A field study was conducted from 1999 through 2001 at Stoneville, MS, to determine the effects of bromoxynil-resistant (BR) and glyphosate-resistant (GR) cotton rotation systems under ultranarrow- (25-cm spacing) and wide- (102-cm spacing) row planting on weed control, weed density and shift, and cotton yield. The four rotations during 3 yr included BR–BR–BR, GR–GR–GR, BR–GR–BR, and GR–BR–GR, all with bromoxynil or glyphosate postemergence (POST) only or following fluometuron plus pendimethalin preemergence (PRE). Control of hemp sesbania, pitted morningglory, prickly sida, and hyssop spurge was ≥97% regardless of row width, rotation, and herbicide program. Control of common purslane, sicklepod, and smooth pigweed was higher with glyphosate POST in GR cotton than with bromoxynil POST in BR cotton. Broadleaf and yellow nutsedge weed biomass were higher with bromoxynil POST in BR cotton than with glyphosate POST in GR cotton. Continuous BR cotton system resulted in higher densities of common purslane, sicklepod, and yellow nutsedge (15.3, 1.5, and 373 plants/m2, respectively) compared with continuous GR cotton (0.7, 0.1, and 1.0 plant/m2, respectively). Seed cotton yield was consistently higher in wide- than in ultranarrow-row cotton. Seed cotton yield was lower in continuous BR cotton than in the other three rotation systems, and yields greatly improved when BR cotton was rotated with GR cotton. During a 3-yr period, seed cotton yields with glyphosate POST only (4,000 to 4,890 kg/ha) or after PRE herbicides (4,480 to 4,860 kg/ha) were similar in GR cotton, whereas in BR cotton, bromoxynil POST only (1,390 to 4,280 kg/ha) resulted in lower yield than did bromoxynil POST after PRE herbicides (2,550 to 4,480 kg/ha). The results indicated that the shift in spectrum of weeds toward more tolerant species and yield decline in continuous BR cotton can be prevented by rotating BR with GR cotton.
Nomenclature: Bromoxynil; fluometuron; glyphosate; pendimethalin; common purslane, Portulaca oleracea L. #3 POROL; hemp sesbania, Sesbania exaltata (Raf.) Rydb. ex A.W. Hill # SEBEX; hyssop spurge, Euphorbia hyssopifolia L. # EPHHS; pitted morningglory, Ipomoea lacunosa L. # IPOLA; prickly sida, Sida spinosa L. # SIDSP; sicklepod, Senna obtusifolia (L.) Irwin & Barneby # CASOB; smooth pigweed, Amaranthus hybridus L. # AMACH; yellow nutsedge, Cyperus esculentus L. # CYPES; cotton, Gossypium hirsutum L. ‘BXN 47’, ‘DP 436 RR’.
Additional index words: Bromoxynil, fluometuron, glyphosate, pendimethalin, rotation, transgenic cotton, weed density, weed shift.
Abbreviations: BR, bromoxynil resistant; GR, glyphosate resistant; POST, postemergence; PRE, preemergence; WAA, weeks after second POST application.