Field research was conducted for a period of 2 yr to evaluate the response of soybean and cotton to simulated drift rates representing 12.5, 6.3, 3.2, 1.6, and 0.8% of the usage rates of 1,120 g ai/ha glyphosate (140, 70, 35, 18, and 9 g/ha, respectively) and 420 g ai/ha glufosinate (53, 26, 13, 7, and 4 g/ha, respectively). Early-postemergence applications were made to 2- to 3-trifoliate soybean and 2- to 3-leaf cotton, and late applications to soybean at first flower and cotton at early bloom. A mid-postemergence application was also made to cotton at pinhead square (first flower bud development). Soybean and cotton injury and height reductions occurred in most cases for only the two highest rates of the herbicides with variation noted between years. Soybean height was reduced by no more than 11%, regardless of herbicide rate or timing. On the basis of visual injury, soybean was more sensitive to glyphosate than to glufosinate when applied early in 1998, but sensitivity was equal for both the herbicides in 1999. When herbicides were applied late, soybean was more sensitive to glufosinate in the first year. Cotton was more sensitive to glufosinate 7 d after application in both years, regardless of timing, but by 28 d differences between herbicides were less apparent. Cotton maturity was not delayed by either herbicide, on the basis of days to first square or flower and nodes above white flower. Both crops were able to recover rapidly from herbicide injury, and yields were not affected negatively.

Nomenclature: Glyphosate; glufosinate; cotton, Gossypium hirsutum L. ‘DPL 33B’, ‘Paymaster 1560B’, ‘Stoneville 474’, ‘Suregrow 125’, ‘Suregrow 747’; soybean, Glycine max (L.) Merr. ‘Asgrow 5959’, ‘Dekalb 5850’, ‘DPL 3588’, ‘Pioneer 9594’, ‘Terral 5893’.

Additional index words: Crop injury, herbicide drift, off-target movement.

Abbreviations: DAT, days after treatment; NAWF, nodes above white flower.