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INTEGRATED PEST MANAGEMENT

Grassland Legume Establishment with Imazethapyr and Imazapic

^a Dep. of Horticulture, Univ. of Nebraska, Keim Hall, East Campus, Lincoln, NE 68583-0937 USA

rmasters1{at}unl.edu

Received for publication June 17, 1998.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion REFERENCES

 
Legumes are important components of grassland communities in North America and have potential to improve grassland productivity and diversity. Weeds can interfere with the establishment of legumes and increase probability of stand failure. Four experiments were conducted from 1994 to 1997 to determine if the imidazolinone herbicides imazethapyr [2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-ethyl-3-pyridinecarboxylic acid] and imazapic [(±)-2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-methyl-3-pyridinecarboxylic acid] applied preemergence at 70 g a.i. ha^-1 could reduce weed interference and improve establishment of seeded grassland legumes. Six native legumes and one introduced legume, crownvetch (Coronilla varia L.), were planted into prepared seedbeds at sites near Mead, NE. Legume response to the herbicides varied among species and sites. Crownvetch, partridgepea [Chamaecrista fasciculata (Michx.) Greene; syn. Cassia chamaecrista L.), and purple prairieclover (Dalea purpurea Vent.) exhibited tolerance to both imazethapyr and imazapic in most experiments and their establishment, as indicated by stem density and/or forage yield collected 14 mo after planting, was improved when treated with the herbicides in weedy environments. Imazapic treatment injured leadplant (Amorpha canescens Pursh), Canada tickclover [Desmodium canadense (L.) DC.], and roundhead lespedeza (Lespedeza capitata Michx.), resulting in lower stem densities and/or forage yields than when imazethapyr was applied. Based on these findings, preemergence application of imazethapyr and imazapic can be used to reduce weed interference and improve the establishment of certain grassland legumes.

Abbreviations: PLS, pure live seed • UAN, urea ammonium nitrate

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion REFERENCES

 
SEVERAL NATIVE LEGUMES are commonly found in tallgrass and mixed-grass prairies of the Great Plains. Native species, such as purple prairieclover and leadplant, are important components of these communities and provide high quality forage and symbiotically fixed N₂ (Becker and Crockett, 1976; Weaver, 1954). Overgrazing, encroachment of undesirable introduced species, and the exclusion of fire have contributed to grassland degradation and a reduction in legume abundance. Herbicides, such as picloram (4-amino-3,5,6,-trichloro-2-pyridinecarboxylic acid) and 2,4-D [2,4-(dichlorophenoxy)acetic acid], that are used to control broadleaf weeds in grasslands can have a negative impact on native broadleaf species (Masters et al., 1992).

Native legumes have potential to improve available forage quality and production, wildlife habitat quality, and grassland community diversity. Purple prairieclover, Illinois bundleflower [Desmanthus illinoensis (Michx.) MacMill. ex B.L. Rob. & Fernald], roundhead lespedeza, and catclaw sensitivebrier [Mimosa quadrivalvis var. nuttallii (DC.) L.S. Beard ex Barneby; syn. Schrankia nuttallii (DC.) Standl.], legumes native to the central Great Plains, improved forage yields and crude protein levels when seeded with native warm-season grasses (Posler et al., 1993). Illinois bundleflower interseeded into established kleingrass (Panicum coloratum L.) improved total yields for 4 yr after establishment (Dovel et al., 1990). Partridgepea, an annual native legume, has been shown to provide N to pearl millet [Pennisetum glaucum (L.) R. Br.] (Redmon et al., 1995), and its seed is an important food for wildlife (Robel et al., 1974; Stubbendieck and Conard, 1989).

Weed interference can reduce the establishment of grassland legumes and other prairie species. Severe annual weed infestations after planting of native grasses can result in poor stands or even complete stand failure (Cox and McCarty, 1958; Martin et al., 1982; Masters et al., 1996). Likewise, many perennial grassland legumes are susceptible to weed competition after planting, because of their slow rate of seedling growth compared with that of annual weeds. Reducing weed interference with herbicides has improved establishment of the introduced perennial legumes crownvetch and cicer milkvetch (Astragalus cicer L.) (Linscott and Hagin, 1974; Moyer 1989).

Methods to reduce weed interference and improve establishment of native prairie species have been investigated, but have focused primarily on grasses. Atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine), metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide], and 2,4-D are herbicides that have been used to improve the establishment of native grasses (Cox and McCarty, 1958; Martin et al., 1982; Masters, 1995). While these herbicides have potential for improving grass establishment, their utility in establishing legumes and other forbs is limited. Forb establishment was reduced where atrazine or 2,4-D was applied, compared with mowed areas (Bragg and Sutherland, 1989). Preemergence application of the imidazolinone herbicides, imazethapyr and imazapic, improved Illinois bundleflower and purple prairieclover as well as big bluestem (Andropogon gerardii Vitman var. gerardii), indiangrass [Sorghastrum nutans (L.) Nash ex Small], and little bluestem [Schizachyrium scoparium (Michx.) Nash] establishment (Masters et al., 1996).

Imidazolinone herbicides control many broadleaf and grassy weeds and exhibit foliar and soil activity (Little and Shaner, 1991; Shaner and Mallipudi, 1991). A wide range of selectivity exists among imidazolinone herbicides. Several legumes, including soybean [Glycine max (L.) Moench], alfalfa (Medicago sativa L.), and peanut (Arachis hypogaea L.), exhibit tolerance to this class of herbicides (Fales and Hoover, 1990; Richburg et al., 1995). Application timing of imidazolinone herbicides can effect weed control efficacy and crop injury. Imazethapyr applied preemergence or preplant-incorporated reduced establishment and increased injury to seeded alfalfa, compared with early postemergence applications (Hartberg and Harvey, 1987; Proost and Buhler, 1987). In contrast, delayed applications of imazethapyr reduced the control of common lambsquarters (Chenopodium album L.) in soybean (Buhler and Proost, 1992), and was less effective for establishment of seeded big bluestem and little bluestem (Masters et al., 1996).

By identifying grassland legumes that are tolerant to imidazolinone herbicides, an important weed control option could be developed that would reduce weed interference and improve the reliability of legume establishment for grassland interseeding, revegetation, and restoration. This research was conducted to determine the influence of imazethapyr and imazapic on establishment of selected native and introduced legumes.

	Materials and methods

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion REFERENCES

 
Four experiments were initiated from 1994 to 1996 at the University of Nebraska Agricultural Research and Development Center near Mead, NE. The soil at all sites was a Sharpsburg silty clay loam (fine, smectitic, mesic Typic Argiudolls). Experiments were designed as randomized complete blocks with four replications in the 1994 and 1996 experiments, and three replications in the experiment initiated in 1995. A factorial treatment arrangement was used in each experiment, with three to six legume species and three weed control treatments.

1994 Experiments
Two experiments were conducted in 1994 on an irrigated and a nonirrigated site that had previously been maintained as tall fescue (Festuca arundinacea Schreb.) sod. On the irrigated site, sprinkler irrigation applied 6 mm of water four times per week, resulting in a minimum of 25 mm of water for the 4 wk after planting by irrigation and/or rainfall. Sites were tilled and cultipacked prior to planting. A 60-cm border of Kentucky bluegrass (Poa pratensis L.) sod was planted around each 1- by 2-m plot. Within each plot, a single species was seeded to a maximum depth of 1.2 cm into three rows that were 20 cm apart. Crownvetch at 11.8 kg pure live seed (PLS) ha^-1 (285 PLS m^-2), purple prairieclover at 8.4 kg PLS ha^-1 (580 PLS m^-2), and Illinois bundleflower at 16.3 kg PLS ha^-1 (260 PLS m^-2) were planted at both sites on 8 June 1994.

Herbicide treatments were applied to individual plots on 10 June 1994.¹ The herbicide treatments were no herbicide and imazethapyr or imazapic applied at 70 g a.i. ha^-1. Methylated sunflower (Helianthus sp.) seed oil and 28% urea ammonium nitrate (UAN) at 1.25% v/v were added to the spray solution to enhance foliar uptake by emerged weeds. Herbicides were applied with a back-pack sprayer that delivered 163 mL m^-2 at 0.24 MPa. The dominant weed species at both sites was smooth crabgrass [Digitaria ischaemum (Schreb. ex Schweig) Schreb. ex Muhl.].

1995 Experiment
An experiment was initiated on an irrigated site that had previously been maintained as a Kentucky bluegrass sod. Site preparation, plot size, planting method, and irrigation method were similar to that of the 1994 irrigated experiment. Crownvetch, Illinois bundleflower, purple prairieclover, Canada tickclover, partridgepea, leadplant, and roundhead lespedeza were seeded to individual plots at 300 PLS m^-2 on 28 June 1995. The planting date was delayed and relatively late, because of excessive rainfall during May and June 1995. Herbicide treatments were applied to individual plots on 29 June 1995. Treatments were no herbicide and imazapic or imazethapyr applied at 70 g a.i. ha^-1. Methylated sunflower seed oil and 28% UAN were added to the spray solution and applied at 380 mL ha^-1. The primary weed species present at the site were swamp smartweed (Polygonum amphibium var. emersum Michx.; syn. P. coccineum Muhl. ex Willd.), horseweed [Conyza canadensis (L.) Cronquist], large crabgrass [Digitaria sanguinalis (L.) Scop.], and smooth crabgrass.

1996 Experiment
An experiment was initiated on an irrigated site that had previously been maintained as a tall fescue sod. Site preparation, plot size, planting method, and irrigation method were similar to that of the 1995 experiment. Purple prairieclover, Canada tickclover, partridgepea, roundhead lespedeza, and leadplant were seeded to individual plots at 300 PLS m^-2 on 19 May 1996. Weed control treatments were no herbicide and imazethapyr or imazapic applied at 70 g a.i. ha^-1 on 20 May 1996. Dominant weeds at the site were common lambsquarters, redroot pigweed (Amaranthus retroflexus L.), and smooth crabgrass.

In the 1994 experiments, smooth crabgrass control was visually estimated at 4 wk after herbicide treatment. Due to nonhomogeneous distribution of weed species, weed control ratings by species at the 1995 and 1996 sites could not be reliably measured. At all sites, legume seedling stands were visually rated on a 0-to-10 scale at 4 wk after herbicide treatment. The seedling stand rating integrated emergence and seedling abundance within the planted rows, with 0 representing no emergence and 10 representing complete, uniform emergence.

Stand establishment of the perennial legumes was assessed about 14 months after planting with density and yield measurements. Stem density was determined by placing a 0.5-m² quadrat across the three rows of each plot and counting the number of rooted stems within the quadrat. Forage yield was estimated by clipping legume herbage rooted within a 0.5-m² quadrat to a 1-cm height and drying the herbage at 60°C to a constant weight. The stem density and yield of partridgepea, the only annual legume in the study, was collected at the end of the first growing season. Yield of partridgepea was not collected in the experiment initiated in 1995, because of the late planting date. Flowering stem density of purple prairieclover, Canada tickclover, and roundhead lespedeza was estimated by counting the number of flowering stems rooted within the quadrat.

To reduce annual grass weed interference during the second growing season, a uniform application of fluazifop-p-butyl{butyl(R)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]propanoate} was applied to all plot areas at 0.4 kg a.i. ha^-1 with a backpack sprayer once annual grasses had reached a 2.5-cm height. This herbicide is registered for use to control annual grass weeds in broadleaf ornamentals and is tolerated by established legumes.

Experiments from each year were analyzed separately, because of significant herbicide x site interactions for several variables. If error variances were determined to be homogeneous according to Hartley's F-max test (Hartley, 1950), data were pooled across irrigated and nonirrigated experiments initiated in 1994. Smooth crabgrass control data from the experiments initiated in 1994 were combined across legume species; all other variables were analyzed by legume species. Arcsine transformation (Lentner and Bishop, 1993) was performed on smooth crabgrass control and seedling stand rating data. Nontransformed data are presented for these variables, because the results between transformed and nontransformed analyses were similar. Purple prairieclover flowering stem density data from the 1994 irrigated experiment, and Illinois bundleflower and Canada tickclover yield and density data from the 1995 experiment were transformed to stabilize error variances among treatments. For these variables, yield data were log-transformed, and density data were square-root-transformed (Lentner and Bishop 1993). Means were separated with Fisher's protected LSD (P 0.05).

	Results and discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion REFERENCES

 
1994 Experiments
Weed interference from smooth crabgrass, the dominant weed species, was severe with 100% foliar cover at 4 wk after planting in the nontreated plots. Analysis of smooth crabgrass control data revealed homogenous error variances and no legume or site interactions; therefore, smooth crabgrass control data were pooled across legume species and the irrigated and nonirrigated sites. At 4 wk after application of the herbicide treatments, imazapic provided greater control of smooth crabgrass (96%) than imazethapyr (66%). Rapid growth of the smooth crabgrass resulted in poor emergence and/or seedling survival of crownvetch and purple prairieclover in the nontreated areas, which resulted in lower seedling stand ratings (Table 1) . Both imazethapyr and imazapic improved seedling stands of crownvetch and Illinois bundleflower, whereas only imazethapyr improved purple prairieclover seedling stands.

Illinois bundleflower stem densities were lower in herbicide-treated areas (Table 3); however, injury symptoms were not observed and many of the plants in the herbicide-treated plots were flowering within 10 wk after planting. In spring 1996, many Illinois bundleflower plants failed to resprout, unlike those in the 1994 experiments. Most of the plants observed in the second year appeared to have originated from seed produced during the first year or from planted dormant seed that germinated in spring 1996. The late planting date (28 June 1995) and an early frost on 21 Sept. 1995 may explain the poor survival of Illinois bundleflower.

1996 Experiment
As with the 1995 site, weed species were not distributed homogeneously and individual weed control ratings were not recorded. Weed interference was moderate, with <50% canopy cover in nontreated areas, which was not as severe as that observed in the 1994-initiated experiments. Injury to purple prairieclover and Canada tickclover seedlings from imazapic resulted in reduced emergence and a lower seedling stand rating, compared with the imazethapyr and no herbicide treatments (Table 4) . Despite initial stunting caused by imazapic, purple prairieclover stem density, forage yield, and flowering stem density (Table 2) did not differ between imazapic and imazethapyr treatments, and were greater than the nontreated control. Treatment with imazethapyr usually resulted in the greatest stem density, flowering stem density, and forage yield of Canada tickclover and roundhead lespedeza. Herbicide treatments did not reduce partridgepea stem density, and forage yields were >9.0 Mg ha^-1 within four months after planting. Leadplant was not evaluated, because the leadplant seedlot was contaminated with alfalfa seed.

In general, imazethapyr was less injurious to the seeded legumes than imazapic. Imazapic injury was most evident in the 1995 site where weed populations were low. Grass establishment studies conducted in eastern Nebraska and Kansas have indicated that increased activity of the imidazolinone herbicides may occur in areas with low weed competition. Imazapyr [(±)-2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-pyridinecarboxylic acid] improved stand establishment of switchgrass (Panicum virgatum L.) at a site with severe annual grass pressure, but reduced establishment at a site with low weed pressure (Masters et al., 1996). Likewise injury of buffalograss [Buchloë dactyloides (Nutt.) Engelm.] seedlings from imazapic was more severe at sites where weed infestations were at a low or moderate level (Fry et al., 1997). Thus, in this region, the rate of imazapic used could potentially be lowered in areas with less weed interference to reduce the risk of injury to planted legumes. Another approach to reducing injury to seeded legumes that merits consideration is early postemergence applications, which have been shown to reduce the risk of crop injury to newly seeded alfalfa treated with imazethapyr (Hartberg and Harvey, 1987; Proost and Buhler, 1987).

This study provides evidence that imidazolinone herbicides are a promising weed management option for establishing certain legumes. Illinois bundleflower, crownvetch, purple prairieclover, and partridgepea establishment was enhanced with imazethapyr or imazapic in sites with moderate to severe weed infestations. Imazethapyr provided less control of smooth crabgrass at two sites, but was also less injurious to Canada tickclover and roundhead lespedeza. A primary benefit of these herbicides is their ability to reduce weed interference and improve establishment in areas such as marginal cropland sites and roadside rights of way, where weed competition is often severe.

Native legumes are usually not incorporated into grassland revegetation programs for various reasons, and key among these are the scarcity of weed control options and limited information on their benefits and management. We determined that the imidazolinone herbicides improved establishment of selected legumes. Given this finding and the documented tolerance of several native warm-season grasses to the imidazolinone herbicides (Masters et al., 1996), these herbicides could be effective components of systems to improve grassland forage resources by facilitating establishment of legumes and warm-season grasses in seeded mixtures. Research is needed to quantify the contribution of native legumes to grassland forage quality and yield and to identify management practices that optimize legume persistence. This information is needed to determine how native legumes can be integrated into grassland improvement strategies.

	ACKNOWLEDGMENTS

 
The authors wish to thank Kevin Grams, Leonard Wit, Fernando Rivas-Pantoja, Brenda Younkin, and LeAnn Beran for their assistance, and the Arthur Sampson Endowment Fund, American Cyanamid, and the Nebraska Turf Foundation for partial support.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion REFERENCES

 
Joint contribution of the Nebraska Agric. Res. Div. and the USDA-ARS, Journal Series no. 12297.

¹ Mention of a particular pesticide does not imply registration under FIFRA, nor does it constitute a recommendation by the Univ. of Nebraska–Lincoln or the USDA-ARS.

	REFERENCES

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