Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Plant, Environmental Management and Soil Sciences

First Advisor

James Griffin


Field experiments were conducted to evaluate the influence of simulated insect defoliation and full season weed competition on soybean (Glycine max (L.) Merr.) growth and yield. Weeds were johnsongrass (Sorghum halepense (L.) Pers.), common cocklebur (Xanthium strumarium L.), and hemp sesbania (Sesbania exaltata (Raf.) Rybd. ex A. W. Hill) at 15, 3, and 12 plants/6 m of row. Simulated defoliation at R2 and R5 soybean growth stages was accomplished by removal of 0, 1, or 2 leaflets per soybean trifoliate, which approximated 0, 33, and 66% defoliation, respectively. Averaged across defoliation levels and stages, johnsongrass, common cocklebur, and hemp sesbania reduced soybean yields 30, 15, and 14%, respectively, in 1994 compared with no weed interference. In 1995, common cocklebur did not affect yield, whereas johnsongrass reduced yield 35%. As defoliation level increased, a linear decrease in soybean yield was observed. Averaged across weeds and defoliation stages, 33 and 66% defoliation reduced soybean yield 6 and 20% in 1994 and 12 and 33% in 1995, respectively. Defoliation at R5 resulted in 10% lower yield than defoliation at R2 in one of two years. Yield reduction due combinations of weeds and defoliation was additive. Field experiments evaluated the influence of hemp sesbania and sicklepod (Senna obtusifolia (L.) Irwin and Barneby) on insecticide deposition within the soybean canopy and resultant soybean looper (Pseudoplusia includens (Walker)) control. Dye-sensitive cards placed in top, middle, and bottom portions of the soybean canopy measured spray droplet deposition for the insecticide thiodicarb applied at 504 g ai/ha in 94 L/ha spray volume with a ground sprayer. Spray droplet deposition was highest on cards placed in the top of the soybean canopy, and weeds reduced deposition 26 to 43% compared with weed-free soybean. Thiodicarb deposition within the middle and bottom levels of the canopy was not reduced by weeds. Weeds, however, did not influence thiodicarb efficacy against soybean looper in the field or in laboratory feeding bioassays. Control of both weeds and defoliating pest is important; however, management strategies for soybean looper may not need to be altered when weeds are present.