Atrazine metabolism was studied in four warm-season forage grasses to determine if metabolism was the basis for differential atrazine tolerance among the grasses. Big bluestem and switchgrass are atrazine tolerant while indiangrass and sideoats grama are atrazine susceptible in the seedling stage. Metabolism of atrazine in big bluestem and switchgrass occurred primarily by glutathione conjugation. The major metabolic product isolated from indiangrass and sideoats grama was the N-deethylated metabolite of atrazine. Glutathione conjugation by big bluestem and switchgrass occurred at a faster rate than N-dealkylation of atrazine in indiangrass and sideoats grama. Differential tolerance to atrazine among the grasses studied was probably due to the metabolic route by which they detoxify atrazine and the rate of metabolism for that specific route. Intraspecific differences in atrazine tolerance in indiangrass were due to the amount of metabolite produced in relationship to the amount of parent atrazine remaining in the shoot tissue. The more tolerant indiangrass lines had a higher metabolite to parent atrazine ratio than susceptible lines. This study confirmed differences in seedling atrazine tolerance of four indiangrass lines observed in previous greenhouse studies.

Experiments were conducted near Scottsbluff, NE in 1991 and 1992 to evaluate the selectivity of imazethapyr for weed control in blue grama, intermediate wheatgrass, little bluestem, Russian wildrye, switchgrass, and western wheatgrass. Imazethapyr reduced weed biomass in all seedings and injured all perennial grasses 12 days after treatment (DAT). Imazethapyr reduced the height of intermediate wheatgrass, Russian wildrye, and switchgrass 35 DAT. Russian wildrye, and switchgrass biomass 50 DAT was reduced by imazethapyr. Weed biomass in new seedings of intermediate wheatgrass and western wheatgrass were reduced more than that of the other grasses.

Quinclorac will control leafy spurge and not injure many established native grasses and forbs. Seeding of desirable species is often required to reestablish native vegetation after an invasive weed-management program, but quinclorac residue may inhibit the reestablishment of native species. Greenhouse studies were conducted to estimate quinclorac dissipation rates in Northern Great Plains soils and the effect of residue on establishment of some native grass and broadleaf plants. Quinclorac 50% dissipation time (DT50) ranged from > 21 to 112 d in four soils from the Northern Great Plains. The quinclorac DT50 was dependent on several factors including soil type, moisture content, temperature, and especially organic matter (OM). Across four different soil textures, quinclorac dissipation generally increased as soil moisture content increased, but moisture had less of an impact in low OM soils. Quinclorac dissipation also increased as temperature increased in the four soils. The most rapid dissipation occurred in soils with higher OM (> 6%), with an average DT50 of < 38 d, at 45% moisture content, held at 16 C. Wild bergamot, purple coneflower, blanketflower, and stiff goldenrod seedling growth were all reduced by quinclorac residue at 6 μg kg−1, the lowest concentration evaluated in the study. The native grass species big bluestem, intermediate wheatgrass, and switchgrass generally were tolerant of quinclorac, but green needlegrass was sensitive, and seedling growth declined as quinclorac residue increased from 6 to 375 μg kg−1. Based on a quinclorac application of 840 kg ha−1 and 150 frost-free d, seeding of sensitive forbs and grasses should be delayed at least 12 mo after herbicide application.

Aphthona spp. flea beetles have established and reduced the density of leafy spurge in much of the western United States. One way to measure the long-term impact and effectiveness of a weed control program is by monitoring the changes in the seedbank over time. The change in leafy spurge stem density and seed in the seedbank were evaluated 5 yr after Aphthona spp. were released to control this weed in the Little Missouri National Grasslands in southwestern North Dakota. Leafy spurge density and seed in both loamy overflow and loamy ecological sites decreased, whereas desirable (high-seral) forbs increased 5 yr after the biological control agents were released. Leafy spurge topgrowth was reduced from an average of over 200 stems/m2 to less than 8 stems/m2 in the most densely infested sites, and leafy spurge seed was reduced from an average of 68% of the seedbank to only 14% in both ecological sites. High-seral forb seed increased by over 300% in the seedbank, which indicated the floristic quality of the sites, was returning to a preinfestation state. Species with the largest increase included western rock jasmine and fringed sage, which increased at least three-fold in both sites. Less desirable low-seral forbs and grasses accounted for about 30% of the seedbank. In a greenhouse study, native grass production was reduced nearly 50% when grown in soil from Aphthona spp. release sites compared to nonrelease sites. Switchgrass production was reduced to a greater extent (66%) than green needlegrass, little bluestem, or western wheatgrass. The cause and extent of reduced native grass production in sites where Aphthona spp. were released has yet to be determined. The decrease in leafy spurge topgrowth and seed in the soil seedbank as desirable species seed increased, should lead to the long-term recovery of the plant community.

Switchgrass is a potential feedstock for cellulosic bioenergy production. Weed competition from annual grass during the establishment year can reduce switchgrass establishment and resulting productivity, but the relationship between early season grass densities and outcomes of competition are not well understood. We measured how a range of giant and yellow foxtail densities in the establishment year influenced switchgrass establishment and resulting productivity in the first production year (second year of the growing season). In two of the three site–yr more than four foxtail plants m−2 reduced switchgrass plant densities below documented thresholds of establishment success. A lesser effect of foxtails in the third site–year suggested that higher switchgrass emergence rates reduced foxtail competitive ability during establishment. Effects on yield were consistent over the three site–yr. The yield (10.96 Mg ha−1 ± 0.77) decreased rapidly as foxtail density increased. One foxtail plant m−2 reduced switchgrass yield in the first production year by 25%, and yield loss was 90% or greater at densities > 50 foxtail plants m−2. Although switchgrass can establish in the presence of foxtail competition, these weed species should be controlled to maximize yields in the first production year.

We tested the effects of seeding date and weed control during switchgrass establishment in a field experiment that was conducted in central Pennsylvania in 2007 and repeated in 2008. Switchgrass was no-till seeded in early May, late May, and mid-June, and three postemergence weed management treatments were evaluated, including Mow (only a single mowing), Broadleaf (2,4-D + dicamba), and Broad Spectrum (2,4-D + dicamba + atrazine + quinclorac). Switchgrass density increased at later seeding dates, except in 2008, when the middle seeding date had the lowest density. In both years, weed biomass in late summer was lowest in the last seeding date of the Broad Spectrum treatment. In contrast, switchgrass biomass in late summer was greatest in the first seeding date of the Broad Spectrum treatment in both years. In the year after establishment (production year), plots were split to test the effects of supplemental weed control, composed of metsulfuron + 2,4-D applied in May, on total aboveground yield. Supplemental control in the production year increased total aboveground yield in the Mow treatment only, indicating that effective weed control during the establishment year might reduce the need for weed control in the following year. Although maximum aboveground yield was achieved when switchgrass was seeded in May and herbicides were used, results from our experiment suggest that seeding switchgrass at a relatively high seeding rate in June in our study region and mowing annual weeds to reduce competition and prevent seed production could be an effective strategy if minimizing herbicide use is a priority.

Interest in the cultivation of bioenergy feedstocks has increased the need for information in this rapidly developing sector of agriculture. Many fast-growing, large-statured perennial grasses have been selected because of their high biomass production potential, competitive nature, and ability to tolerate marginal growing conditions. However, weed pressure in the establishment phase can be detrimental to crop yield. Weed control is one of the most costly and resource-intensive aspects of bioenergy crop establishment. Unfortunately, little information exists on practical weed management techniques for the majority of these new crops. The tolerance of switchgrass, big bluestem, reed canarygrass, sorghum, giant reed, eulaliagrass, and giant miscanthus (sterile and seeded) to 22 PRE and 22 POST herbicides were evaluated. Plants were grown in the greenhouse and evaluated for injury, height, and aboveground biomass after 5 or 7 wk for PRE and POST applications, respectively. PRE and POST application of 2,4-D, bentazon, bromoxynil, carfentrazone, dicamba, halosulfuron, and topramezone did not significantly injure any species. Giant miscanthus was more tolerant to PRE herbicides when established from rhizomes compared with seed establishment. Supporting previous research, all eulaliagrass and switchgrass cultivars demonstrated comparable tolerance to PRE application of all 22 herbicides. With the information gained in this study a suite of herbicides may have potential for use in bioenergy crops; however, they should be tested on larger-scale field trials over multiple growing seasons to validate initial findings.

Research on best management practices, including vegetative filter strips, is needed to evaluate the potential for reducing herbicides in surface runoff. Laboratory studies were conducted to determine the influence of different filter strip components on fluometuron adsorption. Samples were taken from a switchgrass filter strip (1-m wide) established on a Brooksville silty clay. Sampled components included switchgrass stems clipped to 4 cm, plant residue on the soil surface, and topsoil < 5-mm, 0.5- to 1-cm, 1- to 3-cm, and 3- to 5-cm deep. The filter strip topsoil samples contained 1.8, 2.2, 2.3, and 2.8% organic matter for the aforementioned depths, respectively, compared with 1.8% in soil collected from an adjacent cropped area. Fluometuron adsorption on each sample was compared at initial concentrations of 0.017, 0.5, 1.0, 2.0, and 4.0 mg L−1, the last of these representing four times the peak concentration in surface runoff. Fluometuron adsorption was greater in soil from filter strip areas than in soil from cropped areas. Averaged over concentration, the soil–water partition coefficient (K d) value for soil 1- to 3-cm and 3- to 5-cm deep was greater than the value for soil from the cropped area. Stems and residue had K d values 4.9- and 4.1-fold greater, respectively, than soil from the cropped area. Average adsorption coefficients normalized for organic carbon content (K oc) of soil < 5-mm and 3- to 5-cm deep from filter strip areas were greater than the values for soil from cropped areas.

We conducted a field experiment in 2007 and 2008 and repeated it in a separate field in 2008 and 2009 to test the effectiveness of two herbicides applied at two different times on weed control during switchgrass establishment. At 4 and 6 wk after switchgrass planting (WAP), sulfosulfuron was applied at 0.04 and 0.07 kg ai ha−1 with nonionic surfactant and quinclorac was applied at 0.28, 0.42, and 0.56 kg ai ha−1 with methylated seed oil. Herbicides applied at 4 WAP tended to be more effective than at 6 WAP. Sulfosulfuron provided greater control of smooth pigweed; however, quinclorac provided greater control of yellow foxtail, a grass weed that has traditionally been difficult to control with herbicides in switchgrass. Average yellow foxtail control was 73, 87, and 93% for quinclorac at 0.28, 0.42, and 0.56 kg ha−1, respectively, compared to 62 and 60% for sulfosulfuron at 0.04 and 0.07 kg ha−1, respectively. Switchgrass injury (chlorosis and height reduction relative to the untreated control) was observed, but most symptoms were not detectable by 8 wk after treatment (WAT) for most treatments. Plots that received quinclorac at 0.56 kg ha−1 at 6 WAP tended to have relatively low weed biomass and high total aboveground yield in the establishment year and relatively high total aboveground yield in the year after establishment.

Diflufenzopyr is an auxin-transport inhibitor that has increased broadleaf weed control by some auxin herbicides. The effects of auxin herbicides when applied with diflufenzopyr for leafy spurge or Canada thistle control, herbage production, and herbicide absorption and translocation in leafy spurge were evaluated. The influence of diflufenzopyr on leafy spurge and Canada thistle control varied by herbicide. Diflufenzopyr applied with quinclorac increased both Canada thistle and leafy spurge control. Diflufenzopyr added to picloram increased leafy spurge but not Canada thistle control, whereas control of both weeds generally increased when diflufenzopyr was applied with dicamba. Canada thistle control slightly improved when diflufenzopyr was applied with clopyralid or clopyralid plus 2,4-D compared with the herbicides alone. Diflufenzopyr did not influence weed control from imazapic and reduced weed control from glyphosate. Weed control was not influenced by the ratio of diflufenzopyr to herbicide or by whether diflufenzopyr was tank mixed with or applied before the herbicides. Diflufenzopyr did not affect cool-season grass production in the greenhouse or field but did reduce sideoats grama and switchgrass production when applied alone and with dicamba in greenhouse trials. 14C-picloram and 14C-quinclorac absorption was increased when applied with diflufenzopyr in leafy spurge but nearly all absorbed herbicide remained in the treated leaf compared with the herbicides applied alone. 14C-dicamba absorption decreased from 60 to 14% when applied with diflufenzopyr and translocation to the roots decreased by a factor of 10. Overall, the increase in weed control from the addition of diflufenzopyr was much more pronounced with leafy spurge than with Canada thistle.

When invasive weeds are removed with herbicides, revegetation of native species is often desirable. The extended soil activity of aminocyclopyrachlor is important for long-term weed control but could reduce recovery of native species as well. The effect of aminocyclopyrachlor applied alone or with chlorsulfuron on cool- and warm-season grass species commonly used for revegetation was evaluated. The cool-season grasses included green needlegrass, intermediate wheatgrass, and western wheatgrass, whereas the warm-season grasses were big bluestem, sideoats grama, and switchgrass. A separate experiment was conducted for each species. Aminocyclopyrachlor was applied at 91 to 329 g ha−1 alone or with chlorsulfuron from 42 to 133 g ha−1 approximately 30 d after emergence. Warm-season grasses generally were more tolerant of aminocyclopyrachlor than the cool-season grasses evaluated in this study. Switchgrass and big bluestem were the most tolerant of the warm-season species when aminocyclopyrachlor was applied at 168 g ha−1 and averaged 199 and 150% forage production, respectively, compared with the control. Green needlegrass was the most tolerant cool-season grass. Western wheatgrass was the least tolerant species evaluated because forage production only averaged 32% of the control the year after treatment and thus would not be suitable for seeding if aminocyclopyrachlor was applied. The effect of chlorsulfuron applied with aminocyclopyrachlor varied by grass species. For example, green needlegrass injury 8 wk after treatment (WAT) averaged 30 and 48% when aminocylopyrachlor was applied alone, respectively, but injury was reduced to less than 16% when aminocyclopyrachlor was applied with chlorsulfuron. However, injury on the less-tolerant intermediate wheatgrass ranged from 48 to 92% by 4 WAT when aminocyclopyrachlor was applied alone and from 60 to 86% when chlorsulfuron was included in the treatment.

Perennial grasses are expected to comprise a substantial portion of the lignocellulosic biomass to meet renewable energy mandates in the U.S. in the next decade. As many warm-season grasses are slow to establish from seed, plantings are often compromised by weed interference during the establishment year. Greenhouse experiments were conducted to determine the tolerance of switchgrass and prairie cordgrass to several herbicides applied PRE or POST (at four different growth stages). Preemergence atrazine at rates ≤ 1.684 kg ai ha−1 in switchgrass and quinclorac at rates ≤ 0.279 kg ai ha−1 in prairie cordgrass did not significantly reduce emergence, plant height, or biomass yield 8 wk after treatment. When treatments were applied at the two- to three-leaf stage, only atrazine (≤ 0.123 kg ai ha−1) did not reduce switchgrass fresh weight and only 2,4-D ester (≤ 0.533 kg ae ha−1), nicosulfuron (0.018 kg ai ha−1), and quinclorac (0.140 kg ha−1) did not significantly reduce prairie cordgrass yield. Phytotoxic effects decreased for all herbicides with increasing growth stage at the time of treatment for both species. All evaluated herbicides were safe with respect to biomass yield on the respective grasses when applied at the latest growth stage (approximately five-leaf stage). These results show that viable PRE and POST herbicides are available for weed control during establishment of switchgrass and prairie cordgrass; however, all evaluated herbicides would likely reduce biomass yield in a mixture planting of both grasses.