Graduate Thesis Or Dissertation
 

Effect of glyphosate when applied to grass and legume seeds or preemergence to bentgrass (Agrostis tenuis Sibth.)

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  • Paraquat (1,11-dimethy1-4,4'-bipyridinium ion) and glyphosate [N (phosphonomethyl)glycine] are herbicides utilized for chemical seedbed preparation in the establishment of grass and legume seed crops. When these herbicides are applied after seeding the crop, seeds which are not protected by a layer of soil could be exposed to the herbicide. Effects of paraquat on exposed seeds have been studied previously, but more information was needed on effects from glyphosate. Greenhouse studies were conducted using direct applications of glyphosate and paraquat over exposed seeds of six grass species: Kentucky bluegrass, perennial ryegrass, bentgrass, tall fescue, fine fescue, and orchardgrass; and two legume species, alfalfa and red clover. Glyphosate tended to be toxic to all species tested, reducing germination and particularly seedling growth. Paraquat was toxic only to grasses and did not affect legumes. The major detrimental effect of glyphosate on legume seeds and seedlings was caused by direct contact of the herbicide with seeds, but a minor effect was detected when untreated seeds were transferred to treated soil, indicating that glyphosate was not inactivated in soil as fast as paraquat. These results indicate that the use of glyphosate in chemical seedbed preparation may be detrimental to the new crop if the herbicide is applied directly to exposed seeds. Glyphosate has been reported by many research workers to be inactivated almost immediately upon contact with the soil. However, a few growers in western Oregon have reported phytotoxic effects on grass seedlings when glyphosate applications were made shortly before emergence of the crop. Greenhouse studies were conducted to determine whether glyphosate could have herbicidal activity when applied directly to soil prior to bentgrass emergence (from 1 to 5 days) and to examine some soil factors involved. Soil activity of glyphosate differed considerably among three high-organic soils and one mineral soil. The soils in which glyphosate was most active contained the highest levels of organic matter (64% and 28%), but there was no strong correlation with organic matter content in the other soils or in soil blends containing various levels of organic matter. Varying phosphorus levels did not significantly influence activity of glyphosate. There was no evidence that glyphosate formed an inactive complex with Fe or Al. The two soils in which glyphosate was most active, organic soils No. 3 and 4, contained the highest levels of Fe and at least equal levels of Al. Increasing pH increased glyphosate activity markedly in the mineral soil and one high-organic soil, while slightly decreasing activity in two high-organic soils. Although none of these soil factors proved conclusively to be entirely responsible for variations in glyphosate soil activity, general results from the series of experiments indicated that glyphosate can definitely be taken up from soil by crop plants and produce injury. The herbicide remained available in the soil solution for sufficient time to cause significant plant growth reduction. Consequently, a waiting period of several days should elapse from glyphosate treatment to emergence of a crop.
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