Arsenical herbicides have been used extensively, but reports indicate that they have often reduced yields and/or delayed maturity.

We investigated application rates, dates of application, genetic tolerance to MSMA, and possible interactions of Pix with varieties and MSMA. Randomized split-split, plot designs were used in five experiments from 1979 to 1982. Main plots consisted of MSMA overtop at 1.12 kg/ha ai applied "early", 2.24 kg/ha ai applied "early", 1.12 kg/@ia ai applied "early" plus 1.12 kg/ha applied "late", 2.24 kg/ha ai applied "late" and an untreated check. "Early" applications were made to cotton 10 to 12 cm talL. Cotton was 25 to 35 cm tall when the "late" treatments were applied. Subplots were 'Coker 304', 'Deltapine 55', 'Deltapine 61', 'Hancock', 'Stoneville 21.3' and 'Stoneville 825'. Sub subplots consisted of an Untreated check or Pix at 48 g/ha ai except in 1979. All applications were aqueous sprays in a volume of 187 L/ha. Weeds were mechanically controlled to minimize competition effects.