Agronomy Journal 93:428-434 (2001)
© 2001 American Society of Agronomy
PRODUCTION PAPER
Soybean Growth and Development as Affected by Glyphosate and Postemergence Herbicide Tank Mixtures
Kelly A. Nelsona and
Karen A. Rennerb
a Dep. of Agron., Univ. of Missouri, Novelty, MO 63460
b Dep. of Crop and Soil Sci., Michigan State Univ., East Lansing, MI 48824-1325
Corresponding author (nelsonke{at}missouri.edu)
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ABSTRACT
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Field research was conducted to evaluate the effect of glyphosate [N-(phosphonomethyl)glycine] and postemergence herbicide tank mixtures applied to soybean [Glycine max (L.) Merr.] at the V5 growth stage on growth, development, and grain yield of glyphosate-resistant and nonresistant cultivars. Glyphosate at 840 g a.e. ha-1 did not affect vegetative growth, reproductive development, leaf area index (LAI), or grain yield of glyphosate-resistant soybean compared with nontreated glyphosate-resistant soybean. Soybean injury 21 d after treatment (DAT) was 14 to 18% when herbicide tank mixture treatments were applied. Soybean injury caused by postemergence herbicide tank mixtures resulted in delayed vegetative development when measured 7 DAT, delayed reproductive development at 20 and 80 DAT, reduced season-long height of `Asgrow A1900' and `Asgrow AG1901', and reduced aboveground dry weight at 35 and 56 DAT compared with nontreated plants. Leaf area index was reduced by postemergence tank mixtures up to 52 DAT but was greater 70 and 80 DAT, depending on the cultivar. The ratio of red to far red light that reached the soil surface 28 DAT was dependent on cultivar and herbicide selection. Yield of soybean treated with herbicides other than glyphosate timed for weed control and disease suppression was reduced at least 200 and 130 kg ha-1 in 1997 and 1998, respectively, in the absence of weeds and disease.
Abbreviations: COC, crop oil concentrate DAT, days after treatment LAI, leaf area index UAN, 28% urea ammonium nitrate
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INTRODUCTION
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TANK MIXTURES of postemergence herbicides were needed for broad spectrum weed control before the introduction of glyphosate-resistant soybean cultivars (Green, 1991; Hart and Roskamp, 1998; Monks et al., 1993). Research suggests that glyphosate applied to glyphosate-resistant soybean did not result in significant visual injury (Lich et al., 1997) and allowed rapid canopy closure (Nelson and Renner, 1999). In contrast, postemergence applications of thifensulfuron {3-[[[[(4-methoxy-6-methyl-1,3,4-triazin-2-yl)amino]carbonyl]amino]sulfonyl]-2-thiophenecarboxylic acid}, lactofen {(±)-2-ethoxy-1-methyl-2-oxoethyl 5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoate}, and acifluorfen {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid} often cause chlorosis, necrosis, or stunting of soybean (Hart and Roskamp, 1998; Kapusta et al., 1986; Wichert and Talbert, 1993). Injury from these herbicides may persist up to 21 DAT although yield loss generally does not occur (Kapusta et al., 1986). However, early season damage may delay canopy closure and allow weed reinfestation (Mickelson and Renner, 1997; Nelson and Renner, 1999).
A change in canopy development may also influence the development of white mold [Sclerotinia sclerotiorum (Lib.) deBary], a disease prevalent in the north-central region of the USA. White mold is common in narrow-row soybean culture (Grau and Radkey, 1984) and in crop rotations with alternate hosts (Schwartz et al., 1978). Research suggests that lactofen suppresses white mold in soybean (Dann et al., 1999) and affects soybean cyst nematode (Heterodera glycines Ichinohe) reproduction (Levene et al., 1998). Interactions among herbicide treatments, soybean canopy closure, and disease incidence may affect production practices and weed management strategies for soybean.
Soybean restricts light penetration to the soil surface by forming a dense leaf canopy near the upper portion of the crop. The upper 20% of the canopy accounts for only 30% of the total LAI even though it intercepts 90% of total photosynthetically active radiation and produces a majority of the plant's photosynthate (Hatfield and Carlson, 1978; Sakamoto and Shaw, 1967). Light quality may stimulate germination of weed seed and stem elongation of crop or weed plants. Singh et al. (1968) reported a sharp peak of absorption with approximately 12 to 24% intensity in the infrared radiation region at canopy closure. Phytochrome conversion from Pr (red) to Pfr (far red) is necessary for the germination of various weed species. For instance, a low level of red light (3 µmol m-2) was shown to stimulate the germination of buried redroot pigweed (Amaranthus retroflexus L.) (Gallagher and Cardina, 1998).
Soybean cultivars vary in competitiveness with weeds (Bussan et al., 1997). Interaction between a weed and crop species is complex and difficult to predict. Differences in cultivar photosynthetic rates (Johnston et al., 1969), leaf orientation (Blad and Baker, 1972), nodulation (Hunt et al., 1990), and morphology (Huang et al., 1993) have been reported and often result in differential light interception. These differences may alter the competitiveness of soybean with weeds.
Producers have expressed concerns regarding the susceptibility of genetically modified soybean to postemergence herbicides other than glyphosate. Various studies have reported differential sensitivity between soybean cultivars to herbicides; however, sensitivity was not linked to additional herbicide resistance in the cultivar (Connelly et al., 1988; Dayan et al., 1997; Griffin and Habetz, 1989). Other research has evaluated the application of acifluorfen and bentazon [3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] at the V3 (Levene et al., 1998) and V6 (Browde et al., 1994) stage of development and the interaction with nematodes. However, published research evaluating soybean canopy influence as affected by herbicide treatment for weed control and white mold suppression is not available.
The objective of this research was to evaluate effects of glyphosate and postemergence herbicide tank mixtures, timed for weed control and white mold suppression, on vegetative and reproductive development of soybean and on yield of glyphosate-resistant and nonresistant cultivars in a weed- and disease-free environment.
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MATERIALS AND METHODS
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Field research was conducted in 1997 and 1998 at the Bean and Beet Research Farm near Saginaw, MI (43° N, 83° W). The field was fall-plowed and spring field-cultivated in 1997. Soil was a Misteguay clay (Aeric Haplaquept, fine, mixed, mesic) with 3.7% organic matter and a pH of 7.9. In 1998, the field was fall-plowed and spring field-cultivated twice. Soil was a Misteguay silty clay with 1.9% organic matter and a pH of 8.0. The study was arranged in a split-plot design with four replications consisting of soybean cultivars as main plots and herbicide treatments as subplots. On 23 May 1997 and 12 May 1998, `Asgrow A1900', `Asgrow AG1901' (Roundup Ready), `Asgrow AG2701' (Roundup Ready), and `Asgrow A2704' (sulfonylurea-tolerant) soybean were planted with toolbar-mounted International 185 (Case Int., Racine, WI) planter units in 38-cm rows at 350000 seeds ha-1 in plots that were 2.7 by 12.2 m. Soybean was kept weed free throughout the season by hand removal.
Herbicide treatments included a nontreated control; glyphosate (formulated as Roundup Ultra, Monsanto Co., St. Louis, MO) at 840 g a.e. ha-1 plus ammonium sulfate [(NH4)2SO4] at 20 g L-1; the commercial bentazonacifluorfen mixture (formulated as Galaxy, BASF, Research Triangle Park, NC) at 1030 g a.i. ha-1 plus thifensulfuron at 2.2 g a.i. ha-1, sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} at 240 g a.i. ha-1, 28% urea ammonium nitrate (UAN), and crop oil concentrate (COC) [Herbimax (paraffinic oil plus emulsifiers plus surfactants), Loveland Industries, Greeley, CO] at 1.5% (vol./vol.); and the mixture of lactofen at 105 g ha-1 plus bentazon at 1120 g ha-1, clethodim {(E,E)-(±)-2-[1-[[(3-chloro-2-propenyl)oxy]imino]propyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} at 140 g ha-1, UAN, and COC at 1.5% (vol./vol.). Herbicide treatments in this research are commonly used in this area of the USA for weed control. These treatments were selected due to no appreciable soil residual activity and to avoid rotational crop concerns. Herbicides were applied with a compressed-air plot sprayer mounted on tractor and equipped with 8003 flat-fan tips (Spraying Systems Co., Wheaton, IL) delivering 178 L ha-1 at 207 kPa and 6.3 km h-1. Soybean was 23 cm tall and at the V5 growth stage (Fehr and Caviness, 1977) at the time of application. Air temperature was 28 and 22°C, and relative humidity was 40 and 78% in 1997 and 1998, respectively.
Visual estimates of percent soybean injury were recorded 7, 21, and 28 DAT using a scale of 0 to 100% where 0 = no injury and 100 = plant death. Symptoms included leaf necrosis, chlorosis, and stunting. Five photosynthetically active radiation light measurements were recorded at approximately 1.5-m spacings in each plot with a 1-m SunScan Canopy Analysis System (Dynamax, Houston, TX) perpendicular to the soybean row. Measurements were recorded every 6 to 12 d from the time of herbicide application until maturity to estimate soybean LAI. Simultaneous measurements at the soil surface and above the canopy captured the transmitted, diffuse, and incident light, which was used in conjunction with the time of day, latitude, day of the year, and other parameters to calculate soybean LAI in a nondestructive manner. Incident and diffused light measurements have been utilized as an effective nondestructive method to measure soybean LAI (Walker et al., 1988). Three readings (Skye-Probetech, Perkasie, PA) of red/far red light ratio were measured at approximately 2.5-m spacings between soybean rows with a single photocell 14 and 28 DAT. All light measurements were recorded at the soil surface at approximately 1230 h. Vegetative stages at 3, 7, and 14 DAT and reproductive stages at 4- to 9-d intervals were recorded according to Fehr and Caviness (1977) for three randomly sampled plants in each plot. Aboveground dry weight was measured for 1 m of soybean row before herbicide application and then at 35, 56, and 77 DAT using the procedure described by Hunt et al. (1987). Soybean was harvested with a Massey 10 (Kincaid Equipment Manufacturing, Haven, KS) small-plot combine. Final soybean weight was adjusted to 13% moisture.
Data were subjected to analyses of variance with mean separation at P = 0.05 using Fisher's protected LSD. Subsamples were averaged and analyzed as one number before analysis of variance. Data were combined over years, and main effects were presented where interactions were not observed.
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RESULTS
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Glyphosate did not visually injure soybean (Table 1) or affect the LAI of glyphosate-resistant soybean (Fig. 1). Soybean treated with glyphosate had similar vegetative development, reproductive development, dry weight, height, and yield compared with the nontreated soybean; therefore, glyphosate data is not presented.
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Table 1. Soybean injury 21 d after treatment (DAT) and the ratio of red to far red light at the soil surface 14 and 28 DAT in 1997 and 1998.
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Fig. 1. Leaf area index (LAI) for each soybean cultivar and herbicide combined over years. Vertical lines indicate the LSD (P = 0.05). Treatments included nontreated soybean; glyphosate at 840 g ha-1 plus ammonium sulfate at 20 g L-1; bentazonacifluorfen at 1030 g ha-1 plus thifensulfuron at 2.2 g ha-1, sethoxydim at 240 g ha-1, UAN, and COC at 1.5% (vol./vol.); and lactofen at 105 g ha-1 plus bentazon at 1120 g ha-1, clethodim at 140 g ha-1, UAN, and COC at 1.5% (vol./vol.)
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Visual soybean injury averaged over cultivars was 17% from lactofen + bentazon + clethodim and 15% from bentazonacifluorfen + thifensulfuron + sethoxydim 7 DAT (data not presented). By 21 DAT, soybean injury ranged from 14 to 17% for bentazonacifluorfen + thifensulfuron + sethoxydim and from 16 to 18% for lactofen + bentazon + clethodim (Table 1). AG1901 and AG2701 injury from bentazonacifluorfen + thifensulfuron + sethoxydim and lactofen + bentazon + clethodim was greater than injury to A2704. This could be due to increased tolerance of A2704 to components of the postemergence herbicide treatments (Sebastian et al., 1989; Simpson and Stoller, 1996).
The ratio of red to far red light was greatest at the soil surface in the absence of a crop (glyphosate treatment of nonresistant cultivars A1900 and A2704) (Table 1). By 28 DAT, the ratio of red to far red light at the soil surface beneath the canopy of A1900, AG1901, and A2704 treated with bentazonacifluorfen + thifensulfuron + sethoxydim and A2704 treated with lactofen + bentazon + clethodim was similar to nontreated soybean. Similarly, other cultural practices like plant population can affect light quality below the crop canopy (Burkey and Wells, 1991). There was no difference between cultivars in the ratio of red to far red light at the soil surface 14 or 28 DAT in nontreated soybean. Bentazonacifluorfen + thifensulfuron + sethoxydim did not affect the ratio of red to far red light at the soil surface 14 and 28 DAT. However, application of lactofen + bentazon + clethodim to A1900, AG1901, and AG2701 may provide an environment that is favorable for the germination of weed seed because the ratio of red to far red light was greater in these treatments 28 DAT compared with the nontreated control.
Soybean response to herbicides, as measured by LAI, was influenced by cultivar (Fig. 1). Glyphosate did not affect soybean LAI compared with the nontreated control. At 40 and 52 DAT, there was no significant interaction between cultivar and herbicide treatment; however, LAI averaged over cultivar was greatest in the nontreated control compared with bentazonacifluorfen + thifensulfuron + sethoxydim and lactofen + bentazon + clethodim (data averaged over cultivar not presented). The LAI was lower when bentazonacifluorfen + thifensulfuron + sethoxydim and lactofen + bentazon + clethodim were applied and measurements were recorded 7 to 52 DAT. These data also suggest that soybean canopy development was delayed by these herbicide treatments for A1900 and A1901 when measured 70 DAT and for A1900, AG2701, and A2704 when measured 80 DAT. Soybean with delayed canopy development had later leaf abscission (data not presented). In previous research, light interception by soybean cultivars was affected by postemergence herbicides (Ralston and Witt, 1998). However, delayed soybean development with postemergence herbicides has not been reported. Rapid soybean canopy development is important for soybean to be competitive with weed species. Rapid canopy closure can reduce the reproductive potential of weed species through shading (Santos et al., 1997).
Height of AG2701 and A2704 was similar and greater than that of AG1901 or A1900 from 48 DAT until harvest in nontreated soybean (Fig. 2). Postemergence application of Bentazonacifluorfen + thifensulfuron + sethoxydim and lactofen + bentazon + clethodim to A1900 and AG1901 caused season-long stunting compared with nontreated soybean while AG2701 was stunted up to 35 DAT.

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Fig. 2. Height for each soybean cultivar and herbicide treatment combined over years. Vertical lines indicate the LSD (P = 0.05). Treatments included nontreated soybean; bentazonacifluorfen at 1030 g ha-1 plus thifensulfuron at 2.2 g ha-1, sethoxydim at 240 g ha-1, UAN, and COC at 1.5% (vol./vol.); and lactofen at 105 g ha-1 plus bentazon at 1120 g ha-1, clethodim at 140 g ha-1, UAN, and COC at 1.5% (vol./vol.)
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Bentazonacifluorfen + thifensulfuron + sethoxydim and lactofen + bentazon + clethodim reduced soybean dry weight compared with the nontreated control 35 and 56 DAT (Fig. 3). By 77 DAT, differences in dry weight were not apparent among treatments due to earlier senescence in the nontreated control plots. In other research, dry matter production was related to intercepted radiation (Sinclair and Horie, 1989; Shibles and Weber, 1966). A linear relationship between percent light interception and dry matter increase per day was positively correlated (Shibles and Weber, 1965). Canopy development and light interception may be affected by the morphological traits of a soybean cultivar; however, increased yield was not always reported (Huang et al., 1993; Wells et al., 1993).

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Fig. 3. Soybean aboveground dry weight for each herbicide treatment combined over cultivars and years. Vertical lines indicate the LSD (P = 0.05). Treatments included nontreated soybean; bentazonacifluorfen at 1030 g ha-1 plus thifensulfuron at 2.2 g ha-1, sethoxydim at 240 g ha-1, UAN, and COC at 1.5% (vol./vol.); and lactofen at 105 g ha-1 plus bentazon at 1120 g ha-1, clethodim at 140 g ha-1, UAN, and COC at 1.5% (vol./vol.)
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Bentazonacifluorfen + thifensulfuron + sethoxydim and lactofen + bentazon + clethodim delayed vegetative development 7 DAT (Table 2), but by 14 DAT, vegetative stage was not affected by herbicide treatment (data not shown). Reproductive development was delayed compared with nontreated soybean during the transition between full flower and early pod (20 DAT) and physiological maturity (80 DAT) when bentazonacifluorfen + thifensulfuron + sethoxydim or lactofen + bentazon + clethodim were applied (Table 2).
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Table 2. Soybean vegetative stage 7 d after treatment (DAT), reproductive stage 20 and 80 DAT, and yield in 1997 and 1998
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The yield of soybean cultivars differed each year. Bentazonacifluorfen + thifensulfuron + sethoxydim and lactofen + bentazon + clethodim reduced soybean yield from at least 200 and 130 kg ha-1 compared with the nontreated control in 1997 and 1998, respectively (Table 2). Soybean cultivar yield for A1900, A2704, and AG2701 was similar and greater than grain yield for AG1901 in 1997. However, yield for A2704 was greatest, followed by AG2701, A1900, and AG1901, respectively, in 1998. Rainfall in July and August totaled 241 mm in 1997 and 69 mm in 1998 and, similar to other research (Donald, 1998), may be the primary factor that caused lower soybean yield in 1998.
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DISCUSSION
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Factors that reduce or delay canopy development and light interception may also reduce soybean yield. Light interception is influenced by crop species, population (Shibles and Weber, 1966), fertility (Flénet and Kiniry, 1995), planting date (Board and Harville, 1992), maturity group (Board and Harville, 1992), growth stage (Luxmoore et al., 1971), insects (Board et al., 1997), row spacing, leaf orientation (Baker and Meyer, 1966), and leaf morphology (Wells et al., 1993). Soybean development stage affects the effect that some of these factors have on canopy development. Light interception before R5 was considered essential for soybean yield (Board and Harville, 1993). Controversy regarding light interception during R1 to R5 or R5 to R7 and the effect on yield has been argued. Increased soybean yield in narrow-row soybean has been related to increased light interception in early reproductive growth stages compared with wide-row cultures (Board and Harville, 1992; Shibles and Weber, 1965; Shibles and Weber, 1966). However, Egli (1994) reported that soybean yield did not rely on increased light interception in early reproduction stages. In our research, postemergence herbicides applied at the V5 stage of development, before flowering, reduced or delayed season-long canopy development and reduced yield. The degree of this effect may depend on the row spacing, plant population, environmental conditions at the time of application, or cultivar selection.
Soybean height and canopy development were reduced and soybean maturity was delayed when bentazonacifluorfen + thifensulfuron + sethoxydim or lactofen + bentazon + clethodim were applied. Shorter soybean could contribute to increased light penetration, thus resulting in a smaller LAI as indicated in this research. The reduction and delay in soybean development was more evident for early maturing soybean (A1900 and AG1901) than for late Group II cultivars (AG2701 and A2704). Later-maturing soybean had more time to recover from herbicide injury by producing greater LAI. However, delayed soybean development may increase the risk of yield loss due to frost when producers grow late Group II soybean in the northern latitudes. Altered canopy from herbicide injury also affected the ratio of red to far red light reaching the soil surface, which may affect weed seed germination. Altered canopy could also affect canopy microclimate and influence white mold development. Lactofen applied postemergence is currently labeled for white mold suppression in soybean (Anonymous, 1998). A change in canopy development, in addition to the physiological effects (Dann et al., 1999; Levene et al., 1998) as a result of postemergence herbicides, may help reduce incidence of white mold due to increased air movement in an irrigated or nonirrigated soybean culture.
Soybean treated with bentazonacifluorfen + thifensulfuron + sethoxydim and lactofen + bentazon + clethodim had more extensive branching compared with the nontreated control (personal visual observation). Soybean appeared to counteract herbicide injury and stunting by branching, but this did not completely compensate for injury because soybean yield was reduced. The effect of light on canopy development may be observed in the branching characteristics of the soybean plant. For example, soybean planted in wide-row spacings had more branches per plant, which resulted in increased light interception per plant (Shibles and Weber, 1966). The percent main and branched reproductive dry matter in optimal- and late-planted soybean depended on cultivar (Board et al., 1990). The lower portion of the soybean plant, below the dense cover of the soybean canopy, receives limited light. Shaded leaves may continue to photosynthesize when supplemental light is provided (Johnston et al., 1969). An open canopy could reduce the loss of abscised pods and flowers, which could help offset the effect of postemergence herbicides on yield. However, injury may be offset by the control of weeds or the lack thereof compared with glyphosate.
Soybean most often recovers from early season injury caused by current postemergence herbicides registered for use in soybean. Several soybean growth characteristics are altered by a late application of postemergence herbicides for rescue weed control or timing for weed control and white mold suppression. A reduction in yield may occur when tank mixture treatments of bentazonacifluorfen + thifensulfuron + sethoxydim or lactofen + bentazon + clethodim are applied at the V5 stage of development. However, the benefit of white mold suppression and reduced weed seed production may justify such treatments. Soybean that is not injured from herbicide applications has rapid canopy closure, which reduces light quality and quantity at the soil surface. This is important to reduce late-germinating weeds and potential weed seed production and to maximize soybean yield potential. Changes in the time of maximum leaf area and canopy development may affect the microclimate in the canopy and the potential for white mold infection and development. Future research should evaluate the effects of postemergence herbicides on canopy development and the incidence of white mold.
Received for publication January 19, 2000.
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