Managing weeds is one of the most important factors for sustainable switchgrass biomass production. Since switchgrass seedlings develop more slowly than annual weeds, control­ling weeds immediately after planting is critical for successful establishment. Additionally, the economic feasibility of switchgrass for bioenergy is dependent on establishing stands with a harvestable yield in the planting year [42]. Poor establishment caused by weed pressure can delay full production of biomass for two or more years [43]. Well-established switchgrass is less likely to have weed issues.

Weed pressure can be minimized in the establishment year by no-till seeding into glyphosate-tolerant soybean stubble, which provides an excellent seedbed. Switchgrass seed germination is slow and seedling vigor is low compared to annual grassy weeds. Consequently, it is important to plant high quality seed in properly prepared seedbeds [22]. If heavy weed pressure is expected, delay seeding until the first flush of weeds, then apply a broad-spectrum herbicide like glyphosate [N-(phosphonomethyl) glycine) before planting.

Applying pre-emergent and post-emergent herbicides shows a significant effectiveness in controlling and reducing weed populations during the establishment year. Normally, switchgrass establishment is not interrupted by broadleaf weeds. Herbicides, such as 2,4-D (2,4-dichlorophenoxyacteic acid) can control the broadleaf weed effectively, but should not be applied until after the switchgrass seedlings have reached the four — or five-leaf stage [21]. Early grassy weed growth is reduced with herbicides. The herbicides utilized in the forage industry will control weeds but label directions are critical for safe and proper application. The forage industry has used atrazine [2chloro-N-ethy-N/-(1-methylethyl)-1,3,5-triazine — 2,4-diamine] as a pre-emergent to control cool-season annual grasses and broad leaf weeds [44]. Quinclorac (3,7-dichloro-8-quinlinecarboxylic acid) is another common herbicide that can be used as a pre-emergent or post-emergent herbicide to control most of annual warm — season grass weeds [23,45]. Mitchell et al. [23] reported that a combination of atrazine and quinclorac applied immediately after planting provided the best weed control and most rapid establishment for upland and lowland switchgrass ecotypes in Nebraska, South Dakota, and North Dakota, U. S.A. Although some herbicides work well for establishing switchgrass, not all herbicides are labeled for application on the crop in all states. Always read and follow label directions.

Weed competition in post-establishment years is not a major issue for well-established stands, but if stands are poor during the establishment year, they typically have increased weed pressure in subsequent years. Switchgrass stands with seedling densities below 10 plants per square meter are considered to be poor and should be over-seeded or reseeded [21]. As long as adequate switchgrass frequency of occurrence (i. e., >40%) has been achieved in the seeding year, weed control is relatively easy during the post-establishment years. One of the most effective methods to control cool-season annual or perennial weeds is with the application of a broad-spectrum herbicide, such as glyphosate. Switchgrass must be dormant when glyphosate is applied, either prior to spring green-up or after senescence in late summer or early autumn. Once switchgrass starts to grow, canopy development is much faster than annual warm-season weeds and summer annual weeds are usually not an issue.