998 to 54%, 70%, and 80% in 1999, 2000 and 2001, respectively (Table 1). A similar increase in the use of HTCs has occurred in canola and cotton (Table 1). The increase in the use of herbicide-tolerant corn (e.g., Roundup-Ready, Liberty-Link, and Clearfield) has been slower than for soybean or cotton. Only about 7% of corn planting were herbicide-tolerant hybrids in 1998, compared to 8%, 12%, and 15% in 1999, 2000 and 2001 (Table 1). Overall, the most common HTC in the United States is glyphosate-tolerant soybean (29). Although HTCs may have a number of important advantages in an IWM program, there are also risks associated with their use. Therefore, our objective is to provide a brief overview of advantages and disadvantages regarding the use of HTCs. Our goal is to help those involved in weed management at the field level be aware of both benefits and risks associated with this technology.

Table 1. Percent of US acres planted with herbicide-tolerant crops from 1998 to 2001.

Crop	1998	1999	2000	2001
¹Corn	7	8	12	15
²Soybean	41	54	70	80
³Cotton	26	35	46	57
⁴Canola	35	45	55	60

¹Corn includes total of Roundup-Ready, Liberty-Link, and Clearfield hybrids.

²Soybean includes total of Roundup-Ready and STS varieties.

³Cotton includes primarily Roundup-Ready varieties.

⁴Canola includes total of Roundup-Ready, Liberty-Link, and Clearfield hybrids.

Currently Available Herbicide-Tolerant Crops

Herbicide-tolerant crops can be produced by either insertion of a “foreign” gene (transgene) from another organism into a crop, or by regenerating herbicide-tolerant mutants from existing crop germplasm (15,19,24). The first type of HTC is also commonly known as a genetically modified organism, or GMO, while the second type is referred to as a non-GMO variety or hybrid. Examples of GMO crops include canola and soybean varieties or corn hybrids tolerant to glyphosate and glufosinate herbicides. Examples of non-GMO crops include STS-soybeans, Clearfield corn, and Clearfield wheat.

Industry continues to work on the development of new HTCs. For example, it is likely that glyphosate-tolerant spring wheat will be available in 2004 and 2005 for Canadian and US markets, respectively. In addition, Clearfield winter wheat, which is tolerant primarily to imazamox herbicide, is likely to be released for the south-central US in 2002 or 2003. Glyphosate-tolerant alfalfa is currently being evaluated in variety testing trials, indicating potential for release within a few years.

The inclusion of several transgenes in a single hybrid or variety, commonly referred as “stacked genes or stacked traits” is also under development. For example, some corn and cotton hybrids have been genetically engineered to contain two transgenes, one for insect tolerance and another for herbicide tolerance (e.g., Bt/glyphosate, or Bt/glufosinate). Furthermore, some corn hybrids have three traits, two for herbicide tolerance and one for insect tolerance (e.g., Liberty, Clearfield, and Bt).

Advantages

Considering that as much as 90% of soybean planted in some US states are glyphosate-tolerant varieties, soybean producers must clearly realize benefits from this technology. The most commonly cited benefits to the producers include: (I) broader spectrum of weeds controlled, (II) reduced crop injury, (III) less herbicide carry-over, (IV) price reduction for "conventional herbicides", (V) use of herbicides that are more environmentally friendly, (VI) new mode of action for resistance management, and (VII) weed management flexibility and simplicity, especially in no-till systems. Some of these factors contribute to IWM because they are mutually supportive of other weed management tools such as reduced tillage and crop rotation, while others can help improve yields and profit.

(I) Broader spectrum of weeds controlled. Non-selective herbicides such as glyphosate and glufosinate aid in broadening the spectrum of weeds controlled, which is particularly important in no-till systems, and those “weedy” fields. In addition, the systemic activity of glyphosate also helps with the control of perennial weeds and their perennial vegetative structures such as stolons and rhizomes. It is especially true for control of perennial grassy species such as quackgrass (Elytrigia repens (L.) Beauv.), smooth brome (Bromus inermis (Leyss.)), orgchardgrass (Dactylis glomerata L.), foxtail barley (Hordeum jubatum), and johnsongrass (Sorghum halepense (L.) Pers.). In Nebraska, glyphosate can be an effective tool for control of many “hard-to-control” annual grassy species in corn such as longspine sanbur (Cenchrus longispinus (Heck.)), crabgrasses (Digitaria spp.), goosesgrass (Eleusine indica (L.), and woolly cupgrass (Eriochloa villosa). In general, glyphosate is the most widely used herbicide in the world and literature about its use and characteristics is extensive (6,9,31).

(II) Reduced crop injury. Various postemergence type herbicides used for weed control in soybean, canola, or corn can cause crop injury and ultimately yield loss. Crop injury is more severe when the crop is under stress or unfavorable environmental conditions occur. In contrast, crop injury is reduced with the use of HTCs. Both glyphosate and glufosinate cause almost no crop injury, compared to some traditional herbicides (e.g., lactofen, chlorimuron), especially when applied to soybean.

(III) Less herbicide carry-over. Glyphosate and glufosinate have almost no soil residual activity because they are tightly bound to the organic particles in the soil. Hence, there are few restrictions for planting or replanting intervals or injuries to the subsequent crops. This trait facilitates crop rotation by providing flexibility in selection of potential rotation crops.

(IV) Price reduction for "conventional herbicides". Introduction of HTC resulted in a price reduction for conventional herbicides. For example, just a few years ago the cost of weed control with conventional herbicides in soybeans ranged from $40 to $60 per acre compared to the current $20 to $30 per acre. The price reduction is the result of the market adjustment and an attempt by companies to remain competitive with the pricing of herbicides used on non-HTCs.

(V) Use of herbicides that are more environmentally friendly. In general, glyphosate and glufosinate have lower toxicity to humans and animals compared to some other herbicides. Since they are absorbed by the organic particles in the soil and decompose rapidly, they pose little danger for leaching and contamination of ground water or toxicity to wildlife.

(VI) New mode of action for resistance management. Since the discovery and report of triazine resistance almost 40 years ago, weed resistance to herbicides has been well documented (14). For example, there are 40 dicot and 15 monocot species known to have biotypes resistant to triazine herbicides (14). Also, at least 44 weed species have been reported to have biotypes resistant to one or more of 15 other herbicides or herbicide families (14). The list of herbicide-resistant weeds will continue to grow, especially with repeated use of herbicides with the same mode of action. Many of the selective herbicides in corn and soybean have similar or identical mechanisms of action such as the inhibition of enzyme acetolactate synthase (ALS) or the inhibition of acetyl-co-enzyme-A-carboxylase (ACCase). Some common weed species in corn and soybean have become problematic due to developed resistance to ALS-type herbicides. The number of worldwide cases of ALS and ACCase resistance is also increasing (12) and herbicides with alternative sites of action are needed. Therefore, HTCs (e.g., glyphosate and glufosinate) can provide a new mode of action when used in an IWM program as an aid in resistance management. Since single or multiple herbicide resistance in weeds is a serious problem in certain parts of the US and Canada, the use of HTCs can provide solutions.

(VII) Crop management flexibility and simplicity. The HTC technology is simple to use. It requires neither special skills nor training. The technology does not have major restrictions and is flexible, which is probably one of the reasons for such wide adoption by producers. In particular, HTCs tolerant to broad-spectrum herbicides such as glyphosate extend the period of herbicide application for effective weed control, which is helpful in dealing with rainy and windy days during the optimal periods for weed control measures. In contrast, poor weather during the critical period for weed control can greatly limit the effectiveness of more selective herbicides (21).

Disadvantages

A number of concerns (2,8,23) should be considered when deciding whether to use HTCs as a component of an IWM program. These concerns include: (A) yield performance, (B) single selection pressure and weed resistance, (C) shifts in weed species, (D) gene escape, (E) gene flow and contamination of organic crops, (F) drift and non-target movement, and (G) marketing and food labeling in global markets.

(A) Yield performance. Herbicide-tolerant crop varieties or hybrids must achieve yields comparable to conventional varieties to ensure an adequate economic return. Some researchers have identified yield drag and yield lag as two potential concerns (5). Yield drag is a yield reduction due to addition of foreign genes. Yield lag is the potential yield depression due to the age of the variety in which the gene is inserted. Recent University of Nebraska study (5) concluded that soybean varieties with the glyphosate-tolerant gene yielded 5% less than the sister lines without the foreign gene, indicating yield drag. In the same study, glyphosate-tolerant varieties yielded 10% less than the best high-yielding non-HTCs, indicating yield lag. While companies try to incorporate new traits into elite varieties, there can be a time lag in this process. However, as GMO varieties become widely used, as in the case of Roundup-Ready soybean, it is likely that yield lag may diminish.

(B) Single selection pressure and weed resistance. Widespread use of the same HTCs results in repeated use of the same herbicide, creating a single selection pressure on weed population. Repeated use of the same herbicide is the main reason for weed resistance to herbicides worldwide (13). Therefore, special attention should be given to proper management of HTCs to avoid the development of herbicide-resistant weed populations. Indeed, there are already several weed species that are resistant to glyphosate. Examples include: rigid ryegrass (Lolium rigidum) in Australia (22), goosegrass (Eleusine indica) in Malaysia, ryegrass in California, and horseweed (Conyza canadensis) in Delaware and Tennessee (1,32). Resistance in the above cases resulted from repeated use of glyphosate in the absence of an IWM program.

Furthermore, in last few years there were several reports of waterhemp (Amaranthus rubis Sauer) surviving label rates of glyphosate in glyphosate-tolerant soybeans from Iowa, Illinois, and Missouri. Weed scientists from Iowa and Missouri reported that 2.6 and 8 times the label rates of glyphosate, respectively, was needed to control those waterhemp populations. In Nebraska, we observed that we are not controlling waterhemp as well as we did four or five years ago with the label rate, indicating the need for higher rates, repeated applications, and ultimately higher cost of weed control program.

(C) Shifts in weed species. Shifts in weeds are not new. Weed shifts have happened as long as humans have cultivated crops. Weedy and invasive species can easily adapt to changes in production practices in order to take advantage of the available niches (20). Species that do not adapt to management changes become “less frequent” compared to those that do adapt. Weed shifts can also occur both within a population of a certain species (e.g., surviving mutants), or within a plant community (e.g., certain species). Therefore, despite the fact that glyphosate and glufosinate control many weed species, they do not control all plant species. Although glyphosate controls many grasses, certain broadleaf species in major US and Canadian cropping systems are tolerant to label rates of glyphosate. Repeated use of glyphosate can result in a shift in weed species from those easily controlled by glyphosate to those more tolerant of this herbicide. Examples of such species include: wild buckwheat (Polygonum convolvulus), Pennsylvania smartweed (P. pensilvanicum), lady’s thumb (P. lapathifolium), ivyleaf morning glory (Ipomea hederacea), venice mallow (Hibiscus trionum), horseweed (Conyza canadensis), yellow sweetclover (Melilotus officinalis), and field bindweed (Convolvulus arvensis) (32). Furthermore, weeds can survive in crop production systems based on HTCs because of natural tolerance to glyphosate and because of growth types or life cycles that help them avoid being treated (17). Such shifts in weed populations to more tolerant weeds increases weed control costs, even with the use of HTCs.

(D) Gene escape. The potential for the “escape” of genes conferring herbicide resistance via pollen from HTCs to other plant species is another major concern, especially from HTCs to closely-related wild relatives (33). Gene escapes from HTCs are not a new phenomenon. A herbicide resistance gene was naturally transferred via pollen from herbicide-tolerant IMI-wheat to jointed goatgrass (Aegilops cylindrica) in northwestern US (25). Others have also demonstrated that pollen flow was the main reason for naturally occurring multiple resistance of canola (Brassica napus) to three commonly used herbicides (glyphosate, glufosinate and imazethapyr) in Alberta, Canada (10). The probability of gene escape increases if the plant species are closely related (e.g., same genus) because of the possibility of cross pollination (11). The so-called “high risk crops” and their weedy relatives includes: sorghum and its weedy relatives shattercane and johnsongrass; canola and mustards; wheat with jointed goat-grass and quackgrass; rice and red rice; sunflower and wild sunflower.

(E) Gene flow and contamination of organic crops. Gene flow results in the contamination of non-GMO crops by pollen from GMO crops. It is especially a concern for organically-grown crops. For example, widespread use of glyphosate-tolerant soybean can cause problems for the production of organic soybeans due to contamination by glyphosate-resistant genes via cross-pollination from neighboring fields with HTC soybean. The same is true for organic corn production. Because tests can detect very small quantities of cross-contamination, organic farmers are concerned that such cross-contamination will limit their ability to market organic crops which must contain no GMO seeds.

(F) Drift and non-target movement of herbicide. Drift and non-target movement is a general concern when using any herbicide. However the concern becomes greater with the use of non-selective herbicides such as glyphosate and glufosinate. Misapplication and misidentification of fields planted with non-HTCs can occur unless care is taken to identify such fields and to avoid drift onto nearby fields with crops that are not resistant to the herbicide applied.

(G) World markets and food labeling. Current anti-biotechnology sentiment in Europe and Japan have caused a reduction in grain imports from the US to these countries. There is already an estimated 30% reduction in US exports of various products related to glyphosate-tolerant soybeans. This is the result of opposition to biologically engineered crops by consumers in these countries. In addition, many countries are considering, or have implemented, labeling regulations for GMO crops and grain products.

Conclusions

An IWM approach is required to optimize profit by maintaining weed populations below threshold levels. Herbicide-tolerant crops are a relatively new and powerful tool in the IWM toolbox, but they must be used in a mutually supportive fashion with other weed management practices. Therefore, HTC should be used in accordance with the principles of IWM.

In essence, the development of an IWM program is based on a few general rules that can be used on any farm, which include: use agronomic practices that limit the introduction and spread of weeds (preventing weed problems before they start); help the crop compete with weeds; and use practices that do not allow weeds to adapt. Combining agronomic practices based on the above rules will allow agronomists to design an IWM program for any field. The bottom line is that an IWM program is not a "recipe". Rather, it needs to be changed and adjusted to a particular farming operation. The goal is to manage not eradicate weeds. Regardless of whether HTCs or conventional crops are used, there are several things that can be done to give the crop the advantage over weeds and to keep weeds contained. These include: (a) managing fertilizer rates, timing, and placement (3,18); (b) adjusting planting pattern (18) and crop row spacing (16); (c) planting more competitive varieties (18); (d) varying planting dates; (e) rotating crops (7,30); (f) rotating herbicides with different modes of action (13); (g) rotating HTCs that are tolerant to herbicides with different modes of action; (h) scouting fields; (j) using the concept of critical period of weed control to determine timing of weed control (16); and (k) documentation and record keeping. Specific details about these general IWM rules can be found elsewhere (3,4,7,13,16,18,27,28,30).

We believe that HTCs, especially the ones based on glyphosate and glufosinate herbicides, are essential components of the IWM system, but their value can be preserved only by proper management and reduced overuse. This becomes even more important when other HTC crops become more readily available (e.g., Roundup-Ready corn). We believe that it is easy to fall into a trap of overusing, for example, glyphosate when one glyphosate-tolerant crop is grown after another. Therefore, proper use of HTC technology, as a component of IWM program, is the key to preserving the long-term benefits of this technology while avoiding many of the concerns about their use or misuse (e.g., overuse).

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