Graduate Thesis Or Dissertation
 

Sethoxydim herbicide in red fescue and bentgrass : levels and mechanisms of selectivity

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  • The herbicide rate required to reduce growth by 50% (GR₅₀) of 11 cultivars of red fesue (Festuca rubra L. and F. rubra ssp. commutata) and for four cultivars of bentgrass (Agrostis tenuis Sibth. and A. stolonifera L. var. palustris) to sethoxydim (2-[1-(ethoxyimino) butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one) was determined in greenhouse and laboratory experiments. Field experiments compared only red fescue cv.. 'Pennlawn' and bentgrass cv. 'Penncross'. The GR₅₀ values for Pennlawn red fescue in the the field, the greenhouse, and the laboratory were 30 kg/ha, 15 kg/ha, and 5.3 mM, respectively. Penncross bentgrass was 400, 1400, and 12,900 times more susceptible than red fescue in the field, in the greenhouse, and in laboratory experiments, respectively. Part of the difference in tolerance was due to the different stage of growth of the plants. The milli-molar concentration of xylene and surfactant used as a solvent for the technical grade sethoxydim was as toxic to red fescue without sethoxydim as when sethoxydim was included. Cultivars of red fescue differed in their response to sethoxydim, but bentgrass cultivars did not. The GR₅₀'s of other species tested in the greenhouse were > 4 kg/ha for annual bluegrass (Poa annua L.), 0.22 kg/ha for tall fescue (F. arundinacea Schreb.), 0.37 kg/ha for downy brome (Bromus tectorum L.), and 0.017 kg/ha for Italian ryegrass (Lolium multiflorum Lam.). Translocation and metabolism of ¹⁴C sethoxydim was studied in whole plants of Pennlawn red fescue and Penncross bentgrass and contrasted with that of soybeans (Glycine max L.) cv. 'Amsoy'. More ¹⁴C translocated from the treated leaf in soybeans than in red fescue or bentgrass. Little movement of ¹⁴C occurred in bentgrass during 7 days after application of herbicide. The four major metabolites measured in these experiments constituted over 80% of the total activity applied. The principal solvent system for differentiating metabolites was a 1:4 (v/v) benzene:acetone mixture. The metabolite with an Rf value 0.5 appeared more in red fescue than in bentgrass, but the quantity of the metabolite was insufficient to ascribe all the tolerance of red fescue to the appearance of metabolites. Not all the ¹⁴C radioactivity applied to red fescue could be accounted for in the plants and in the nutrient solution, whereas all the radioactivity could be accounted for in bentgrass and in soybean. Some of the radioactivity in red fescue was given off as a gas and was trapped in monoethanolamine, but not enough was captured to account for all the ¹⁴C applied to the plants. The loss of ¹⁴C suggests that red fescue tolerance is physiological, either a lack of a site of activity or an effective method of detoxifying sethoxydim.
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