Conifer phytotoxicity and vegetation control efficacy of ten selected herbicides Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/3484zk42m

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  • Ten herbicides (atrazine, azafenidin, chlorsulfuron, clopyralid, hexazinone, imazapic, imazapyr, metsulfuron, pendimethalin, and sulfometuron) were evaluated for phytotoxicity in first-year seedlings of eight conifer species (Douglas-fir, grand fir, noble fir, redwood, Sitka spruce, western hemlock, western red cedar, and western white pine). Six of these herbicides (atrazine, clopyralid, hexazinone, imazapyr, metsulfuron, and sulfometuron) were also evaluated for vegetation control efficacy. Azafenedin, chiorsulfuron, imazapic, imazapyr, metsulfuron, pendimethalin, and sulfometuron were applied as fail site preparation treatments prior to spring conifer planting. Atrazine, clopyralid, hexazinone, and sulfometuron were applied as spring release treatments after conifer planting. All herbicides were applied at an operational rate (lx), as determined by label recommendations and industry standards, and a rate equal to three times the operational rate (3x). Treatments were replicated at three sites in western Oregon and Washington. Most of the herbicides tested in this study exhibited high levels of conifer safety. At the operational rate, hexazinone and sulfometuron were the only herbicides to cause significant injury to any of the conifer species tested. The hexazinone lx treatment caused mortality in western red cedar. The spring sulfometuron lx treatment caused mortality in grand fir and both sulfometuron lx treatments caused growth reductions in western red cedar. In addition, sulfometuron produced a subtle but non-significant trend of growth inhibition in five of the remaining six species, and appears to be a mild growth inhibitor in most conifers. Otherwise, none of the herbicides caused significant injury at the operational rate. Hexazinone and sulfometuron afforded the best overall weed control of the herbicides tested, Sulfometuron provided equal or better weed control as a fall site preparation treatment than as a spring release treatment. Atrazine provided the best combination of weed control efficacy and overall conifer safety. Clopyralid provided good control of weeds in the Asteraceae and Polygonaceae families, but was generally ineffective against other species. Metsulfuron and imazapyr were generally ineffective against the herbaceous species present in this study. Herbicide recommendations for various combinations of conifer crop and target weed species are provided in convenient reference tables. These tables facilitate the design of herbicide prescriptions for a variety of reforestation scenarios in the Pacific Northwest. Few of these herbicides are acutely toxic to any of the conifer species tested. We specify the conifer/herbicide combinations that should be avoided. Otherwise, foresters have a great deal of flexibility in developing vegetation management prescriptions with these chemicals.
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