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© 2007 Plant Management Network. Influence of Glyphosate Tank-mix Combinations and Application Timings on Weed Control and Yield in Glyphosate-resistant Soybean Kevin W. Bradley, Assistant Professor, Nicholas H. Monnig, Graduate Research Assistant, Travis R. Legleiter, Graduate Research Assistant, and Jimmy D. Wait, Research Associate, Division of Plant Sciences, University of Missouri, Columbia 65211 Corresponding author: Kevin W. Bradley. bradleyke@missouri.edu Bradley, K. W., Monnig, N. H., Legleiter, T. R., and Wait, J. D. 2007. Influence of glyphosate tank-mix combinations and application timings on weed control and yield in glyphosate-resistant soybean. Online. Crop Management doi:10.1094/CM-2007-0419-01-RS. Abstract Field trials were conducted at two locations in Missouri during 2004 and 2005 to determine the effects of glyphosate and glyphosate tank-mix combinations on late-season weed control and yield in glyphosate-resistant soybean. Glyphosate was applied alone at 1.1 kg/ha or in combination with various tank-mixtures to weeds that averaged 10 cm in height (early timing) or 20 cm in height (late timing). Other than morningglory species, few differences in visual weed control or weed density at harvest were observed with glyphosate tank-mix combinations compared to applications of glyphosate alone, regardless of the application timing. However, when compared to the standard rate of glyphosate alone, lower morningglory density was observed at harvest with early applications of glyphosate at 1.1 kg/ha plus imazamox at 0.02 kg/ha or plus cloransulam at 0.009 and 0.018 kg/ha. Soybean yield losses were also incurred with several treatments as a result of applying herbicides at the later compared to the earlier timing. Introduction A decade after the introduction of glyphosate-resistant soybean, these varieties now comprise approximately 89% of the total soybean hectareage in the United States (13). This illustrates the widespread acceptance of glyphosate-resistant soybean by farmers throughout the United States. Although there are many benefits associated with this technology, many have observed shifts toward weeds that are difficult to control with glyphosate. For example, in a survey conducted across eleven states, all weed scientists surveyed felt that weed shifts were occurring in glyphosate-resistant soybean, and that morningglory species, common waterhemp, and common lambsquarters were some of the species that were becoming most problematic (2). Additionally, a number of glyphosate-resistant weed biotypes such as common ragweed, palmer amaranth, and common waterhemp have been identified in recent years, primarily in continuous glyphosate-resistant cropping systems like cotton or soybeans where glyphosate is used as the only active ingredient (8). One of the most effective ways to manage weed shifts or glyphosate-resistant weed biotypes is to apply an herbicide other than glyphosate that acts at an alternative site of action. Due to the broad-spectrum activity and economic feasibility of glyphosate, it seems likely that most growers will apply an alternative herbicide tank-mixed with glyphosate rather than abandoning the glyphosate-resistant soybean system altogether. In fact, Johnson and Gibson (9) found that 90% of the growers surveyed in Indiana were willing to use tank mixtures with glyphosate as a resistance management strategy, and that some larger-scale growers were already doing so. Although it seems that many growers are willing to apply a glyphosate tank mixture, conflicting information exists as to the utility of glyphosate tank-mix combinations. For example, Ellis and Griffin (5) observed no differences in pitted morningglory control with fomesafen plus glyphosate tank-mix combinations compared to glyphosate alone while Norris et al. (11) reported antagonistic effects from this same combination and Shaw and Arnold (12) observed a synergistic response. Other researchers have also found that simply increasing the rate of glyphosate will provide as good or better morningglory control than certain tank-mix combinations (3), and that the ultimate utility of a tank-mix combination might be determined by the size of the weeds at the time of the application (5). The objectives of these experiments were to determine the effects of glyphosate and glyphosate tank-mix combinations on late-season weed control and yield in glyphosate-resistant soybean when applied at two application timings. Measuring the Effect of Glyphosate Tank-mix Combinations and Application Timing Separate field experiments were conducted in central Missouri at the Bradford Research and Extension center and in northeast Missouri at the Greenley Research Center during the 2004 and 2005 growing seasons resulting in a total of four site-year combinations. The soil type at both locations in both years was a Mexico silt loam (fine, smectic, mesic Aeric Vertic Epiaqualfs). In 2004, the soil at the central location contained 2.4% organic matter and had a pH of 6.3. In 2005 the central location had a pH of 6.4 with 3.0% organic matter. At the northeast location the organic matter content was 2.3% and the pH was 6.0 in 2004. In 2005, the site had a pH of 5.9 with 3.3% organic matter. At all locations, Dekalb 3852 Roundup Ready soybean were planted into a conventionally tilled seedbed in rows spaced 76 cm apart at a seeding rate of 395,000 seeds/ha. Fertilizer applications were made according to soil test recommendations provided by the University of Missouri Soil and Plant Testing Laboratory. Dates of planting and other major field operations for each location are presented in Table 1 while the average monthly precipitation and temperature at each experimental location is presented in Table 2. Table 1. Dates of major field operations at the central and northeast Missouri research sites during the 2004 and 2005 growing seasons.
Table 2. Average monthly temperature and total monthly precipitation at each experimental location during the 2004 and 2005 growing seasons.
All herbicide treatments were applied at a constant speed of 5 km/h with a hand-held CO2-pressurized research backpack sprayer containing 8002 flat fan nozzle tips (Spraying Systems Co., Wheaton, IL) that delivered 140 liter/ha. The treatments evaluated included glyphosate alone at 1.1 kg ai/ha or in combination with acifluorfen at 0.15 and 0.21 kg/ha, thifensulfuron at 0.09 kg/ha, imazamox at 0.016 and 0.02 kg/ha, bentazon at 0.56 kg/ha, lactofen at 0.1 kg/ha, cloransulam at 0.009 and 0.018 kg/ha, chlorimuron at 0.004 kg/ha, fomesafen at 0.1 kg/ha, and flumetsulam at 0.007 kg/ha. Additionally, glyphosate was applied at a slightly higher rate of 1.2 kg ai/ha for comparison. All treatments were applied at two distinct application timings: when the average size of weeds present was approximately 10 cm in height, and when the average size of weeds present was approximately 20 cm in height. The specific sizes and density of weeds at the time of each application are recorded in Tables 3 and 4. At the 10-cm application timing, soybeans were in the V3 to V4 stage of growth and ranged from 13 to 20 cm in height regardless of location or year. At the 20-cm application timing, soybean were in the V6 to V7 stage of growth and ranged from 25 to 40 cm in height. Table 3. Average height of weeds at the early and late application timing at each experimental location.
x Weed abbrebiations: AMATA = common waterhemp; AMBEL = common ragweed; CHEAL = common lambsquarters; IPOSP = morningglory species; POLPY = Pennsylvania smartweed; SETSP = foxtail species; SIDSP = prickly sida; XANST = common cocklebur. Table 4. Average density of weeds at the early and late application timing at each experimental location.
x Weed abbrebiations: AMATA = common waterhemp; AMBEL = common ragweed; CHEAL = common lambsquarters; IPOSP = morningglory species; POLPY = Pennsylvania smartweed; SETSP = foxtail species; SIDSP = prickly sida; XANST = common cocklebur. Treatments were arranged factorially in a randomized complete block design and were replicated four times. Individual plots were 3 by 12 m in size. Visual weed control and soybean injury ratings were taken at regular intervals throughout the growing season. Visual ratings were based on a scale of 0 to 100, with 0 equal to the vigor and weed ground cover observed in the untreated control plots or no soybean injury and 100 equal to complete weed control or complete soybean death. Just prior to harvest at each location, counts of all weed species within the center 18-m² area of each plot were recorded. The predominant morningglory species at all locations was ivyleaf morningglory, Ipomoea hederacea. However some pitted and tall morningglory (I. lacunosa and I. purpurea, respectively) also occurred sporadically, therefore all morningglory species were grouped into one category and recorded collectively. Similarly, the predominant foxtail species at all locations was giant foxtail, Setaria faberi, but some green and yellow foxtail (S. viridis and S. glauca, respectively) also occurred sporadically. Therefore, all foxtail species were also grouped into one category and recorded collectively. At each location, soybeans were harvested from the two center rows in each plot with a small plot combine and yields were adjusted to 13% moisture content. All data were analyzed using the Proc Mixed procedure in SAS (SAS Institute Inc., Cary, NC). As suggested by Carmer et al. (1), each year-location combination was considered an environment sampled at random. Fixed effects in the model were herbicide treatment and application timing. Random effects included environment, replications (nested within environments), and all interactions within environment and replications. Considering environments at random enables inferences about the treatments to be made over a range of environments (1,6,7). Individual treatment differences were detected by using Fishers protected LSD at P < 0.05. Nontransformed means are presented because transformations did not alter the data interpretation. Visual Weed Control Ratings Herbicide application timing did not interact with treatments to affect weed control. Therefore, visual control ratings for all weed species were averaged across timings to display significant differences in treatments. All treatments evaluated in these trials resulted in less than 5% visual soybean injury when evaluated 30 days after the last herbicide application (data not shown). All treatments evaluated in these trials also provided good (> 80%) visual control of common waterhemp, common ragweed, common lambsquarters, morningglory species, Pennsylvania smartweed, foxtail species, prickly sida, and common cocklebur when evaluated 30 days after the last herbicide application (Table 5). No tank-mix combinations increased the visual control of any weeds except morningglories compared to standard applications of 1.1 kg glyphosate alone. For these species, the addition of acifluorfen at 0.21 kg/ha, fomesafen at 0.1 kg/ha, or the increased rate of glyphosate at 1.2 kg/ha provided better morningglory control than applying 1.1 kg glyphosate alone. Similarly, few tank-mixtures decreased visual weed control compared to glyphosate alone except 0.21 kg acifluorfen, which reduced prickly sida control compared to glyphosate alone, and 0.56 kg bentazon, which reduced common waterhemp, common ragweed, and common lambsquarters control compared to glyphosate alone. Weed Density at Harvest Herbicide application timing did not
interact with treatments to affect any of the weed density counts at harvest
except for the morningglory species. Therefore, densities of all species other
than morningglories were averaged across timings. For the morningglory species
density, there was a significant treatment by timing interaction, therefore
results are presented separately by timings. All treatments reduced the density
of common waterhemp, common ragweed, common lambsquarters, Pennsylvania
smartweed, foxtail species, and common cocklebur at harvest compared to the
untreated control (Table 6). However, the addition of a tank mixture to
glyphosate at 1.1 kg/ha neither increased nor decreased the density of common
waterhemp, common ragweed, common lambsquarters, Pennsylvania smartweed, foxtail
species, prickly sida, or common cocklebur remaining at harvest compared to
applications of glyphosate alone. Similarly, glyphosate at 1.2 kg/ha did not
reduce the density of these weeds at harvest compared to glyphosate at 1.1 kg/ha
alone. At harvest, most of the remaining weeds appeared to have survived the
initial herbicide applications, rather than emerging later in the season,
although late-season weed emergence in response to treatment was not measured in
these experiments. These results indicate that the tank combinations evaluated
in these trials are not likely to improve control of larger weeds or weeds that
are often difficult to control with glyphosate alone.
Many treatments applied at the later application timing did not reduce morningglory density at harvest compared to the untreated control (Table 7). Other researchers have also observed poor morningglory control with late compared to early applications (5). When applied at the early application timing, 1.2 kg glyphosate reduced morningglory density compared to the untreated control but the 1.1-kg/ha glyphosate treatment did not. All tank mixtures except acifluorfen at 0.21 kg/ha also reduced morningglory density compared to the untreated control when applied at the early application timing. Conversely, at the late application timing only acifluorfen at 0.21 kg/ha, lactofen at 0.1 kg/ha, cloransulam at 0.004 kg/ha, and flumetsulam at 0.007 kg/ha reduced morningglory density at harvest compared to the untreated control. Neither glyphosate rate reduced morningglory density compared to the untreated control when applied at the late application timing. For many of the treatments evaluated in these trials, the early application timing typically resulted in fewer morningglory plants at harvest compared to the same treatment applied at the later application timing. These responses are likely due to the larger size of the morningglory plants at the time of the late compared to the early application timing, as late-emerging seedlings were not observed in these trials. Table 7. Influence of glyphosate tank-mix combinations and application timings on morningglory species density at soybean harvest averaged across four environments.
x Plus signs indicate tank-mix combinations of the herbicide in question with glyphosate at 1.1 kg/ha. All treatments applied with 2.8 kg of ammonium sulfate per hectare. y Means followed by the same letter are not significantly different and can be used to make all treatment by timing comparisons. z IPOSP = morningglory species. Soybean Yields Herbicide application timing did interact with treatments to affect soybean yields, therefore yields are presented separately by timing for each treatment. All treatments increased soybean yields compared to the untreated control (Table 8). Yield losses were also incurred for several treatments as a result of applying herbicides at the later compared to the earlier timing. For example, soybean yields were significantly lower with late compared to early applications of glyphosate at 1.1 and 1.2 kg/ha, and with the glyphosate tank-mixes acifluorfen at 0.21 kg/ha or lactofen at 0.10 kg/ha. Other researchers have also observed similar reductions in yield as a result of late versus early applications of glyphosate in glyphosate-resistant soybean (4,10). Table 8. Influence of glyphosate tank-mix combinations and application timings on yield of glyphosate-resistant soybean averaged across four environments.
x Plus signs indicate tank-mix combinations of the herbicide in question with glyphosate at 1.1 kg/ha. All treatments applied with 2.8 kg of ammonium sulfate per hectare. y Means followed by the same letter are not significantly different and can Conclusions Collectively, these results suggest that the ultimate utility of a tank-mix combination is primarily determined by the specific weed species in question and the timing of application. Few differences in weed control were observed in these trials with glyphosate tank combinations compared to standard or higher rates of glyphosate alone, but the morningglory species responded differently from the other weeds evaluated. When compared to the standard rate of glyphosate alone, lower morningglory density was observed at harvest with early applications of glyphosate at 1.1 kg/ha plus imazamox at 0.02 kg/ha or cloransulam at 0.009 and 0.018 kg/ha. These results indicate that for some harder-to-control species like the morningglories, glyphosate tank-mix combinations can enhance control compared to applications of glyphosate alone as well as provide an additional mode of action to prevent weed shifts and/or the development of glyphosate-resistant weeds. Additionally, results from these experiments suggest that growers who wait to apply their herbicides to weeds that are 20 vs. 10 cm in height are likely to incur soybean yield reductions because of early season weed competition. Literature Cited 1. Carmer, S. G., Nyquist, W. E., and Walker, W. M. 1989. Least significant differences for combined analysis of experiments with two or three-factor treatment designs. Agron. J. 81:665-672. 2. Culpepper, A. S. 2006. Glyphosate-induced weed shifts. Weed Technol. 20:277-281. 3. Culpepper, A. S., Agustin, E., Gimenez, A. C., York, R. B., Batts, and Wilcut, J. W. 2001. Morningglory (Ipomoea spp.) and large crabgrass (Digitaria sanguinalis) control with glyphosate and 2,4-DB mixtures in glyphosate-resistant soybean (Glycine max). Weed Technol. 15:56-61. 4. Dewell, R. A., Johnson, W. G., Nelson, K. A., Li, J., and Wait, J. D. 2003. Weed removal timings in no-till, double-crop, glyphosate-resistant soybean grown on claypan soils. Online. Crop Management doi:10.1094/CM-2003-1205-01-RS. 5. Ellis, J. M., and Griffin, J. L. 2003. Glyphosate and broadleaf herbicide mixtures for soybean (Glycine max). Weed Technol. 17:21-27. 6. Hager, A. G., Wax, L. M., Bollero, G. A., and Stoller, E. W. 2003. Influence of diphenylether herbicide application rate and timing on common waterhemp (Amaranthus rudis) control in soybean (Glycine max). Weed Technol. 17:14-20. 7. Hasty, R. F., Sprague, C. L., and Hager, A. G. 2004. Weed control with fall and early-preplant herbicide applications in no-till soybean. Weed Technol. 18:887-892. 9. Johnson, W. G., and Gibson, K. D. 2006. Glyphosate-resistant weeds and resistance management strategies: an Indiana grower perspective. Weed Technol. 20:768-772. 10. Knezevic, S. Z., Evan, S. P., Mainz, M. 2003. Yield penalty due to delayed weed control in corn and soybean. Online. Crop Management doi: 10.1094/CM-2003-0219-01-RS. 11. Norris, J. L., Shaw, D. R., and Snipes, C. E. 2001. Weed control from herbicide combinations with three formulations of glyphosate. Weed Technol. 15:552-558. 12. Shaw, D. R., and Arnold, J. C. 2002. Weed control from herbicide combinations with glyphosate. Weed Technol. 16:1-6. |
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