HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (2) Citing Articles via Google Scholar Google Scholar Articles by Ziska, L. H. Articles by Goins, E. W. Search for Related Content PubMed Articles by Ziska, L. H. Articles by Goins, E. W. Agricola Articles by Ziska, L. H. Articles by Goins, E. W. Related Collections Agroclimatology Hillslope Analysis Ecological Risk Assessment

CROP ECOLOGY, MANAGEMENT & QUALITY

Elevated Atmospheric Carbon Dioxide and Weed Populations in Glyphosate Treated Soybean

Crop Systems and Global Change Laboratory, USDA, Agricultural Research Service, Building 1, Room 323, 10300 Baltimore Avenue, Beltsville, MD 20705

^* Corresponding author (lziska{at}asrr.arsusda.gov)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Although rising atmospheric carbon dioxide (CO₂) is known to stimulate the growth of agronomic weeds, the impact of increasing CO₂ on herbicide efficacy has not been elucidated for field-grown crops. Genetically modified soybean [Glycine max (L.) Merr.] (i.e., Round-up Ready soybean) was grown over a 2-yr period at ambient and projected levels of atmospheric carbon dioxide (CO₂, 250 µmol mol^–1 above ambient), with and without application of the herbicide, glyphosate [N-(phosphonomethyl)glycine], to assess the impact of rising atmospheric carbon dioxide concentration [CO₂] on chemical efficacy of weed control. For both years, soybean showed a significant vegetative response to elevated [CO₂], but no consistent effect on seed yield. For 2003, weed populations for all treatments consisted entirely of C₄ grasses, with no [CO₂] effects on weed biomass (unsprayed plots) or glyphosate efficacy (sprayed plots). However, in 2004, weed populations were mixed and included C₃ and C₄ broadleaves as well as C₄ grasses. In this same year, a significant increase in both C₃ broadleaf populations and total weed biomass was observed as a function of [CO₂] (unsprayed plots). In addition, a [CO₂] by glyphosate interaction was observed with significant C₃ broadleaf weed biomass remaining after glyphosate application. Overall, these data emphasize the potential consequences for CO₂–induced changes in weed populations, biomass, and subsequent glyphosate efficacy in Round-up Ready soybean.

Abbreviations: AHI, apparent harvest index • ai, active ingredient • DAS, days after sowing

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
ATMOSPHERIC CARBON DIOXIDE concentration has shown an increase of about 21% from 315 to 379 µmol mol^–1 since the late 1950s (cdiac.ornl.gov/trends/co2/sio-mlo.htm; verified 10 February 2006). Although the rate of increase is variable, levels are projected to exceed 600 µL L^–1 by the end of the 21st century (Houghton et al., 2001).

Overall, current and projected increases in global atmospheric [CO₂] are likely to change the biology of agricultural weeds in two fundamental ways. The first is related to climate stability. Evaluations by the Intergovernmental Panel on Climate Change (IPCC) based, in part, on an assessment by the U.S. National Academy of Sciences has indicated that the rise of [CO₂] and associated "greenhouse" gases could lead to a 1.4 to 5.8°C increase in global surface temperatures, with subsequent consequences on precipitation frequency and amounts (IPCC, 2001). Temperature and water availability remain key factors in determining weed species growth and success (Patterson and Flint, 1990; Patterson, 1995a). The second likely impact is the [CO₂] "fertilization" effect. That is, plants evolved at a time when the atmospheric [CO₂] appears to have been four or five times present values (Bowes, 1996). Because CO₂ remains the sole source of carbon for plant photosynthesis, and because at present, [CO₂] is less than optimal, as atmospheric [CO₂] increases, photosynthesis and growth will be stimulated accordingly. Although, in general, the relative effect of increasing [CO₂] is greater for C₃ than C₄ species, species-specific responses demonstrate a wide range of relative enhancement within C₃ and C₄ weeds (Patterson and Flint, 1980).

Weed management efforts, in turn, will be altered both by climatic uncertainty and rising carbon dioxide levels (Ziska, 2004). To date, our understanding of how rising CO₂ affects chemical weed management has focused exclusively on reductions in efficacy for individual weeds or monocultures (Ziska et al., 1999, 2004). Data on how elevated CO₂ could alter weed populations (and subsequent chemical control) are not available in genetically modified crops, that is, crops designed to be treated with herbicides. Our specific objective in the current study, therefore, was to quantify changes in weed populations and potential changes in chemical control efficacy as a function of [CO₂] for Round-up Ready (Monsanto Corp., St. Louis, MO) soybean grown with and without application of commercially formulated glyphosate.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Experimental Treatments
The experiment was conducted over a 2-yr period at a 0.3-ha plot at Beltsville, MD. Field soil was classified as a Cordurus silt-loam (Cordurus harboro), pH 5.5 with high availability of potash, phosphate, and nitrate. Twelve experimental aluminum chambers (3 m in diameter and 2.25 m in height) covering an area of 7.2 m² were placed at regular intervals within the field. Because of the chamber size, a modified suspended chamber top was necessary to prevent wind intrusion and to maintain a stable CO₂ concentration. For each year of the study, individual chambers were assigned one of two CO₂ treatments, either ambient or ambient +250 µmol mol^–1 CO₂; and one of two herbicide treatments, either sprayed at manufacturers recommended dosage, or unsprayed. CO₂ treatments were maintained 24 h d^–1 from germination until maturity. Air was supplied through perforations in the inner wall of the lower half of the chamber. Air was adjusted to the desired [CO₂] with pure CO₂ supplied from a 5 Mg liquid CO₂ tank. Gas samples were withdrawn from all elevated and one ambient CO₂ chamber at 4-min intervals at canopy height and adjustments to the elevated [CO₂] were made daily. Carbon dioxide concentration, determined by an absolute CO₂ analyzer (Li-Cor 6252, Li-Cor Corp., Lincoln, NE USA), indicated average daytime [CO₂] (0600–1900 h) values of 401 ± 21, 384 ± 14, and average nighttime values of 542 ± 35, 527 ± 26 µmol mol^–1 for the ambient CO₂ treatment in 2003 and 2004, respectively, and corresponding CO₂ values of 624 ± 18, 631 ± 23 (daytime) and 745 ± 32, 753 ± 38 µmol mol^–1 (night-time) for the elevated CO₂ treatment over the same time period.

Growth Conditions
Integrated day-time micrometeorological conditions of photosynthetic photon flux indicated that the chamber transmitted 90% of all incoming light, with an average daytime temperature increase of 0.8 and 1.3°C above the outside ambient temperature for 2003 and 2004, respectively. Overall, average temperatures during the growing season were 1.1°C below and 0.4°C above the 100-yr average for Maryland in 2003 and 2004, respectively. Precipitation values for this same period were 833 and 543 mm. The 2003 year was the wettest in Maryland since the onset of record keeping in 1895.

Before planting and chamber placement, the top 20 cm of soil was removed over the experimental field, bulked, and mixed for each year of the experiment. Subsamples placed in flats indicated uniform mixing, as determined by germination, and the subsequent presence of approximately 25 different annual and perennial weeds. Following chamber placement, soybean ‘Ascro’ (Ag3002, ‘Round-up Ready’, Maturity Group III, determinate) was planted within the chambers and in border rows surrounding the chambers on 27 June and 14 May for 2003 and 2004, respectively. The later planting date in 2003 was necessitated by excessive moisture during May and early June (i.e., the presence of standing water in the field plots during this period). Row widths were 30 cm with all plants thinned to 1 plant per 10 cm of row following emergence.

Timing of glyphosate application coincided with the period just before canopy closure of soybean rows (as per the recommendations of the Maryland Cooperative Extension Service). Glyphosate was applied as a isopropylamine salt with standard surfactant. Spraying occurred approximately 54 and 48 d after sowing (DAS) for 2003 and 2004, respectively, for half of the experimental chambers (i.e., three ambient and three elevated). A pressurized backpack sprayer was used to apply 2.24 kg ai ha^–1 to each of the treated plots. The other six plots received water only. No effort was made to control weeds on the water sprayed plots.

Vegetative and Reproductive Measurements
Soybean was considered mature when >95% of the leaves had senesced and pods were noticeably brown. Because of differential planting dates (because of the high precipitation in 2003), maturity occurred by late October and late September for 2003 and 2004, respectively. At maturity, four center rows from each chamber (i.e., excluding border rows) were cut at the base of the plant and harvested. At harvest, individual pods were counted and separated by treatment. Pods were air-dried and aboveground shoot dry matter (i.e., stems, petioles, peduncles) was oven-dried at 65°C for 72 h or until a constant dry weight was observed, then weighed. Pods were threshed by hand and seed collected and weighed. A subsample of 50 seeds was used to determine individual seed mass and to estimate seeds per pod. Because of leaf senescence and drop, harvest index was calculated as the ratio of seed mass to the sum of stem plus pod mass at maturity. This is typically done for commercial soybean and is referred to as the apparent harvest index (AHI) (Schapaugh and Wilcox, 1980).

Weed species were identified by chamber just before application of either water or glyphosate and again at soybean harvest. At soybean harvest, weeds within the harvested rows were cut at their bases, sorted into three general categories: C₃ broadleaf, C₄ broadleaf, or C₄ grass (no C₃ grasses were observed), dried, and weighed. No new species were observed between glyphosate application (i.e., canopy closure) and harvest. C₃ broadleaves were composed almost entirely (>95%) of three species; lambsquarters (Chenopodium album L.), velvetleaf (Abutilon theophrasti Medik.) and Virginia copperleaf (Acalypha virginica L.); C₄ broadleaves were exclusively redroot pigweed (Amaranthus retroflexus L.) and C₄ grasses consisted of barnyard grass [Echinochloa crus-galli (L.) P. Beauv.], Bermuda grass [Digitaria sanguinalis (L.) Scop.] and foxtail (Setaria spp.).

Statistical Analysis
The experiment was arranged in a completely randomized block at the field site with three replications of [CO₂] with and without glyphosate application (three replications x two [CO₂] x two glyphosate treatments). Vegetative and reproductive characteristics were determined for each year of the experiment by a two-way ANOVA with [CO₂] and glyphosate as the classification variables (Statview, Cary, NC, USA). Treatment comparisons were made using a Fisher protected least significant difference. Unless otherwise mentioned, differences for any measured parameter were determined as being significant at the P < = 0.05 level.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Increasing the [CO₂] by 250 µmol mol^–1 resulted in consistent increases in above-ground vegetative biomass, particularly stem weight, for soybean for both seasons (Table 1). However, the effect of elevated [CO₂] on seed yield (with glyphosate application) was inconsistent. Although individual seed weight tended to increase with [CO₂], the effect of [CO₂] was only observed for seed yield in 2004, primarily as a result of increased pod number (Table 1). Overall, the impact of elevated [CO₂] was greater on stem weight than seed yield, with a subsequent reduction in AHI (significant in 2004) (Table 1).

View larger version (17K): [in this window] [in a new window]   Fig. 1. Quantification of above ground weed biomass in three general categories, (C3 broadleaf, C4 broadleaf and C4 grass) when grown at ambient and elevated (+250 µmol mol–1) [CO2] in Round-up Ready soybean without glyphosate application in 2004. Variation for a given weed category was tested by a one-way ANOVA, with three replicates. *, P < 0.05; **, P < 0.01; ***, P < 0.001.

View larger version (16K): [in this window] [in a new window]   Fig. 2. Soybean seed yield (g m–2) as a function of increasing weed biomass at either ambient (solid line) or elevated (ambient +250 µmol mol–1, dashed line) CO2 for 2003 and 2004. No differences in the slope of the regression as a function of [CO2] were observed (ANCOVA).

View larger version (13K): [in this window] [in a new window]   Fig. 3. Quantification of above ground weed biomass in three general categories, (C3 broadleaf, C4 broadleaf and C4 grass) when grown at ambient and elevated (+250 µmol mol–1) [CO2] in Round-up Ready soybean, but after application of recommended amounts of glyphosate (Round-up). Note that significant amounts of C3 broadleaf weeds were still present at the elevated CO2 treatment after glyphosate application. Variation for a given weed category was tested by a one-way ANOVA, with three replicates. *, P < 0.05; **, P < 0.01; ***, P < 0.001.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

If glyphosate (Round-up) is not applied, how does increasing [CO₂] alter the growth of weed populations within the soybean canopy? Given that weed seeds were uniformly distributed within the seedbank before the start of each field season, the impact of CO₂ on weed populations may be dependent on those environmental factors that altered the establishment of C₃ and C₄ weeds. One such factor may be precipitation, which is generally recognized as a significant environmental factor in weed establishment, i.e., higher precipitation favors anoxic conditions and the establishment of shallow rooted grasses or adapted species (Patterson, 1995b). This is consistent with observations from the first year of the current study: specifically, that high precipitation rates (the highest recorded in Maryland since 1895), resulted in the sole recruitment of C₄ grasses. In this circumstance, elevated [CO₂] had no effect on weed biomass at maturity. However, for near normal precipitation in the following year, a range of weed species, including C₄ broadleaf and grasses, and C₃ broadleaves, was observed. In this year, a significant effect of elevated [CO₂] on total weed biomass was noted, due primarily to an approximate 5x increase in the amount of C₃ broadleaf biomass relative to ambient conditions.

How do differences in [CO₂] alter weed–crop competition and crop losses? The decrease in seed yield per increase in weed biomass (i.e., the slopes in Fig. 2) did not vary between years or as a function of [CO₂], suggesting that reductions in soybean productivity were not significantly altered by weed species per se. This has been observed previously for field grown soybean in competition with a C₃ and C₄ weed population (Ziska, 2000).

But is weed–crop competition even likely if weeds are controlled chemically? Commercially, one of the advantages of using a genetically modified crop such as Round-up Ready soybean is the nonselective application of herbicides for weed control. Previous experimental data have indicated that the effectiveness of glyphosate could be reduced for individual C₃ weeds at elevated [CO₂] under glasshouse conditions (Ziska et al., 1999; Ziska and Teasdale, 2000); however, it was uncertain if similar results would be obtained at commercial application rates in situ.

In the current field study, the overall efficacy of glyphosate application in response to elevated [CO₂] was reduced in 2004. Could greater growth of soybean in response to elevated [CO₂] be reducing spray coverage of glyphosate? This seems unlikely since elevated [CO₂] resulted in greater vegetative biomass in both 2003 and 2004. Alternatively, previous research on individual plants suggested that reduced glyphosate efficacy at elevated [CO₂] was associated primarily with C₃ weeds (Ziska et al., 1999). Such a finding is consistent with the reduction in efficacy observed concurrently with the stimulation of C₃ weeds in field grown soybean for 2004.

If reduced chemical efficacy in response to rising [CO₂] is a function of the relative proportion of C₃ vs. C₄ weeds, then the current study also suggests that those environmental factors that influence the ratio of C₃:C₄ species could play a role in [CO₂] response and subsequent chemical efficacy. For example, if high precipitation results in anoxic conditions and greater grass formation (with an over-representation of the C₄ pathway), the effect of [CO₂] on plant growth could be minimal and glyphosate efficiency unimpaired. Alternatively, if high soil nitrogen increases the population of C₃ relative to C₄ species, then the impact of [CO₂] may be considerable, with subsequent reductions in chemical efficacy.

The mechanistic basis for the reduction in glyphosate efficacy at elevated [CO₂] for C₃ species has not been entirely explained. Previous work with lambsquarters (C₃ broadleaf) suggested that CO₂ induced increases in biomass, while a factor, did not entirely account for the reduction in chemical efficacy (Ziska et al., 1999). Recent work with Canada thistle [C₃ broadleaf, Cirsium arvense (L.) Scop.] grown in monoculture under field conditions indicated that in addition to growth stimulation, a greater root to shoot ratio and subsequent below-ground dilution of glyphosate increased glyphosate tolerance at elevated relative to ambient [CO₂] (Ziska et al., 2004). In any case, differences in plant size, absorbance characteristics, or dilution effects were not determined in the current experiment in part because of concerns regarding sampling of weeds within the soybean canopy and physical disturbance effects on soybean seed yield.

But even if increasing [CO₂] alters glyphosate efficacy, is there cause for concern? It could be argued that chemical management could adapt to any CO₂ induced changes in weed control. For example, glyphosate could be applied earlier in the season, or alternatively, herbicide concentration or spraying frequency could be increased. However, if glyphosate application is too early (i.e., before canopy closure), then weed regrowth could occur; similarly, changes in the frequency of application or concentration of glyphosate, while increasing weed control, would also increase economic and/or environmental costs. From an economic perspective, it may be worth noting that any profits associated with greater seed yield at elevated [CO₂] could, potentially, be offset by additional costs of weed control.

While the response of agronomic species to rising atmospheric [CO₂] has been confirmed in literally hundreds of studies (e.g., Kimball, 1983), it is becoming increasingly evident that [CO₂] will also benefit agronomic and invasive weeds as well (Ziska and George, 2004). The argument that rising atmospheric [CO₂] will reduce weedy competition because the C₄ photosynthetic pathway is over represented among weed species (e.g., Holm et al., 1977) does not consider the range of available C₃ and C₄ weed species present within the agronomic seed bank, nor those environmental factors (e.g., precipitation) that may influence their relative proportion following emergence. Overall, the data presented here suggest that, depending on weed species (C₃ vs. C₄), elevated [CO₂] can increase weed biomass, decrease yields, and reduce glyphosate efficacy for Round-up Ready soybean.

	ACKNOWLEDGMENTS

 
The Authors thank Dr. Joel Swerdlow and Dr. Jeff Baker for useful comments and additions to the manuscript, and to Ms. Daniel Reed and Dr. Kate George for technical assistance.

Received for publication October 18, 2005.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

• Ainsworth, E.A., P.A. Davey, C.J. Bernacchi, O.C. Dermody, E.A. Heaton, D.J. Moore, P.B. Morgan, et al. 2002. A meta-analysis of elevated [CO₂] Effects on soybean (Glycine max) physiology, growth and yield. Glob. Change Biol. 8:695–709.[CrossRef]
• Baker, J.T., L.H. Allen, Jr., K.J. Boote, P. Jones, and J.W. Jones. 1989. Response of soybean to air temperature and carbon dioxide concentration. Crop Sci. 29:98–105.[Abstract/Free Full Text]
• Bowes, G. 1996. Photosynthetic responses to changing atmospheric carbon dioxide concentration. p. 387–407 In N.R. Baker (ed.) Photosynthesis and the environment. Kluwer Publishing, the Netherlands.
• Holm, L.G., D.L. Plucknett, J.V. Pancho, and J.P. Herberger. 1977. The Worlds Worst Weeds. Distribution and Biology. University of Hawaii Press, Honolulu.
• Houghton, J.T., Y. Ding, D.J. Griggs, M. Noguer, P.J. Van der Linden, and D. Xiaosu (ed.). 2001. Climate change 2001: The scientific basis. Cambridge Univ. Press, Cambridge, UK.
• Intergovernmental Panel on Climate Change Third Assessment Report WG I. 2001. In J.T. Houghton et al. (ed) Climate change 2001: The scientific basis. Cambridge Univ. Press, Cambridge, UK.
• Kimball, B.A. 1983. Carbon dioxide and agricultural yield: An assemblage and analysis of 430 prior observations. Agron. J. 75:779–788.[Abstract/Free Full Text]
• Patterson, D.T. 1995a. Weeds in a changing climate. Weed Sci. 43:685–701.
• Patterson, D.T. 1995b. Effects of environmental stress on weed/crop interactions. Weed Sci. 43:483–490.
• Patterson, D.T., and E.P. Flint. 1980. Potential effects of global atmospheric CO₂ enrichment on the growth and competitiveness of C₃ and C₄ weed and crop plants. Weed Sci. 28:71–75.
• Patterson, D.T., and E.P. Flint. 1990. Implications of increasing carbon dioxide and climate change for plant communities and competition in natural and managed ecosystems. 83–110. In B.A. Kimball (ed.) Impact of carbon dioxide, trace gases and climate change on global agriculture. ASA Spec. Publ. 53, ASA, Madison, WI.
• Schapaugh, W.T., Jr., and J.R. Wilcox. 1980. Relationships between harvest indices and other plant characteristics in soybeans. Crop Sci. 20:529–533.[Abstract/Free Full Text]
• Ziska, L.H. 2000. The impact of elevated CO₂ on yield loss from a C₃ and C₄ weed in field- grown soybean. Glob. Change Biol. 6:899–905.[CrossRef]
• Ziska, L.H. 2004. Rising carbon dioxide and weed ecology. p. 159–176 In Inderjit (ed.) Weed biology and management. Kluwer Publishing, the Netherlands.
• Ziska, L.H., and J.R. Teasdale. 2000. Sustained growth and increased tolerance to glyphosate observed in a C₃ perennial weed, quackgrass (Elytrigia repens), grown at elevated carbon dioxide. J. Plant Physiol. 27:159–166.
• Ziska, L.H., and K. George. 2004. Rising carbon dioxide and invasive, noxious plants: Potential threats and consequences. World Resour. Rev. 16:427–447.
• Ziska, L.H., J.R. Teasdale, and J.A. Bunce. 1999. Future atmospheric carbon dioxide concentrations may increase tolerance to glyphosate [N-(phosphonomethyl) glycine] in weedy species. Weed Sci. 47:608–615.
• Ziska, L.H., J.A. Bunce, and F.A. Caulfield. 2001. Rising atmospheric carbon dioxide and seed yield of soybean genotypes. Crop Sci. 41:385–391.[Abstract/Free Full Text]
• Ziska, L.H., S.S. Faulkner, and J. Lydon. 2004. Changes in biomass and root:shoot ratio of field-grown Canada thistle (Cirsium arvense) with elevated CO₂: Implications for control with glyphosate. Weed Sci. 52:584–588.[CrossRef]

This article has been cited by other articles:

				L. H. Ziska and A. McClung Differential Response of Cultivated and Weedy (Red) Rice to Recent and Projected Increases in Atmospheric Carbon Dioxide Agron. J., August 11, 2008; 100(5): 1259 - 1263. [Abstract] [Full Text] [PDF]

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (2) Citing Articles via Google Scholar Google Scholar Articles by Ziska, L. H. Articles by Goins, E. W. Search for Related Content PubMed Articles by Ziska, L. H. Articles by Goins, E. W. Agricola Articles by Ziska, L. H. Articles by Goins, E. W. Related Collections Agroclimatology Hillslope Analysis Ecological Risk Assessment