Agronomy Journal 93:408-412 (2001)
© 2001 American Society of Agronomy
PRODUCTION PAPER
Glyphosate-Resistant Soybean Cultivar Yields Compared with Sister Lines
Roger W. Elmorea,
Fred W. Roetha,
Lenis A. Nelsonb,
Charles A. Shapirod,
Robert N. Kleinc,
Stevan Z. Knezevicd and
Alex Martinb
a Univ. of Nebraska, South Central Res. and Ext. Center, Clay Center, NE 68933
b Dep. of Agronomy, Univ. of Nebraska, Lincoln, NE 68583
c Univ. of Nebraska-Lincoln, West Central Res. and Ext. Center, North Platte, NE 69101
d Univ. of Nebraska, Northeast Res. and Ext. Center–Haskell Ag Lab, Concord, NE 68728
Corresponding author (relmore1{at}unl.edu)
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ABSTRACT
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Herbicide-resistant crops like glyphosate resistant (GR) soybean [Glycine max (L.) Merr.] are gaining acceptance in U.S. cropping systems. Comparisons from cultivar performance trials suggest a yield suppression may exist with GR soybean. Yield suppressions may result from either cultivar genetic differentials, the GR gene/gene insertion process, or glyphosate. Grain yield of GR is probably not affected by glyphosate. Yield suppression due to the GR gene or its insertion process (GR effect) has not been reported. We conducted a field experiment at four Nebraska locations in 2 yr to evaluate the GR effect on soybean yield. Five backcross-derived pairs of GR and non-GR soybean sister lines were compared along with three high-yield, nonherbicide-resistant cultivars and five other herbicide-resistant cultivars. Glyphosate resistant sister lines yielded 5% (200 kg ha-1) less than the non-GR sisters (GR effect). Seed weight of the non-GR sisters was greater than that of the GR sisters (in 1999) and the non-GR sister lines were 20 mm shorter than the GR sisters. Other variables monitored were similar between the two cultivar groups. The high-yield, nonherbicide-resistant cultivars included for comparison yielded 5% more than the non-GR sisters and 10% more than the GR sisters.
Abbreviations: a.e., acid equivalent • GR, glyphosate resistant • LL, liberty link • NEREC-HAL, Northeast Research and Extension Center–Haskell Agric. Lab. • PRR, phytophthora root rot • R1, flowering • R7, physiological maturity • R8, harvest maturity • SCREC, South Central Research and Extension Center • STS, sulfonylurea-resistant soybean • WCREC, West Central Research and Extension Center
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INTRODUCTION
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SOYBEAN improvement through the incorporation of genetic resistance or tolerance is an accepted practice in soybean cultivar development for yield-limiting factors such as diseases (Athow, 1987) and nematodes (Riggs and Schmitt, 1987). A goal of plant breeders is to maintain the productivity of the parent line in the absence of the yield-limiting factor. Comparisons of near-isogenic lines with and without the tolerance or resistance genes are important to ascertain if grain yields are suppressed.
Phytophthora root rot (PRR, caused by Phytophthora megasperma f. sp. glycinea Kuan and Erwin) was one of the most destructive diseases of soybean (Athow, 1987). It provides a good case study for this discussion. In the early 1960s genetic resistance to PRR was incorporated into several cultivars through backcrossing programs resulting in near-isogenic lines (Athow, 1987). Several researchers using near-isogenic lines have reported that PRR-resistant lines perform the same as PRR-susceptible lines in the absence of PRR (Caviness and Walters, 1971; Singh and Lambert, 1985; Wilcox and St. Martin, 1998). Singh and Lambert (1985) also reported no deleterious pleiotropic effects of the insertion of the gene for PRR resistance. Thus, no yield suppression was associated with the incorporation of the PPR genes into soybean cultivars.
Herbicide-resistant crops like glyphosate resistant (GR) soybean are gaining widespread acceptance in U.S. cropping systems. This technology has promise in areas where other herbicides cannot effectively control weeds. However, potential yield suppression associated with GR cultivars is a concern of producers and seed companies. Data from university soybean cultivar performance trials in several states suggest a yield suppression may exist with GR soybean (Minor, 1998; Nielsen, 2000; Nelson et al., 1997, 1998, 1999; Oplinger et al., 1998; H.C. Minor, Univ. of Missouri, personal communication, 1999). However, Delannay et al. (1995) stated that no yield suppresion was associated with the GR gene. This statement was based on unpublished research where six pairs of isopopulations with and without the GR gene were compared (X. Delannay, personal communication, Dec. 1999). He concluded that GR cultivars should perform as well as conventional cultivars of equivalent maturities. The GR gene, CP4 EPSPS, from breeding line 40-3-2 tested in the Delannay et al. study remains as the source for resistance in current GR cultivars (X. Delannay, personal communication, Dec. 1999).
Yield suppression may result from either the GR gene/gene insertion, glyphosate (both individually or collectively are termed yield drag), or cultivar genetic differentiation (yield lag). Yield lag represents yield suppression due to the genetics of the cultivar or line in which the GR gene is inserted. Thus, yield of GR cultivars may lag behind that of other cultivars simply because the GR gene was inserted in lower yielding or older cultivars. Yield drag can result from either the GR gene or its insertion (GR effect) or the application of glyphosate (herbicide effect). We reported that glyphosate did not suppress grain yield of GR soybean cultivars and hence did not contribute to a yield drag (Elmore et al., 2001). Yield drag could also result from the GR gene or its insertion process (GR effect); evidence of this has not been reported.
We designed two experiments to test for yield drag: the effect of GR gene insertion on GR (reported in this paper) and the effect of glyphosate (Elmore et al., 2001). To evaluate the GR effect on yield and agronomic traits, field experiments were conducted at four Nebraska locations on five pairs of GR, non-GR sister lines in 1998, and on four pairs of GR, non-GR, sister lines in 1999. Eight other cultivars were included for comparison. We could not discern between yield drag associated with the GR gene itself or effects of its insertion in this study. Thus, reference to the GR effect could mean either or both of these possibilities.
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MATERIALS AND METHODS
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Field experiments were planted at four Nebraska locations in 1998 and 1999 (Tables 1 and 2). Corn was grown before the experimental year in both years at all locations. Subplots were 4 to 0.76 m rows by 9.1 m in length. Seeding rate was 370000 seed ha-1. Field preparation activities varied by location and year: Lincoln Agronomy farm 1998 and 1999—disk and field cultivate in spring; North East Research and Extension Center–Haskell Ag Lab 1998—disk and field cultivate in spring 1999—fall disk, spring disk and field cultivate; South Central Research and Extension Center 1998—two passes of mulch master (John Deere, Moline, IL) in spring; 1999—rototilled in spring; West Central Research and Extension Center: 1998 and 1999—ridge till. Plots were sprayed with the preemergence herbicide combination of metolachlor and metribuzin to help control weeds. Glyphosate was also applied at the West Central Research and Extension Center (WCREC) location to control emerged weeds (Table 3). The experiments were maintained weed-free by hand weeding. We used a randomized complete block experimental design with four replications at all locations except only three replicates were used at WCREC in 1999.
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Table 3. Preplant and preemergence herbicide application information by location and year. Nebraska, 1998–1999 (see Table 1 for location abbreviations)
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Cultivars grown are shown in Table 4. Entries 1 to 3 were included based on their tolerance to glufosinate (Liberty Link, LL) and chlorimuron/thifensulfuron (sulfonylurea-resistant soybean, STS). Entries 4 to 6 were included because of their high yield in the Univ. of Nebraska's 1997 cultivar tests (Nelson et al., 1997). Entries 7 to 8 were included since they were also in the companion study (Elmore et al., 2001); these cultivars were provided by two of the major seed companies in Nebraska based on their maturities and yield. Entries 1 to 8 were included as checks. Backcross-derived BC3 and BC4 sister line pairs were provided by two seed companies. The lines were chosen based on appropriate maturities for our locations. Unfortunately, isogenic lines of non-GR and GR cultivars or lines were not available.
Flowering date (R1), physiological maturity (R7), and harvest maturity (R8) (Ritchie et al., 1996) were recorded at several of the locations. In addition, stand counts were taken during the vegetative stages, plant height at R7, and lodging scores were recorded at R8. Seed weights were recorded at three locations in 1999. The center two rows of each plot were harvested with a small plot harvester for yield and seed weight determination.
Data were processed with SAS mixed models procedures (Littell et al., 1996). Cultivar was considered a fixed effect. Locations and replicates and their interactions with the fixed effect were treated as random effects. Single-degree-of-freedom comparisons were used to isolate cultivar grouping differences: LL/STS vs. STS; STS vs. High yield; LL/STS vs. High yield; LL/STS vs. all GR; STS vs. all GR; High yield vs. non-GR sisters; non-GR sisters vs. GR sisters; GR cultivars (7 and 8) vs. GR sisters; High yield vs. all GR; High yield vs. GR (7 and 8); 9 vs. 10; 11 vs. 12; 13 vs. 14; 15 vs. 16; and 17 vs. 18. We also used correlation to compare grain yields of GR and non-GR sister lines.
Three sets of analyses were used for each variable. The first compared all entries except 13 and 14 over both years. The second and third analyses included all entries in 1998 and 1999, respectively, since entries 13 and 14 were not available in 1999. Data presented are least squares adjusted means. Differences mentioned are significant at P 
0.05.
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RESULTS AND DISCUSSION
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On average, non-GR sister lines yielded 5% (200 kg ha-1) more than the GR sisters when averaged over all locations and both years (Table 5). Non-GR sister grain yields were greater than those of their associated GR sisters in two of the five pairs (Table 4). Results were similar in the single-year analyses (data not shown). Grain yields of sister-line pairs are shown in Fig. 1. The greater number of data points to the right of the 1:1 ratio line indicates that the non-GR sisters yielded more on the average than their GR sister counterparts. This reinforces the previous statement on the average soybean yields of the non-GR sisters relative to the GR sisters in the individual years and in the 2-yr analysis (Table 5).
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Table 5. Non-glyphosate-resistant sister lines (non-GR sisters) yielded more than their GR sisters averaged over all locations and 2 yr. Growth and development of these two variety groups differed
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Fig. 1. Yield of non-GR (glyphosate resistant) sisters compared with their respective GR sisters at four locations in 2 yr. Each marker represents yield data of sister line pairs from the same replicate, location, and year. University of Nebraska, 1998 and 1999
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Seed weight of the non-GR sisters was greater than that of the GR sisters (in 1999) and the non-GR sister lines were 20 mm shorter than the GR sisters (Table 5). Other variables monitored were similar between the two cultivar groups. The GR sisters yielded the same as the average of entries 7 and 8, two GR cultivars included in the study for comparison (data not shown). Entries 7 and 8 yielded the same as other GR cultivars in another study with other GR cultivars (Elmore et al., 2001). Yield of the GR check cultivar entry 8 was similar to those of the high-yield, nonherbicide-resistant cultivars (entries 4, 5, and 6). In addition, although no statistical comparisons were possible, yields of the highest yielding GR cultivars in the other study (Elmore et al., 2001) were similar to those of the high-yield, nonherbicide-resistant cultivars in this study (data not shown). The high-yield, nonherbicide-resistant cultivars (entries 4, 5, and 6) yielded 5% more than the non-GR sisters (Tables 5 and 6). This 5% difference is a yield lag. The GR gene in the GR sisters therefore reduced soybean yield 5% compared to the non-GR sisters and 10% when compared to high-yield, nonherbicide-resistant cultivars.
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Table 6. Herbicide-resistant varieties yielded less than the nonherbicide-resistant check varieties averaged over locations and 2 yr
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The high-yield, nonherbicide-resistant checks in the study (entries 4, 5, and 6) also yielded the same or more than the other herbicide-resistant cultivars included in the experiment (Tables 4 and 6). The average yield of all seven GR cultivars was similar to that of the LL/STS cultivar (entry 1), and greater than that of the average of the two STS cultivars (entries 2 and 3). A comparison of the means of the STS cultivars shows that entry 3 yielded less than the other STS cultivar, entry 2, as well as the other herbicide-resistant cultivars (Table 4). Herbicide-resistant cultivars yielded from the same to 15% less than the nonherbicide-resistant cultivars included in these studies (Table 4 and 6).
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CONCLUSIONS AND IMPLICATIONS
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Yields were suppressed with GR soybean cultivars. Our other work showed that there was no effect of glyphosate on GR cultivars (Elmore et al., 2001). The work reported here demonstrates that a 5% yield suppression was related to the gene or its insertion process and another 5% suppression was due to cultivar genetic differential. Producers should consider the potential for 5 to 10% yield differentials between GR and non-GR cultivars as they evaluate the overall profitability of producing soybean. Cultivar choices are best based on (i) previous weed pressure and success of control measures in specific fields, (ii) the availability and cost of herbicides, (iii) availability and cost of herbicide-resistant cultivars, and (iv) yield, and not solely on whether cultivars are herbicide resistant. Based on our results from this study and those of Elmore et al., 2001, the yield suppression appears associated with the GR gene or its insertion process rather than glyphosate itself.
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ACKNOWLEDGMENTS
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We thank the Nebraska Soybean Board, who partially supported the research. Technical support personnel at the various locations were: Clay Center—Sharon Hachtel, George Hoffmeister, Jr., Ralph Klein, Perry Ridgeway, Irv Schleufer, and Sandy Sterkel; Lincoln—Greg Dorn and John Eis; North Platte—Jeff Goulis; Concord—Lisa Lunz and Ray Brentlinger. We appreciate their efforts!
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NOTES
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Univ. of Nebraska, Lincoln, Agric. Res. Div. J. Ser. no. 13050.
Received for publication June 12, 2000.
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REFERENCES
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TOP
ABSTRACT
INTRODUCTION
