Herbicide-tolerant crops have been the most widely used application of agricultural biotechnology in the United States. Currently, crops have been modified to be tolerant to three herbicides: bromoxynil, glyphosate and glufosinate. Bromoxynil controls broadleaf weeds only while glyphosate and glufosinate are broad-spectrum with activity on both grass and broadleaf weeds.Herbicide-tolerance results from three mechanisms: metabolic detoxification, resistance at the site of action and prevention of the herbicide from reaching the site of action. Bromoxynil-tolerant crops were developed by introducing a gene that encodes for bromoxynil-specific nitrilase from a soil bacterium, Klebseilla ozaenae (Stalker et al., 1996). Crop tolerance to bromoxynil results from metabolic detoxification. While introduction of the glyphosate-insensitive EPSPS from Agrobacetrium sp. strain CP4 into crops by genetic modification techniques was successful in conferring glyphosate tolerance (Padgette et al., 1996), glufosinate tolerance was achieved through the use of bar gene isolated from another soil bacteria, Streptomyces hygroscopicus. The bar gene encodes for an enzyme, phosphinothricin acetyl transferase, which detoxifies the herbicide glufosinate (Vasil, 1996).
Commercialised herbicide-tolerant crops to date include bromoxynil-tolerant cotton, glyphosate-tolerant soybean, cotton, corn, sugarbeet and canola, and glufosinate-tolerant corn and canola (Table 61.1). Bromoxynil-tolerant cotton was introduced in 1995, while glyphosate-tolerant soybean, cotton, corn, sugarbeet and canola have been available in the United States since 1996, 1997, 1998, 1999 and 1999, respectively. Glufosinate-tolerant corn and canola were commercialised in 1997 and 1999, respectively.
The adoption of bromoxynil-tolerant cotton has been low in the United States (Table 61.1) due to several reasons. Although bromoxynil provides effective control of problem weeds such as morning glory and cocklebur, it is weak on sickle pod, which is a key weed species in several cotton growing states and has no activity on grass weeds. Marketability of bromoxynil-tolerant cotton is further limited, as the herbicide-tolerance trait has not been stacked with the insect-resistance trait.
On the other hand, the commercial adoption of glyphosate-tolerant soybean, cotton and canola has been the most rapid cases of technology diffusion in the history of agriculture. In 2001, glyphosate-tolerant soybean, cotton and canola were planted on approximately 68, 70 and 50% of the total planted acreage, respectively (Table 61.1). Herbicide-tolerant (glyphosate and glufosinate tolerant included) corn was planted on only 7% of the total acreage in 2001. Lack of approval for biotechnology-derived glyphosate-tolerant corn imports into the European Union and non-availability of herbicide-tolerance trait in popular varieties adapted for various corn growing regions have hindered the adoption of herbicide-tolerant corn thus far. Although glyphosate-tolerant sugarbeet has been available for commercial planting since 1999, adoption has been zero due to issues related to marketing.
A traditional weed control program in conventional crops involves the use of several herbicides, targeted to a specific weed or groups of weeds. Herbicides are typically applied either as preplant incorporated (PPI) treatments prior to planting, pre-emergence (PRE) applications at planting or before crop emergence, post-emergence (POST) applications after the crop has emerged or a combination of PRE followed by POST applications. Several constraints limit the success of PPI and PRE herbicide applications. Preplant incorporated and PRE treatments involve guesswork as herbicide applications are made anticipating the weed species that may emerge. The efficacy of soil-applied PRE herbicides is highly dependent on rainfall, with poor weed control under extremely low or high rainfall conditions. As a result, there is an increasing trend towards total POST herbicide programmes. Herbicide-tolerant crops, on the other hand, facilitate the use of POST herbicides, such as glyphosate and glufosinate, wherein herbicides are selected based on emerged weed species in the field within the limits of crop and weed growth stages.
Conventional herbicides pose carryover concerns resulting in planting restrictions as many of them have long soil persistence periods. For example, herbicide labels suggest that crops such as alfalfa, dry beans, cabbage, lima beans, muskmelon, onions, peas, peppers and pumpkins should not be planted for 18 months following the application of a premix of atrazine/rimsulfuron/nicosulfuron (trade name: Basis Gold) in corn. Similarly, 26 months should elapse after imazethapyr application when planting potato, and 40 months for tomato, watermelon, squash and pumpkin following imazethapyr application in soybean (Pest Management Recommendations for Field Crops, 2000). Herbicide-tolerant crops resolve this problem because herbicides used in biotechnology-derived crops, such as glyphosate and glufosinate, have no residual activity and thus no carryover potential.
Crop injury from herbicide applications is a major concern in crop production. The potential for crop injury is generally greater with certain conventional herbicides in crops such as cotton and soybean. For instance, herbicides, such as acifluorfen and 2,4-DB, can cause substantial injury to conventional soybean leading to yield losses (Kapusta et al., 1986; Wax et al., 1973). Weed control is compromised if herbicide rates are decreased to lessen crop injury. Herbicide-tolerant crops offer growers remarkable flexibility because crop injury is practically non-existent.
Herbicide-tolerant crops add flexibility to weed management as they offer programmes that are less restricted by crop growth stage, weed species, weed size, tank-mix partners and adjuvant type. Herbicides used in conjunction with herbicide-tolerant crops can be applied at later crop growth stages compared to conventional herbicides, and the maximum height at which they are effective on weed species is greater than that of currently used herbicides. For example, glyphosate can be used up to the 4-leaf stage on cotton, 6-leaf stage on canola and up to flowering on soybean. These application windows are much wider than those with conventional herbicides. Labels instruct that maximum height up to which glyphosate applications can be made for the control of foxtail and fall panicum, two problem weeds in corn, is 6 inches in contrast to 3 inches with the premix of conventional herbicides atrazine/rimsulfuron/nicosulfuron (Curran et al., 1999).
Herbicides used in herbicide-tolerant crops are broad-spectrum and non-selective in activity. As a result, control of both annual and perennial broadleaf and grass weeds can be obtained with one herbicide application alone and with no need for a tank-mix partner in most situations. This is in contrast to intense weed management programmes used in crops such as cotton, which on average receive three herbicide applications consisting of three active ingredients in combination with one to three cultivations. This simplicity in weed control coupled with no crop injury is the reason cited most often by growers for the adoption of herbicide-tolerant crops.
Perennial weeds are a major issue in crop production as they are difficult weeds to control. The difficulty arises due to their propagation behaviour that includes both vegetative and reproductive methods. Many of the currently available conventional herbicides are not effective on perennial weeds. Though herbicides such as clopyralid are effective, high cost limit their use. Herbicide-tolerant crops provide a viable choice for perennial weed management as herbicides such as glyphosate provide excellent perennial weed control in addition to control of other weeds and are cost-effective.
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