Agronomy Journal 92:1248-1255 (2000)
© 2000 American Society of Agronomy
CORN
Vegetation Control for No-Tillage Corn Planted into Warm-Season Perennial Species
Malcolm L. Broomea,
Glover B. Triplett, Jr.a and
Clarence E. Watson, Jr.b
a Plant and Soil Science Dep., Mississippi State Univ., Box 9555, Mississippi State, MS 39762 USA
b Experimental Statistics and Plant and Soil Science Dep., Mississippi State Univ., Box 9555, Mississippi State, MS 39762 USA
mbroome{at}pss.msstate.edu
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ABSTRACT
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The success of no-tillage corn (Zea mays L.) planted into sod in the Southeast depends on control of warm-season perennials. Preemergence (PRE) and postemergence (POST) herbicides were evaluated for control of warm-season perennial and annual species in no-tillage corn production. Vegetation spectrum differences at the various locations influenced both herbicide rate and timing of application required for 90% control by imazapyr {(±)-2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-pyridinecarboxylic acid}, imazethapyr {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1 H-imidazol-2-yl]-5-ethyl-3-pyridinecarboxylic acid}, glufosinate [2-amino-4-(hydroxymethylphosphinyl) butanoic acid], or imazapyr + imazethapyr. Imazapyr controlled established bermudagrass (Cynodon dactylon L.), broomsedge (Andropogon virginicus L.), johnsongrass [Sorghum halepense (L.) Pers.], tall fescue (Festuca arundinacea Schreb.), and dallisgrass (Paspalum dilatatum Poir.) in all instances. Several herbicides were used in selected combinations for PRE and/or POST applications in discrete rate trials. These were imazapyr, imazethapyr, glufosinate, imazapyr + imazethapyr, paraquat (1, 1'-dimethyl-4,4'-bipyridinium ion), atrazine [6-chloro-N-ethyl-N1-(1-methyethyl)-1,3,5-triazine-2,4-diamine], and glyphosate [N-(phosphonomethyl)glycine]. The glufosinate-resistant corn hybrid in 1997 at Holly Springs, MS, with glyphosate PRE at 0.84 kg a.i. ha-1 followed by glufosinate POST (28 DAP) at 0.45 kg ha-1 produced a maximum yield of 10.2 Mg ha-1. Results from this study indicate that vegetation can be controlled for corn production in untilled sod comprised of warm-season perennials when transgenic hybrids are grown.
Abbreviations: CRP, conservation reserve program • DAP, days after planting • fb, followed by • IR, imadazolinone resistant • POST, postemergence • PRE, preemergence • T, soil loss tolerance level
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INTRODUCTION
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RAINFALL frequency, amount, and intensity in the southeastern USA coupled with percent slope create a greater erosion hazard than in any other region in the continental USA (Wischmeier and Smith, 1978). In the 1980s, millions of hectares of highly erodible farmland was enrolled in the Conservation Reserve Program (CRP) and maintained in permanent vegetative cover. The first CRP contracts have matured and some of this land has been returned to crop production. High commodity prices could lead to another cycle of conversion of these highly erodible lands to annual crop production, as occurred in the 1970s. Research has shown that no-tillage planting offers opportunities for production of annual crops while maintaining yields and protecting soil resources (Bruce et al., 1995; Dabney et al., 1993; Dick et al., 1986; Triplett and Dabney, 1995). The question then becomes one of how to initiate no-tillage culture directly into sod. Such a system of crop culture must meet the needs of the crop for planting and stand establishment, for weed control, and for application of nutrients to ensure optimum productivity.
Poor weed control is often the limiting factor in the adoption of conservation tillage crop production systems (Gebhardt et al., 1985; Triplett, 1985; Koskinen and McWhorter, 1986). The reduction or elimination of tillage not only reduces weed control options but also alters interactions among crops, weeds, and herbicides (Triplett, 1985). Early no-tillage work in Virginia (Jones et al., 1968), Ohio (Triplett et al., 1964), and Kentucky (Blevins et al., 1971) all involved planting into killed sod that was comprised mostly of cool-season perennial grasses. However, planting into killed perennial vegetation has not been developed in the mid-South region of the USA, primarily because of difficulty in vegetation control without tillage. Warm-season perennial grasses such as johnsongrass, bermudagrass, broomsedge, bahiagrass [Paspalum notatum (L.) Flugge], and dallisgrass that commonly occupy noncropped sites may be dormant when crops such as corn are planted. Thus, the vegetation will not be controlled by application of PRE herbicides such as atrazine that have little or no activity on these species, and they may not be controlled with currently available selective POST herbicides.
A recently published survey of vegetation present on CRP sites in Kentucky (Martin et al., 1996) reported vegetation similar to that in the lower mid-South. Williams and Wicks (1978) listed bermudagrass and johnsongrass as major detriments to no-tillage with herbicides available at that time. Both weeds were also listed by Dowler (1997) in a weed survey of southern states as 2 of 10 most troublesome weeds in corn in Mississippi.
The selection of effective herbicides and the appropriate timing of application are important factors in controlling perennial species. Combinations of herbicides are almost always required for complete control of species present and to provide residual activity in the soil to control weeds that germinate during the growing season (Triplett, 1985). The introduction of genetically modified crops resistant to several herbicides will increase the potential for successful no-tillage planting into sod in the mid-South. The imidazolinone herbicides and glufosinate provide broad spectrum control and acceptable crop tolerance when appropriate transgenic cultivars are planted (Orwick et al., 1983; Shaner et al., 1983; Smith, 1988, 1989). Studies in Louisiana (Lanie et al., 1994) indicated that glufosinate controls a number of weeds as effectively as glyphosate or paraquat. Research by Worsham and Saunders (1987) in North Carolina support similar conclusions. Performance of herbicides applied to untilled vegetation cannot be extrapolated from experiences with conventional tillage (Triplett, 1985).
Objectives were to evaluate application rates and timing of herbicide combinations for control of untilled perennial and annual vegetation as a means of producing corn with glufosinate and imidazolinone herbicide-tolerant hybrids and a nontransgenic hybrid.
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Materials and methods
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Field experiments were conducted to evaluate various herbicide combinations, application timings, and application rates for corn production in untilled sod from 1995 through 1997 at the Mississippi Agricultural and Forestry Experiment Station's North Mississippi Branch at Holly Springs, Black Belt Branch at Brooksville, and Coastal Plain Branch at Newton. Soil at Holly Springs was a Grenada silt loam (fine-silty, mixed, thermic Glossic Fragiudalfs) with a pH 5.7 and 2.7% organic matter on a 2 to 5% slope; at Brooksville the soil was a Brooksville silty clay (fine, montmorillonitic, thermic Aquic Chromuderts) with a pH 6.8 and 4.1% organic matter on a 1 to 3% slope; at Newton the soil was a Prentiss fine sandy loam (coarse loamy siliceous, thermic Glossic Fragiudults) with a pH 6.3 and 2.0% organic matter on a 2 to 5% slope. Vegetation present at each site is listed in Table 1
. Weed cover and weed species contributions to total cover were estimated visually and were expressed in percentage of plot area covered by each weed species using a scale of 0 to 100% where 0 = no weeds and 100 = total plot coverage (Table 1). Weed densities were evaluated by counting stems of different species in five randomly selected 1 m2 quadrants in all plots (Table 1). A four-row planter was used to plant into a predominantly bermudagrass sod at Newton, bermudagrass and broomsedge at Holly Springs, and tall fescue and dallisgrass at Brooksville. Site preparation involved clipping vegetation to a 15-cm stubble height 10 to 14 d before planting or herbicide application. This was done to remove overgrown, dead material that would interfere with herbicide coverage.
Both discrete rate and logarithmic (variable) rate herbicide applications were made on four-row plots with 76-cm spacing by 9 m long. The center two plot rows were sprayed using a pressurized backpack sprayer using four 8002 regular fan nozzles calibrated to deliver 165 L water ha-1 at 207 kPa.
All plots treated with imidazolinone herbicides, either alone or in combinations, were planted to Pioneer 3245IR imidazolinone resistant (IR) corn. Plots treated with glufosinate were planted to DeKalb 689 Liberty Link (glufosinate–ammonium resistant) corn. All other plots were planted to the nontransgenic hybrid Pioneer 3165. In 1997, Pioneer 3395IR replaced 3245IR at all locations. At Holly Springs and Brooksville in 1997, an experimental dual-gene hybrid (Garst Seeds, Slater, IA) was included. This hybrid contained resistance to both imidazolinone and glufosinate herbicides but was not considered adapted to the Southern region (Hawkins, personal communication, 1997). All hybrids were planted at 60500 seed ha-1 and all seed lots had an average germination of at least 90%. Lime, N, P, and K fertilization for a projected yield of 11 Mg ha-1 were based on soil test recommendations. Nitrogen at 168 kg ha-1 was applied POST when corn was 20 to 25 cm tall.
Logarithmic Rate Studies
Herbicide rate was reduced 50% for each 1.5 m of plot length sprayed. Logarithmic applications provide a continuum of rates that can provide effects ranging from complete control to no activity on vegetation. Herbicide rate required for 90% vegetation control was determined by measuring from the point spraying was initiated to the point of 90% control, and this distance was used to calculate the herbicide rate. The first herbicide application was made immediately following planting. In 1995, applications were made on 27 April after warm-season perennials had initiated growth, and at succeeding 14 d intervals for a total of three applications. In 1996 and 1997, applications were made at 2-wk intervals on three dates, beginning in mid-March (Date 1) when warm-season perennial grasses were dormant. By 1 April, perennials had initiated growth (Date 2), and by mid-April, perennials were actively growing (Date 3). The 1996 study included a hybrid tolerant to glufosinate that permitted POST application after corn emergence.
In 1996, an experimental product X-996 containing 17.5% imazapyr and 52.5% imazethapyr active ingredients (a.i.) labeled for IR corn was included. The initial rate of each herbicide applied was: 0.9 kg a.i. ha-1 glufosinate, 2.24 kg a.i. ha-1 glyphosate, 0.56 kg a.i. ha-1 imazethapyr, 1.12 kg a.i. ha-1 imazapyr, and 0.36 kg a.i. ha-1 imazapyr + imazethapyr mixture. Glyphosate was applied PRE only in the tall fescue plot due to no glyphosate resistant varieties. The other herbicides were repeated approximately 14 and 28 d after initial application for a total of three application dates. In the CRP plot, glufosinate and imazapyr + imazethapyr mixture had three application dates, while imazapyr and imazethapyr were applied twice.
In 1997, the logarithmic rate plot on tall fescue was moved to a location not harvested for hay during the previous year. Initial herbicide rates for both locations were: 1.1 kg ha-1 imazapyr, 1.1 kg ha-1 imazethapyr, 1.1 kg ha-1 glufosinate, and 0.2 kg ha-1 imazapyr + 0.6 kg ha-1 imazethapyr mixture. POST treatments were applied 14 and 28 d later at Holly Springs; 15 and 30 d later at Brooksville.
Discrete Rate Studies
To develop suitable weed management practices, discrete rate field experiments with 29 herbicide treatments were conducted in 1995, 1996, and 1997. In 1995, the experiment was planted at Newton on 17 April, Holly Springs on 25 April, and Brooksville on 27 April. In 1996, the study was planted at Newton on 16 April, Holly Springs on 10 April, and Brooksville on 9 April. In 1997, the study was planted at Newton on 7 April, Holly Springs on 24 April, and Brooksville on 25 April. POST treatments were applied to actively growing vegetation 21 to 35 DAP, depending on growing conditions for the plots.
Herbicides applied PRE at all three locations in 1997 were 0.28 and 0.56 kg ha-1 imazapyr; 0.56 and 0.84 kg ha-1 imazethapyr; and 0.18 and 0.28 kg ha-1 imazapyr + imazethapyr. Each rate of these three herbicides was based on previous results and was considered the minimum rate for 90% vegetation control and a rate twice as high as the minimum rate. Herbicides were applied alone or with 0.56 kg ha-1 paraquat. Additional treatments included paraquat (0.56 kg ha-1) fb glufosinate (0.28 kg ha-1) POST at 14 and 28 d, glyphosate (0.84 kg ha-1) fb glufosinate (0.45 kg ha-1) at 21 d, paraquat (0.56 kg ha-1) fb imazethapyr (0.08 kg ha-1) plus glufosinate (0.45 kg ha-1) POST at 14 d, and fb glufosinate (0.34 kg ha-1) at 35 d (treatment not present at Newton).
Control Ratings and Statistical Analysis
Visual estimates of percent vegetation control were recorded 56 DAP using a scale of 0 to 100 where 0 = no control and 100 = plant death. Corn was harvested in 1996 and 1997 at all locations for the discrete rate plots except for Newton in 1996. Individual ears were hand harvested and weights recorded for the 1997 logarithmic rate study at Holly Springs and Brooksville (CRP Study). All yields were adjusted to 15.5% moisture. Covariance analysis was applied to all corn yields using stand and ear weight as covariates. There was significant interaction between yield and stand, but not between yield and ear weight.
The experimental design was a randomized complete block with three replications in all studies except at Brooksville in 1995 (four replications in the discrete rate plot). Data were subjected to analysis of variance and means separated using Fisher's Protected LSD at
(SAS Inst., 1990; Steel et al., 1997). In the logarithmic rate studies, treatment x date of herbicide application interactions were significant; consequently, all treatments are reported separately by weed species. In 1997, data were subjected to regression analysis and fitted to a quadratic equation. The 1997 discrete rate plots at all three locations evaluated paraquat combined with imazapyr, imazethapyr, imazapyr + imazethapyr mixture, and glufosinate. Each herbicide was also used alone at two different rates. Because of the number of different treatments in 1995 and 1996, data were not combined across years and locations. In 1997, there was a significant treatment x location interaction; however, there was little or no rank-change interaction. Therefore, the data from the three locations in 1997 were combined for analysis.
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Results and discussion
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Logarithmic Rate Studies
Herbicide effectiveness was influenced by herbicide characteristics, species present, and application rate and timing (Erbach and Lovely, 1975; Triplett, 1985). The 1996–1997 studies included imazapyr, imazethapyr, glufosinate, and imazapyr + imazethapyr. Since the weed spectrum differed between locations, both in species present and density, the rate needed to control 90% of the vegetation was also different among the four herbicide treatments (Table 2)
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Table 2 Minimum rate of herbicide required for 90% vegetation control (56 DAP). Herbicide applied with indeterminate logarithmic sprayer (1996–1997)
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Imazapyr was the only herbicide to control bermudagrass at less than the initial rate in virtually all circumstances (Table 2). Regardless of the weed species, imazapyr controlled all species except the warm-season annual grass southern crabgrass [Digitaria ciliaris (Retz.) Koel.] (Table 1), which may have successive flushes of germination following application. Southern crabgrass emerged somewhat later and was less affected by the residual activity of imazapyr. Effective rates with different timings of application for imazapyr were very similar for control of broomsedge, johnsongrass, tall fescue, and other perennial species. With some species, the rate required for control on the early April date was higher than when applied 2 wk earlier or later. This pattern was repeated for the other two imidazolinone herbicides in the study, but not necessarily for the same weed species.
Rates of several imidazolinone herbicides and glufosinate required for 90% perennial control when applied alone were on the threshold of causing crop injury, even though IR and glufosinate-resistant hybrids were used (Table 3)
. Regression of herbicide rates on corn ear weights indicated injury occurred for herbicide rates and application dates when the rate necessary for weed control was exceeded by 25%. For example, imazapyr on all three application dates at Brooksville reached a maximum rate calculated from the regression equations, and as that rate was exceeded, ear weights declined even though weed control was excellent (Fig. 1)
. This suggests that even though hybrids are resistant to imidazolinone herbicides and corn vegetative growth exhibited no visual injury symptoms, phytotoxicity is likely when application rates approached or exceeded those required for control of perennial vegetation (Table 3). However, glufosinate applied at 1.12 kg ha-1 on 15 March and 1 April at Brooksville failed to reach a maximum calculated rate for 90% control (Fig. 2)
. At this rate of glufosinate, no injury symptoms were evident (Krausz et al., 1999) but weed control was poor as evidenced by lower ear weights.
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Table 3 Herbicide rate causing reduction of grain yield on three dates at two locations determined from logarithmic rate sprayer application
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With imazapyr, when no injury was apparent and weed control was excellent, individual ear weights were 200 g or higher (Fig. 1). Conversely, as the herbicide rate decreased, ear weights decreased, indicating the effect of increased weed interference. At higher rates injury was evident from visual inspection of ears where kernels were aborted, ear length reduced, and ears were malformed. Potential for injury was minimized when the rate necessary for 90% control was not exceeded.
The mixture of imazapyr + imazethapyr reflected the effects of both herbicides combined, especially at Brooksville. At Holly Springs, imazapyr and imazethapyr, when applied alone, required lower rates for control on 1 April and 15 April than did the mixture. This indicates that the lower rates in the mixture were less effective than an individual herbicide applied at a higher rate (Table 4)
. Heat unit accumulation, on the same calendar dates, was greater at Brooksville, which may have caused increased vegetative development and increased herbicide activity. Since the mixture is labeled for corn and imazapyr is not, it holds promise as an effective herbicide when used on susceptible weed species, especially if combined with a complimentary herbicide such as glufosinate and applied to hybrids resistant to both herbicides.
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Table 4 Herbicide rate required for 90% vegetation control on three dates at two locations determined from logarithmic rate sprayer application
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Discrete Rate Studies
PRE and POST treatments containing imazapyr, except when applied tank mixed with paraquat PRE in 1997, controlled at least 90% of the tall fescue, bermudagrass, and broomsedge (Table 5)
. The cool-season species (winter annuals) present were controlled effectively with most herbicides at lower rates, less than required to control warm-season species, and were not competitive with the corn (data not shown). Hybrids tolerant or resistant to both imidazolinone and glufosinate herbicide would complement the strengths of each herbicide and broaden the spectrum of vegetation control, including grass and broadleaf species (Culpepper and York, 1997 and 1999).
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Table 5 Effect of discrete rate PRE and POST herbicide treatments on broomsedge and bermudagrass control at three locations and fescue control at one location
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Imazapyr, imazethapyr, or glufosinate either alone or combined with other herbicides controlled at least 90% of either broomsedge or bermudagrass (Table 5). Control varied by treatment and year, but not by location. Glufosinate applied POST (28 DAP) provided 90% control of broomsedge in 1996 and 1997 with no injury to transgenic corn. Paraquat, glyphosate, and glufosinate do not have soil residual activity, and escaped vegetation included both summer annuals and perennials that recovered following the initial application. For effective weed control with nonresidual herbicides, correct timing of application is critical and all PRE applications required a POST application to achieve effective weed control (Krausz et al., 1999). Warm-season annuals, primarily southern crabgrass and yellow foxtail [Setaria glauca (L.) Beauv.], posed a problem at all locations, especially when no soil residual herbicides were used or when POST applications were improperly timed (Table 1). Similar results have been reported previously (Buhler, 1991; Buhler and Daniel, 1988; Spandl et al., 1998). Paraquat, glyphosate, and glufosinate applied PRE gave excellent control of southern crabgrass early but were unsatisfactory by 56 DAP. Atrazine PRE combined with imazapyr, imazethapyr, or nonresidual herbicides controlled 90% of southern crabgrass. Imazapyr POST also controlled (90%) southern crabgrass. Without the use of a residual herbicide, warm-season annuals emerged and in many cases competition reduced corn yield (Table 6)
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Effective control of warm-season perennials was provided with imidazolinone herbicides used alone, but at rates that may cause crop injury. Imazapyr at 0.56 kg ha-1 controlled weeds more effectively than 0.28 kg ha-1, although corn yield following the lower rate was the highest in all locations during 1997 (Table 6). This suggests some injury with the higher rate, even though no vegetative injury symptoms were observed. Planting a glufosinate-resistant hybrid and the dual gene (imidazolinone plus glufosinate) hybrid allowed the application of label rates. In 1997 at Holly Springs, glyphosate PRE at 0.84 kg ha-1 followed by glufosinate POST (28 DAP) at 0.45 kg ha-1 produced a yield of 10.2 Mg ha-1. The PRE application of paraquat at 0.50 kg ha-1 and imazapyr + imazethapyr mixture at 0.18 kg ha-1 was the second highest yield at 9.9 Mg ha-1 (Table 6). Unless the imazapyr + imazethapyr could be used on a dual gene hybrid, rates required for 90% control were well above labeled rate of 0.09 kg ha-1. Imazethapyr also required rates above the 0.07 kg ha-1 label rate except on the dual gene hybrid. These herbicide-resistant systems should allow corn to achieve maximum yields with satisfactory weed control and no injury to the corn.
Results from this study indicate efficacious herbicides are now available for no-tillage corn planting into warm-season perennial vegetation or sod when used in conjunction with transgenic corn hybrids that tolerate PRE and POST applications. Through the use of PRE residual herbicides such as atrazine with glyphosate or paraquat followed by glufosinate POST, corn can be grown successfully in perennial sod. Dual-gene hybrids that tolerate both glufosinate and imidazolinone herbicides will further increase success of no-tilling corn into perennial sod. Timely planting of corn requires satisfactory control of unwanted vegetation. The combination of residual and contact herbicides applied at planting suppressed perennial vegetation. A POST application within 28 DAP extended the weed control and allowed no detrimental competition throughout the season. No-till planting into sod is possible in the mid-South and this will preserve soil improvements accumulated under perennial vegetation and increase sustainablility while producing a corn crop.SAS Institute 1990
Received for publication September 20, 1999.
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TOP
ABSTRACT
INTRODUCTION
