Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Plant, Environmental Management and Soil Sciences

First Advisor

James L. Griffin


In field studies conducted in Louisiana in 1990 and 1991, area of influence, density, and duration of interference methodologies were used to evaluate wild poinsettia (Eupborbia heterophylla L.) interference in soybean (Glycine max (L.) Merr.). In 1990, soybean seed yield within 10 cm of wild poinsettia was similar to distances of 10 to 20 and 20 to 40 cm, but was less than that for distances greater than 40 cm from the weed. In contrast, soybean yields in 1991 growing within 10 cm of the weed were less than at greater distances. Weed interference resulted in a 9.4 and 18% reduction in soybean seed yield within 10 cm of the weed in 1990 and 1991, respectively, when compared with 80 to 100 cm. Full-season interference of 8 wild poinsettia per 6 m of row reduced soybean yield 17%. Yield was also reduced 26% from a natural infestation of wild poinsettia after 10 wk of competition. Postemergence applications of chlorimuron, imazaquin, fomesafen, and acifluorfen following preemergence applications of clomazone, metribuzin, and metribuzin plus chlorimuron at labeled rates enhanced wild poinsettia control when compared with the preemergence herbicides applied alone. Soybean seed yields in both years were generally not increased with additional postemergence applications, but foreign material in harvested seed was reduced both years. In other studies, fomesafen, acifluorfen, imazaquin, lactofen, and chlorimuron applied early postemergence when wild poinsettia plants were 2 to 8 cm tall, improved weed control compared with a late application at 10 to 15 cm in 1 of 2 yr. Soybean seed yields in 1990 for all herbicide treated plots were at least 55% higher than the untreated check. Percent foreign material and moisture in harvested seed were reduced by at least 33 and 25%, respectively, compared with the untreated check. An early freeze in 1991 negated differences in foreign material and moisture content. In greenhouse and field studies, imazaquin, fomesafen, acifluorfen, and chlorimuron, applied postemergence, provided similar control when applied to weeds 5 to 7 cm and 8 to 10 cm tall, but weed control was less when herbicides were applied at 15 to 20 cm tall. Herbicide application to weeds 8 to 10 cm tall reduced both new weed emergence observed with the earlier application and weed regrowth that is common following the late application. Wild poinsettia seed production was greatest with the later herbicide application.