Doctoral Dissertations

Date of Award

12-2000

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Plant, Soil and Environmental Sciences

Major Professor

Thomas C. Mueller

Committee Members

Bob Hayes, Neil Rhodes, William Hart, David Coffey

Abstract

Field trials were conducted near Milan, TN in 1998 and 1999 to evaluate weed control, crop response, lint yield and quality, and net returns of standard, bromoxynil resistant, and glyphosate tolerant cotton (Gossypium hirsutum) management systems. Smooth pigweed (Amaranthus hybridus), common cocklebur (Xanthium strumarium), entireleaf momingglory (Ipomoea hederacea var. integriuscula), and large crabgrass (Digitaria sanguinalis) were controlled > 90 % by all weed control programs. Goosegrass (Eleusine indica) was controlled greater than 97 % with all programs that included a soil-applied herbicide (fluometuron and/or pendimethalin) or a late post- directed (PD) application of cyanazine + MSMA. Red sprangletop (Leptochloa filiformis) was controlled at least 93 % in all systems that included a soil-applied herbicide. The total postemergence (POST) treatment of glyphosate followed by (fb) glyphosate in glyphosate tolerant cotton controlled goosegrass and red sprangletop 80 to 82 % at 8 to 10 wk after planting. Red sprangletop control in the total POST system of glyphosate fb cyanazine + MSMA was poor (33 %).

Lint yields obtained from weed free checks were similar for the three varieties 'STV 474,' 'STV BXN47,' and 'Paymaster 1220RR.' With the exception of bromoxynil early POST fb cyanazine + MSMA, lint yields were similar for all systems that included soil-applied herbicide(s) or two applications of glyphosate. Lint yields in the glyphosate tolerant systems that included a soil-applied herbicide were 130 kg/ha greater than the bromoxynil system. Lint yield from the bromoxynil system was equivalent to both standard programs and the total POST programs in glyphosate tolerant cotton.

Net returns were highest for the pendimethalin fb glyphosate fb glyphosate program, but this program was not different from the total POST program of glyphosate fb glyphosate or pendimethalin + fluometuron fb glyphosate fb cyanazine + MSMA. The addition of soil-applied herbicides to the total POST glyphsate fb cyanazine + MSMA increased net return. However, no increase in net return was obtained by the addition of pendimethalin to the total POST glyphosate fb glyphosate program. Net returns following the pyrithiobac standard system and the bromoxynil system were significantly lower than those obtained from glyphosate systems.

A cotton variety decision aid model was created using weed control efficacy information reported on herbicide labels and in university extension publications. In fields with a history of weed problems that include weedy Amaranthus species (Amaranthus hybridus, A. retroflexus, and A. palmeri, respectively), spotted spurge (Euphorbia maculata), sicklepod (Senna obtusifolia), or purple and yellow nutsedges (Cyperus rotundus and C. esculentus), the model did not recommend bromoxynil resistant cotton varieties. Likewise, glyphosate tolerant varieties were not recommended for areas that have a history of weed problems including morning glories (Ipomoea spp.), hemp sesbania (Sesbania exaltata), Florida pusley (Richardia scabra), or spreading dayflower (Commelina diffusa). Fields with a history of weed problems that included a cross section of those weeds listed for bromoxynil resistant and glyphosate tolerant were recommended to be planted to conventional varieties. In addition, producers are advised not to use pyrithiobac in areas with a history of major weed problems resulting from spotted spurge, sicklepod, purple or yellow nutsedge, common lambsquarters (Chenopodium album), common ragweed (Ambrosia artemisiifolia), tall morningglory (Ipomoea purpurea), or tropic croton (Croton glandulosus var. septentrionalis). Growers were also recommended not to use pyrithiobac in fields with a history of weeds resistant to acetolactate synthase (ALS) inhibiting herbicides. The decision aid presented utilizes a "decision tree" approach where the cotton variety is selected to optimize weed control, minimize herbicide resistance development, and maximize producer profitability.

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