Published in Crop Sci. 43:1584-1589 (2003).
© 2003 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
SPECIAL SYMPOSIUM—2001 CSSA MEETING (Charlotte)
Challenges in Comparing Transgenic and Nontransgenic Soybean Cultivars
Paul L. Raymer* and
Timothy L. Grey
The Univ. of Georgia, Griffin Campus, 1109 Experiment St., Griffin, GA 30223
* Corresponding author (praymer{at}griffin.uga.edu).
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ABSTRACT
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Genetically modified soybean [Glycine max (L.) Merr.] cultivars containing herbicide resistance have become the predominant cultivars marketed in the USA. This technology dramatically changed weed management strategies in the USA and created controversy and new challenges within various segments of the industry. This review seeks to summarize many of the issues that arose within the soybean industry surrounding the introduction of the world's first major transgenic crop. Hopefully, the experiences gained during this transition period will benefit future efforts to commercialize transgenic crops. Initially, the new transgenics made the already difficult, yet critically important, task of cultivar selection even more arduous and confusing. Lack of university data, high cultivar turnover rates, and the added burden of factoring in technology fees, yield lag, and weed control costs into the decision of which cultivar to plant were frustrating to most producers. Industry's early insistence on separate cultivar performance trials for Roundup Ready (RR) cultivars taxed limited testing resources and increased the size of the already massive data sets generated. Most often, the end result did not provide a direct comparison with popular conventional cultivars. As this new technology has become fully accepted and assimilated into our industry, comparisons with conventional cultivars are less important since most farmers are selecting only among RR cultivars. The current literature indicates that soybean variety test (SVT) programs with limited resources can effectively compare soybean cultivar performance using combined trials treated only with conventional herbicides with little risk of yield loss or herbicide x cultivar interactions. However, the use of a systems approach or separate RR trials to evaluate the performance of soybean cultivars is now commonplace in the USA due to the sheer dominance of RR cultivars. Many larger SVT programs will soon consider discontinuing separate trials for conventional cultivars as their numbers and importance diminish.
Abbreviations: GMO, genetically modified organism • RR, Roundup Ready • SVT, soybean variety test
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INTRODUCTION
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THE PHRASE genetically modified organism, or its more commonly known acronym GMO, is now a household word. Ten years ago, few people in the USA knew the meaning of the term; now most Americans have at least some understanding of the term and the many consumer and environmentally related issues that surround it. GMO crops, also known as transgenic crops, are now commonplace in American agriculture. These terms broadly refer to crop cultivars that were developed using recombinant DNA techniques and that contain genetic material from another organism. The use of transgenic crops that express traits considered advantageous for pest management has risen dramatically since their commercial introduction in the mid-1990s. In soybean, the dominant transgenic technology currently in use is the RR system. Roundup Ready soybean cultivars contain a bacterial transgene, commonly known as CP4, that confers resistance to glyphosate (N-phosphonomethyl glycine), the active ingredient in Roundup. Glyphosate-resistant soybean cultivars were first commercially introduced in the USA in 1996 (Fig. 1) . More than 400 thousand hectares of RR soybean were planted in the USA the first year it was commercially available (Fernandez-Cornejo and McBride, 2000). In 2001, the USDA estimated 68% of the 30 million hectares of soybean in the USA were planted to RR cultivars (National Agricultural Statistics Service, 2001).
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Fig. 1. Adoption of Roundup Ready soybean in the USA, 1996 to 2001. Source: National Agricultural Statistics Service, 2001.
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The rapid adoption rate of this system infers soybean producers believe it offers advantages over the conventional weed control systems they were using. Results of a farmer survey conducted by the USDA regarding adoption of genetically engineered crops were summarized by Fernandez-Cornejo and McBride (2000). Sixty-five percent of growers surveyed indicated they used the RR soybean system because of the potential for increased yields through improved pest control. Another 20% indicated the primary reason was decreased pesticide cost. Certainly this system does offer the potential for enhanced weed control. Roundup is a broad-spectrum herbicide that controls most grasses and many problematic broadleaf weeds common in soybean production (Delannay et al., 1995). It is also possible for growers using this system to reduce herbicide costs (Fernandez-Cornejo and McBride, 2000) and improve flexibility in timing of herbicide application (Vangessel et al., 2000) since many weed species can be controlled much later in the growing season than was previously possible using conventional herbicides. Soybean lines with the RR system have good crop tolerance to glyphosate and therefore, afford less opportunity for herbicide-related crop injury. The combination of the above factors makes this system of weed control easier for the farmer to manage. Ease of management is most likely the actual reason this technology was adopted so quickly.
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Separate Trials for Roundup Ready Cultivars
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Shortly after the introduction of the RR system, several industry-related issues arose. The first issue was precipitated by Monsanto's early insistence that only Roundup should be used on RR cultivars. Monsanto justified this position by contending conventional herbicides injured soybean and reduced seed yield. Therefore, the full benefit of the RR system could not be realized unless only Roundup was used on RR cultivars. As a result of this position, Monsanto-owned seed companies refused to enter RR cultivars in conventional trials or in any trials that herbicides other than Roundup would be used. Many agronomists within the soybean industry viewed this position as a corporate attempt to conceal deficiencies in cultivars containing this new technology.
This issue created a serious dilemma for SVT programs throughout the country. If a SVT program failed to modify its structure to specifically accommodate RR cultivars, most seed companies would not enter their cultivars and limited RR cultivar performance information would be obtained. This compromised the very purpose of university-operated testing programs, since their primary mission is to provide unbiased cultivar performance data to aid growers in selecting cultivars.
On the other hand, if an SVT program established separate RR cultivar trials, they were in essence duplicating their existing programs and taxing limited program resources. This is what most university testing programs eventually did, although with some delay. Even though RR cultivar performance data were being generated and disseminated, its usefulness to growers was greatly diminished. The establishment of separate RR trials precluded valid direct comparisons between conventional and RR cultivars. At the time, a direct comparison between RR and conventional cultivars was the single biggest piece of information that growers needed to determine how this new technology compared with the technology that they were currently using.
A third option first used by testing programs in Missouri and later in Georgia and a few other states was to include conventional cultivars as checks for comparison in Roundup-treated trials (Minor, 1998). This approach represented a compromise between the first two options discussed previously and required that conventional checks be protected when Roundup was applied. Initially this appeared to be a logical way around the problem. Only Roundup was applied to RR cultivars, and yet comparisons with conventional cultivars were possible. However, this protocol proved difficult to manage and often produced unreliable data. Conventional check plots required spot treatment with conventional herbicides or hand weeding. Untimely weed control often resulted in competition that biased results. Furthermore, the end result of even the most well-managed trial was a limited set of comparisons between the RR cultivars and a few conventional checks.
The University of Illinois program (Table 1)
illustrates the impact of the introduction of RR cultivars on the structure and size of programs (R. Esgar, 2001, personal communication). In 1996, at the time of the introduction of RR technology, the University of Illinois offered soybean cultivar tests at nine locations. In 1997, even though RR cultivars were widely available to producers, the University of Illinois still offered only conventional trials at all nine locations. Of the 607 cultivars tested by the University of Illinois in 1997, only three (0.5%) were RR. In 1998, separate RR trials were established at five of the nine locations, increasing the total number of trials offered to 14. Seed companies responded quickly to these additional trials. The number of RR cultivars tested increased from three the previous year to 328 and the total number of cultivars tested increased from 607 the previous year to 807. The University of Illinois modified its testing structure in 1999 to offer both RR and conventional trials at each of the nine locations. The number of RR cultivars tested increased to 451 and surpassed the number of conventional cultivars tested. In 2000, the number of test locations increased to 12 with both RR and conventional trials offered at each location. This brought the total number of trials offered to 24, an increase of 266% since the introduction of the RR cultivars. In 2001, 75.9% of the 681 soybean cultivars tested by the University of Illinois program were RR.
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Table 1. Number of trial locations and number of conventional and Roundup Ready cultivars entered in the University of Illinois Soybean Trials, 1996 to 2001.
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Yield Depression of Roundup Ready Cultivars
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A second issue that quickly arose following the introduction of RR cultivars was related to concerns that yields of RR cultivars were not competitive with yields of conventional cultivars. Raymer (1997) reported on the performance of RR cultivars entered in university SVT conducted in nine southern states during 1995 and 1996. Although yields of some RR cultivars in some trials were often above the mean yield of the trial, seed yields of the population of RR cultivars as a whole averaged 2.9 units below the mean (Fig. 2)
. In comparison, seed yields of conventional southern regional check cultivars averaged 1.0 unit above the trial mean and 3.9 units higher than the average of all RR entries. Minor (1998) reported similar findings from southern SVTs.
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Fig. 2. Frequency distribution of 269 individual comparisons of the seed yield of Roundup Ready (RR) soybean cultivars relative to the test mean. Data are from soybean variety test programs conducted by nine southern states in 1995 and 1996. Each unit on the x-axis represents a difference of 67.19 kg ha-1 (one bushel per acre).
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In 1999, the 1995 and 1996 data discussed above were compared with performance data from university trials conducted in 1998 (Raymer and Minor, 1999). Analysis of the 1998 data indicated that average performance of RR cultivars had improved. Seed yields of RR cultivars in 1998 were only 0.5 units below the trial mean as compared with 2.9 units below the trial mean in the 1995-1996 data set. However, the average yield of RR cultivars was still 1.5 units below that of the conventional check cultivars. Examination of the 1998 data by maturity group indicated yield parity for RR Maturity Group IV cultivars had been reached in that they performed as well as conventional cultivars. There was still evidence of a small yield lag persisting in Maturity Groups V through VIII. It was also noted that disease and nematode resistance were often compromised in RR cultivars, as many RR cultivars lacked the level of disease resistance or nematode resistance of their conventional counterparts. On the basis of the trend indicated by these data, the authors concluded that with continued breeding efforts, RR cultivars would soon become fully competitive with conventional cultivars.
Oplinger et al. (1998) reported significantly lower yields of RR cultivars when the top five RR cultivars were compared with the top five conventional cultivars entered in the 1998 SVT programs of nine northern states. Christmas of Purdue University made similar comparisons using 1999 data from eight northern state SVT programs and reported a lower but still significant yield penalty for RR cultivars when compared with non-RR cultivars (Nielsen, 2000).
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Technology Lag or Technology Drag
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Once it became apparent that lower yields were often associated with RR cultivars, many in the industry began to propose possible explanations. Most explanations fell into two main areas and the issue became popularly referred to as technology lag vs. technology drag. The term technology lag implies a temporary or transient problem associated with the introduction of a new technology, in this case resistance to glyphosate. Eventually the industry would be expected to overcome the problem. The potential for a yield lag would be expected to increase when (i) the trait of importance is from a single genetic source, and (ii) it is rapidly introgressed into a broad range of highly advanced germplasm. In the case of glyphosate resistance, the original registered transformation event was performed on ‘Asgrow A5403’, a well-adapted and high yielding cultivar. This cultivar became the only source of glyphosate resistance for all breeding programs.
Some speculated that at least a portion of the problem could be attributed to old genetics. Many of the initial RR cultivars released were developed using a backcross breeding approach to introgress the glyphosate resistance gene into proven top performing cultivars. A backcross program is a 3- to 5-yr process for even the fastest breeding program. The original cultivar used as the recurrent parent in many cases was 10 yr old or more by the time the RR version became commercially available. It is possible that the newly released RR cultivar with old genetics had now been surpassed in performance by the most recently developed conventional cultivars. Still others insisted the evaluation period for the first RR cultivars was abbreviated and yield too often compromised by breeders and seed companies in their haste to commercialize the first wave of RR cultivars.
The term technology drag implies an inherent yield reduction associated with the technology itself. The concept of yield drag is certainly of more concern than yield lag because drag implies a permanent and perhaps unavoidable problem directly related to the technology. Possible explanations for yield suppression that fall into the technology drag category are: (i) the presence of the gene (CP4 conferring glyphosate resistance) reduces fitness of the plant, (ii) normal genetic or physiological processes were disrupted by the transformation process, and (iii) the application of glyphosate causes yield suppression.
Delannay et al. (1995) reported results from over 60 field trials and conclusively stated that glyphosate application did not affect seed yield of RR soybean lines and that the RR soybean lines were not different in seed yield than their nontransgenic parental lines. These findings provided strong evidence against the possibility of a technology drag. Elmore et al. (2001a) also showed that no yield effect was associated with application of glyphosate on RR cultivars. However, in a companion paper, Elmore et al. (2001b) reported that RR sister lines yielded 5% less than the non-RR sisters. They also included three high-yield, non-RR cultivars, and five other RR cultivars in the comparison and reported that the high-yield, non-RR cultivars yielded 5% more than the non-RR sisters and 10% more than the RR sisters. On the basis of these findings they concluded that a 5% yield suppression was related to the gene or its insertion process and another 5% suppression was due to cultivar genetic differential. On the basis of these recent reports, it appears that the existence of a technology drag associated with the RR system will continue to be debated.
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Cultivar Selection and Transgenics
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The primary objective of university-operated SVT programs is to provide accurate and unbiased performance information for comparing the cultivars that are available to producers. The SVT-generated data provides a multiyear database that hopefully will aid the producer in selecting the best-suited cultivar and improving profits. Initially, the new transgenics made the already difficult yet critically important task of cultivar selection even more arduous and confusing. This process suddenly became more difficult for both the extension specialist who attempted to present data to facilitate the decision-making process as well as for the producer who was making the decision.
First, SVTs generated limited performance data on RR cultivars in 1996, 1997, and 1998 because few RR cultivars were entered. As a result, growers often did not have the information they needed to make educated decisions regarding cultivar selection. Once RR cultivars began to be entered in SVT programs, many programs noted that cultivar turnover rates increased dramatically. In the Georgia SVT, the percentage of cultivars entered for two consecutive years decreased from 70% in 1994 to 46% in 1998 (Day, 2001, personal communication). Companies were rushing to get their latest, best-yielding RR cultivars out to growers, and often RR cultivars were only marketed for 1 yr before being replaced. High turnover rates made it difficult to amass multiyear data that is often necessary to make accurate decisions on cultivar performance (Bowman, 1998).
The introduction of herbicide-resistant technology greatly complicated the actual process of cultivar selection. With the advent of herbicide-tolerant cultivars, growers now had to factor into the cultivar decision-making process such things as technology fees, premiums on seed costs, and potential differences in weed control costs. Concerns that growers might give up yield potential or needed pest resistance if they selected a RR cultivar were also complicating issues.
With the introduction of herbicide-tolerant cultivars, seed-associated costs and weed-control costs were no longer separate issues, they were now interrelated and must be considered together. Even when total seed-associated costs and weed-control costs appeared comparable between RR and conventional systems, soybean farmers often gave up yield potential by selecting less competitive RR cultivars, and as a result actually reduced their potential for profit. Growers desiring to use the RR system must select the highest-yielding and best-adapted RR cultivars available to maintain maximum profitability.
The cultivar selection decision before the introduction of RR cultivars was relatively straightforward. Among all available cultivars, producers selected cultivars within maturity groups most adapted to their area, and then within that set of cultivars they selected those cultivars with the pest resistance(s) needed. Within that subset of cultivars they selected the highest yielding cultivar(s) for their farm. The decision of what weed control program to use was an unrelated and separate process.
Now with the large-scale adoption of RR cultivars and the availability of yield-competitive RR cultivars comes an opportunity to alter the decision-making process and to simplify things greatly by placing the decision of what weed control system to use at a level above the cultivar selection decision (Fig. 3)
. Using this approach, producers first select the weed control program for their farm. That decision should be field-specific, based on the weed spectrum present, and consider the associated costs and the level of management the producer desires. Once the weed control system is determined, only cultivars that are appropriate for that system are considered for planting. Producers should strive to select the most cost-effective weed control program and then select the highest-yielding cultivars that are available for use in that program. This approach negates the need for direct comparisons of RR cultivars to conventional cultivars or certainly makes those comparisons less important than they were when RR cultivars were first introduced.
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Fig. 3. Schematic representation of the cultivar selection process before and after large scale adoption of Roundup Ready (RR) soybean.
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Injury of Roundup Ready Soybeans by Conventional Herbicides
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Probably the only valid justification for separate cultivar trials or a system approach for the evaluation of RR cultivars is the contention that conventional herbicides reduce yields of conventional as well as RR soybean cultivars. Starting in 1999, journal articles addressing this issue began to appear in the literature. Comparisons among glyphosate only, residual herbicides only, the combination of these two systems, and in some cases, comparisons with hand-weeded controls have been reported (Gonzini et al., 1999; Krausz and Young, 2001; Krausz et al., 2001). Glyphosate alone (Gonzini et al., 1999; Akin and Shaw, 2001; Culpepper et al., 2001; Krausz and Young, 2001; Krausz et al., 2001; Wiesbrook et al., 2001) and in combinations with alachlor (Methoxymethyl-2',6'-diethylanilide chloroacetate) (Gonzini et al., 1999; Krausz et al., 2001), chlorimuron {2-[[[[(4-chloro-6-methoxy-2-pyrimidinyl) amino] carbonyl] amino] sulfonyl] benzoic acid ethyl ester} (Akin and Shaw, 2001; Gonzini et al., 1999), clomazone [2-((2-chlorophenyl) methyl)-4,4-dimethyl-3-isoxazolidinone] (Wiesbrook et al., 2001), cloransulam-methyl{N-(2-carbomethoxy-6-chlorophenyl)-5-ethoxy-7-fluoro(1,2,4) triazolo-[1,5-c]pyrimidine-2-sulfonamide} (Gonzini et al., 1999; Krausz and Young, 2001), imazaquin[2-(5-isopropyl-5-methyl-4-oxo-2-imidazolin-2-yl)] (Akin and Shaw, 2001; Krausz and Young, 2001), imazethapyr [2-(4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl)-5-ethyl-3-pyridinecarboxylic acid] (Gonzini et al., 1999; Krausz and Young, 2001; Wiesbrook et al., 2001), metolachor [2-Chloro-6'-ethyl-N-(2-methoxy-1-methylethyl)-o-acetoluidide] (Akin and Shaw, 2001), metribuzin [4-Amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one] (Gonzini et al., 1999), pendimethalin [N-(Ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine] (Gonzini et al., 1999; Krausz and Young, 2001; Wiesbrook et al., 2001), and sulfentrazone (3-Phenoxybenzyl 2,2-dimethyl-3-(2,2-dichlorovinyl) cyclopropanecarboxylate) (Gonzini et al., 1999; Akin and Shaw, 2001) have also been investigated.
Yield reductions associated with the use of residual herbicides on RR soybean cultivars have been most often reported when using imidiazolinone chemistry. Gonzini et al. (1999) reported that average soybean yield for imazethapyr alone (preemergent) treatments was 79% of the hand-weeded control. Wiesbrook et al. (2001) reported that yield of imazethapyr (postemergence) in combination with pendimethalin treatment was 83% of the glyphosate treatment. Akin and Shaw (2001) noted imazaquin with metolachlor averaged 82% of the yield of the glyphosate treatment. Other reports indicated little or no yield reduction associated with the use of imidazolinone chemistry (Krausz and Young, 2001; Krausz et al., 2001).
Although some reports, as referenced above, support claims of yield reduction due to injury by conventional herbicides, Wiesbrook et al. (2001) pointed out that yields may be reduced as a result of increased weed competition in glyphosate-tolerant soybean systems that utilize only residual herbicides for weed control. To conclusively determine if yield losses are due to injury by conventional herbicides rather than differences in weed competition between the two systems, hand-weeded residual herbicide treatments must be compared directly to hand-weeded glyphosate treatments. Such comparisons have not been reported.
Contrary to the above concerns of injury from conventional herbicides, a number of published reports suggest that the use of conventional herbicides in combination with glyphosate may improve weed control and yield. Some weed species, specifically morning-glory (Ipomoea spp.), have some level of tolerance to glyphosate (Culpepper et al., 2001; Wiesbrook et al., 2001; Grey and Raymer, 2002). Weed escapes can occur in a glyphosate-only system due to the lack of residual activity, and these weedy escapes can reduce yield (Swanton et al., 2000). Thus, efforts to control some weed species have emphasized the use of soil residual herbicides in combination with glyphosate. Other research indicates that use of preplant soil-applied herbicides can also be advantageous in the RR system by providing a wider glyphosate application window (Vangessel et al., 2000).
Regardless of whether the use of conventional herbicides on RR cultivars causes injury and reduced yield or actually complements glyphosate use; this issue has largely disappeared from the industry. Monsanto has now modified its position on this matter as evidenced by the facts that it has labeled a number of Roundup-tank mixes, and seed companies are now very willing to enter RR cultivars in combined SVT trials where only conventional herbicides are used.
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Current Status of Roundup Ready Cultivars in Soybean Variety Tests
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Of 27 states surveyed, 20 now offer separate RR trials and seven offer only combined trials where RR and conventional cultivars are tested side-by-side using only conventional herbicides. With the exception of Arkansas, most of the SVT programs that offer only combined trials are relatively small and have difficulty justifying separate trials.
In 1997, only conventional trials were offered at Illinois (Table 1) and 607 different cultivars were tested in their program. Total number of cultivars tested peaked at 807 in 1998, the first year RR-only trials were offered. Total cultivars tested declined from 1999 to 2001 and are now approaching levels tested before the establishment of separate RR trials. The number of conventional entries declined as the proportion of RR cultivars entered increased from 1997 to 2001. In 2001, 76% of the cultivars entered in the Illinois SVT were RR cultivars. Although separate trials for RR soybean cultivars may or may not be justified based on the potential for herbicide injury as discussed above, separate trials may be well justified by the sheer dominance of RR cultivars entered in SVTs.
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Concluding Comments
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The transgenic nature of glyphosate resistance is irrelevant to all the issues discussed above. Most of these issues were not created because of the transgenic nature of the herbicide resistance trait, but rather were the direct result of the unprecedented marketing approach and corporate policies surrounding the commercialization of this technology. Fortunately, the majority of these issues have proven to be transient and inherently associated with the transition from the use of conventional cultivars to the use of herbicide-resistant systems. Hopefully, the experiences gained during the introduction of RR soybean cultivars will benefit future efforts to commercialize other transgenic traits.
Research findings in the weed science literature indicate that it is possible for SVT programs to effectively determine soybean cultivar performance in combined trials with little risk of yield loss or herbicide x cultivar interactions due to the use of conventional herbicides. The current dominance of RR cultivars in U.S. production minimizes the importance of early arguments against the systems approach (separate RR trials) to cultivar evaluation. Direct comparisons with conventional cultivars are now seldom needed since most soybean farmers are selecting only among RR cultivars. The use of a systems approach by SVT programs is now commonplace in the USA. This approach does provide an effective means of generating data for direct comparisons among RR cultivars. As the number and importance of available conventional soybean cultivars diminishes in the U.S. soybean industry, many larger SVT programs will now consider if they can continue to justify separate trials for conventional cultivars.
Received for publication July 5, 2002.
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REFERENCES
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ABSTRACT
INTRODUCTION
