Herbicides for Establishment

Switchgrass has small seed (~600,000 to 900,000 seeds kg-1) that often are reported to be slow to establish (Aiken and Springer 1995). This characteristic provides a competitive advantage to weeds, resulting in excessive competition during establishment (Masters et al. 2004; Boydston et al. 2010; Mitchell et al. 2010a). Controlling weeds during the establishment year improves establishment and increases biomass production in subsequent years (Schmer et al. 2006; Mitchell et al. 2010a). Using current agronomic recommendations, it is feasible to produce 50% of the yield potential of the cultivar to be available for harvest after a killing frost in the planting year, and produce and harvest 75-100% of the yield potential of the cultivar in the first full growing season after planting (Mitchell et al. 2010a; Mitchell et al. 2012).

Warm-season annual grass weeds are the most detrimental to successful switchgrass establishment, since broadleaf weeds can easily be controlled with 2,4-D amine 2,4-dichlorophenoxyacetic acid, when switchgrass reaches the 4-leaf stage (Vogel 2004; Anonomous 2008b). Cool-season weeds are relatively easy to control since they can be controlled with glyphosate prior to planting (Sanderson et al. 2012). Use of a pre-emergence herbicide is typically recommended as an aid in establishing warm-season grasses. For example, application of metolachlor and/or atrazine [6-chloro-N-ethyl- N-(1-methylethyl)-1,3,5-triazine-2,4-diamine] was reported to improve biomass yield in big bluestem (Andropogon gerardii Vittman) during the second year (Masters 1997). In three environments in the central and northern Great Plains, pre-emergence application of atrazine and quinclorac (3,7-Dichloro-8-quinolinecarboxylic acid) resulted in acceptable stands and high biomass yields (Mitchell et al. 2010a). No differences were detected among switchgrass lowland and upland ecotypes for tolerance to atrazine and quinclorac. The use of a pre-emergence herbicide to control such weeds needs evaluations in other environments.

There are very few herbicides currently labeled for use during switchgrass establishment. The scientific literature provides limited information on the phytotoxicity and efficacy of the herbicides used in other warm-season grasses when used for weed control in switchgrass establishment. Currently only quinclorac (Paramount®; Anonymous 2008c,

2010) is labeled in the USA, while nicosufuron (Accent®; Anonymous 2008a) has a supplemental label in the state of Tennessee for weed control during switchgrass establishment once it reaches the 2-leaf stage. In non-crop areas and Conservation Reserve Program (CRP) sites, sulfosulfuron (Outrider®; Anonymous 2011a) controls johnsongrass, and nutsedge (Cyperus sp.) when applied to newly seeded switchgrass after the 3-leaf stage. Use of atrazine, which is labeled for corn (Zea mays L.) and CRP plantings of switchgrass, has led to successful establishment of upland switchgrass as a companion crop in corn fields (Hintz et al. 1998). Although atrazine can improve switchgrass establishment by controlling broadleaf weeds and cool-season grasses (Martin et al. 1982; Bahler et al. 1984), it does not control warm — season annual grass weeds (Boydston et al. 2010; Mitchell et al. 2010a). Injury to switchgrass is reported to differ with herbicide used, application rates, growth stage at application, and the ecotype of switchgrass being evaluated. Research results have varied and are sometimes contradictory. Mitchell et al. (2010a) reported that lowland and upland ecotypes had comparable tolerances to atrazine and quinclorac that effectively controlled weeds and resulted in acceptable plant stands in both switchgrass ecotypes. ‘Pathfinder’, an upland ecotype, is reported to have greater tolerance to pre-emergent applications of atrazine, and the use of atrazine aids its establishment (Martin et al. 1982; Vogel 1987; Masters et al. 1996; Hintz et al. 1998). However, despite this tolerance, there are reports of increasing injury in Pathfinder as atrazine application rate increases from 1.1 to 2.2 kg ha1 (Martin et al. 1982; Vogel 1987; McKenna et al. 1991; Masters et al. 1996; Hintz et al. 1998). Imazapic (2-[4,5-dihydro-4-methyl-4-(1-ethylethyl)- 5-oxo-1H-imidazol-2-yl]-5-methyl-3-pyridinecarboxylic acid) often reduced switchgrass stands and is not recommended for switchgrass establishment (Mitchell et al. 2010a).

Despite these successes for atrazine use in upland ecotypes (Bovey and Hussey 1991), recommended that atrazine should not be used when establishing Alamo, a lowland switchgrass ecotype, due to excessive injury. Furthermore, the phytoxicity of atrazine may also be site specific. Bahler et al. (1984) reported that atrazine application reduced switchgrass seedling density, with the degree of damage being greater in loamy sandy soil than in silty clay loam soil. The upland switchgrass cultivar ‘Cave-in­Rock’ tolerated atrazine (1.1 kg a. i. ha-1), while a lowland strain derived from Alamo was killed by atrazine (T. J. Butler, unpublished data). Time to rainfall after planting appears to mediate atrazine activity on lowland switchgrass. In an Oklahoma study, atrazine application followed by rainfall the succeeding day resulted in complete lowland switchgrass mortality. The second year, however, rainfall did not occur for two weeks upon atrazine application, and the lowland switchgrass had only transient injury (T. J. Butler, unpublished data).

Some alternatives to atrazine have been evaluated. For example, an application of 1.6 kg a. i. siduron ha-1 effectively controlled large crabgrass with no effect on ‘Caddo,’ an upland switchgrass ecotype (McMurphy 1969). However, subsequent work indicated that a pre-emergence application of 2.2 kg ai siduron ha-1 caused significant injury to Alamo, a lowland ecotype (Bovey and Hussey 1991). Although Mitchell et al. (2010a) reported that both upland and lowland switchgrass ecotypes tolerated 560 g a. i. ha-1 quinclorac applied pre-emergence in the central and northern Great Plains, similar quinclorac pre-emergence applications in the southern Great Plains have reduced lowland switchgrass emergence (T. J. Butler, personal comm.). Masters et al. (1996) reported that imazethapyr improved big bluestem establishment (77-94%) similar to atrazine (18-95%) and proved a suitable replacement for atrazine when establishing big bluestem; however imazethapyr generally reduced switchgrass establishment (stand frequency) of the upland ecotype ‘Trailblazer’. Although the concern for using atrazine is valid in some regions, atrazine has been used effectively in hundreds of small plot trials and production scale fields on all available upland and lowland switchgrass strains in the central and northern Great Plains.

Post-emergence herbicides have also demonstrated mixed results. However, in well-managed established stands, such herbicide application is seldom needed (Mitchell et al. 2010a). Applications of quinclorac at 0.56 kg a. i. ha-1 or pendimethalin at 1.1 kg a. i. ha-1 at the 1-2 leaf stage have been shown to improve weed control but reduce switchgrass in irrigated stands in the arid west (Boydston et al. 2010). Post-emerge quinclorac applications reduced switchgrass biomass at establishment by 33% compared to a control receiving pre-mergence atrazine only, but this effect was less than an yield 89% reduction with post-emerge pendimethalin application (Boydston et al. 2010). However, in rain-fed production in the central and northern Great Plains, the application of quinclorac to established upland and lowland switchgrass strains has not been observed to reduce stands. Work by Curran et al. (2011) showed that quinclorac applied 4 wk after planting achieved better weed control in Cave-in-Rock switchgrass than when applied 6 wk after planting. Additionally, application of 2.2 kg a. i. ha-1 MSMA to greenhouse grown lowland switchgrass at the 3-to-4-leaf stage did not cause significant injury compared to the control (Bovey and Hussey 1991). Kering et al. (2012b) evaluated lowland switchgrass establishment with competition from large crabgrass [Digitaria sanguinalis (L.) Scop.], broadleaf signalgrass [Urochloa platyphylla (Munro ex C. Wright) R. D. Webster], Johnsongrass [Sorghum halepense (L.) Pers.], and Texas panicum [Urochloa texana (Buckley) R. Webster] and reported that switchgrass establishment was improved with a combination or quinclorac + foramsulfuron + pendimethalin at the 1-2 leaf stage (13-26% stand) and MSMA at the 3-4-leaf stage (7-35% stand) compared to an untreated control (0-3% stand). However such results are less than satisfactory based on a minimum goal of 40% coverage at the end of the first season (Vogel 1987; Masters 1997).

The best outcomes for switchgrass establishment result from using sound agronomic practices for weed control. For example, when grass weeds are controlled the previous season, especially where glyphosate — tolerant soybeans, corn, or cotton are grown, switchgrass has a much greater chance of successful establishment (Christensen and Koppenjan 2010; Mitchell et al. 2010a). Mitchell et al. (2010a, 2012) provided recommendations that produce harvestable yields after a killing frost in the planting year if precipitation is adequate: 1) develop a good seedbed (no-till seed into soybean stubble or clean till and pack to leave a faint footprint); 2) plant within 3 weeks before or after the optimum maize planting date; 3) use high quality certified seed of adapted material; 4) plant at least 300 PLS m-2; 5) use a planter that controls depth and plant seeds 0.6 to 1.2 cm deep; 6) manage weeds with a pre-emergent application of 1.1 kg ha-1 of atrazine plus 560 g ha-1 of quinclorac then mow or spray broadleaf weeds with