Switchgrass - Bioenergy Crop Plants - page 572

Most dicot weeds can be controlled with 2,4-D (2,4-dichlorophenoxyacteic acid). Generally, 2,4-D
should be applied after switchgrass seedlings have approximately four to five leaves. Atrazine
[6-chloro- N -ethyl- N ′-(1-methylethyl)-1,3,5-triazine-2,4-diamine] has been used to improve
establishment of switchgrass by controlling broadleaf weeds and C 3 weedy grasses (Martin et al.
1982; Bahler et al. 1984). Switchgrass can metabolize atrazine (Weimer et al. 1988). Acceptable
stands of switchgrass could be established at a reduced seeding rate of 107 pure live seed per
m 2 when weed interference was reduced following atrazine application at time of planting (Vogel
1987). Imazethapyr {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-ethyl-
3-pyridine carboxylic acid}, applied at 70 g ai/ha before the grass seedlings emerged, provided
excellent weed control and enabled excellent stands of switchgrass to be obtained within 1 year of
planting (Masters et al. 1996). The postplant, pre-emergence application of a tank mix of quinclorac
(Paramount®; 3,7-dichloro-8-quinolinecarboxylic acid) at 1.1 kg ai/ha plusatrazine at 1.12 kg ai/ha
has provided excellent weed control in switchgrass seedlings in Nebraska, North Dakota, and South
Dakota (Mitchell, unpublished data). This herbicide treatment controlled broadleaf weeds and
weedy grasses and resulted in acceptable stands and high biomass yields. Application of imazapic
{2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-methyl-3-pyridine
carboxylic acid} on switchgrass, although effective in some trials, has resulted in significant stand
reductions in other tests. The labeled use of imazethapyr and quinclorac on switchgrass as a pre- or
postemergent herbicide varies with state or region and year. The efficacy of these herbicides does
not change, only the regulations. Maize has been successfully used as a cover crop for switchgrass
(Hintz et al. 1998). Atrazine is applied for weed control after both crops are planted. Corn is harvested
for grain and is the primary crop the year of establishment. Herbicides should be used only in
geographical regions and applications for which they are labeled.
In addition to herbicides that can be used during establishment, other herbicides are available
for use on established stands of switchgrass. Switchgrass stands are not affected by metolachlor
[2-chloro- N -(2-ethyl-6-methylphenyl)- N -(2-methoxy-1-methylethyl)acetamide] applied at rates
needed to control annual weedy grasses (Masters et al. 1996). Commercial products containing both
atrazine and metolachlor are labeled for use in seed production in some regions. Metasulfuron
(methyl 2-[[[[(4-methoxy-6-methyl-1,3,5-trizin-2-yl)-amino]carbonyl]-amino]sulfonyl]benzoate) and
clopyralid (3,6-dichloro-2-pyridinecarboxylic acid) plus 2,4-D can be used for weed control in
established stands (Anonymous 2002).
22.5.2 f Ertility
Switchgrass can tolerate low fertility conditions but it responds to fertilizer (Rehm et al. 1976; Jung
et al. 1988). It responds to N fertilization with significant increases in forage and biomass yield
(McMurphy et al. 1975; Rehm et al. 1976, 1977; Perry and Baltensperger 1979; Hall et al. 1982;
Rehm 1984; Madakadze et al. 1999a; Sanderson et al. 1999; Vogel et al. 2002a). Recommended N
fertilization rates vary with location and are primarily dependent upon precipitation, cultivar, and
harvest management. In the eastern Great Plains and the Midwest, United States, recommended
annual rates of N vary from 90 to 110 kg/ha when switchgrass is managed for hay or pasture whereas
further west, where there is less precipitation, rates of 45-70 kg/ha are used. When switchgrass is
managed for optimal biomass production in the Midwest, approximately 10-12 kg/ha N needs to
be applied for each Mg/ha of biomass yield (Vogel et al. 2002a). At fertility rates above this level,
nitrates accumulated in the soil profile. In South Dakota Conservation Reserve Program (CRP) lands
dominated by switchgrass, the application of 56 kg N/ha increased total biomass, but there was no
benefit to applying more N (Mulkey et al. 2006). Switchgrass may respond to P fertilization if the
availability of P in the soil is low (Rehm et al. 1976; Rehm 1984). Switchgrass and other C 4 grasses
should be fertilized in late spring when they are initiating growth. Early spring fertilization will
stimulate invasion by C 3 grasses and forbs (Rehm et al. 1976). Nitrogen fertilization increases the