WEED SCIENCE Pyrithiobac and Bromoxynil Combinations with MSMA for Improved Weed Control in Bromoxynil-Resistant Cotton

Pyrithiobac (Staple herbicide) and bromoxynil (Buctril herbicide) were registered for weed control in cotton in the mid-1990s. Pyrithiobac can be applied pre-emergence or post-emergence to any variety of cotton, whereas bromoxynil can be applied post-emergence only to bromoxynil-resistant cotton. Both herbicides are options for post-emergence control of several troublesome broadleaf weeds in Georgia and Florida cotton. Bromoxynil and pyrithiobac do not control sicklepod, a common weed in cotton in Georgia and Florida. Prickly sida must be small (three or fewer leaves) for effective control. Sicklepod control can be improved by post-emergence-directed application of MSMA, but successful post-emergence-directed herbicide application requires a height differential between the cotton and the weeds, and this is often difficult to obtain with sicklepod. Experiments were conducted in Georgia and Florida to determine whether control of sicklepod by bromoxynil or pyrithiobac could be increased by mixing these herbicides with MSMA at rates suitable for overtop application, and to determine whether control by these mixtures would be adequate for cotton. MSMA at 0.5 to 0.75 lb a.i. acre $1 mixed with bromoxynil or pyrithiobac increased control of sicklepod and yellow and purple nutsedge compared with bromoxynil or pyrithiobac applied alone and did not adversely affect cotton yield. Sicklepod control often was as good with MSMA alone as with the combinations. However, MSMA alone has a more limited spectrum of control than the combinations. ABSTRACT Field studies were conducted to determine the tolerance of bromoxynil (3,5-dibromo-4- hydroxybenzonitrile)-resistant cotton (Gossypium hirsutum L. 'BXN 57') and weed control with pyrithiobac {2-chloro-6-((4,6-dimethoxy-2- pyrimidinyl)thio)benzoic acid}, bromoxynil, pyrithiobac plus MSMA (monosodium methylarsonate), and bromoxynil plus MSMA applied post-emergence to one- to three-leaf cotton. MSMA at 0.6 to 0.8 kg a.i. ha $ $$ $1 added to pyrithiobac at 70 g a.i. ha $ $$ $1 or bromoxynil at 0.6 kg a.i. ha $ $$ $1 increased control of sicklepod (Senna obtusifolia (L.) H. S. Irwin & Barneby), yellow nutsedge (Cyperus esculentus L.), and purple nutsedge (Cyperus rotundus L.) compared with pyrithiobac or bromoxynil applied alone without adversely affecting cotton yield. MSMA did not improve prickly sida (Sida spinosa L.) control by pyrithiobac or bromoxynil.