Field research was conducted during 3 yr to evaluate response of rice and corn 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, and 4 g/ha, respectively). Early-postemergence applications were made to two- to three-leaf rice and six-leaf corn, and late-postemergence applications to rice at panicle differentiation and to corn at nine-leaf stage (1 wk before tasseling). Crop injury was generally greater for the two highest rates of both herbicides when applied early. Little to no reduction in rice or corn height was observed with glufosinate. Glyphosate consistently reduced rice plant height when the two highest rates were applied early, and heading was delayed 2 to 5 d. In 2 of 3 yr, the highest rate of glyphosate reduced rice yield 99 and 67% when applied early and 54 and 29% when applied late. Germination of rice seeds from glyphosate-treated plants was reduced in 1 of 2 yr and for only the highest rate. For glufosinate, rice yield was reduced 30% and in only one year when applied late at the highest rate. Early application of glyphosate reduced corn yield an average of 22 to 78% for the three highest rates, but only for the highest rate at the late timing (33%). Corn yield was reduced an average of 13 and 11% for the highest rate of glufosinate at the early and late timings, respectively. In greenhouse studies, five rice varieties were equally sensitive, as were five corn varieties, to reduced rates of glyphosate and glufosinate.
Nomenclature: Glufosinate; glyphosate; corn, Zea mays L. ‘Asgrow 897’, ‘Dekalb 687’, ‘Mycogen 8460’, ‘Pioneer 3223’, ‘Terral 2930’; rice, Oryza sativa L. ‘Bengal’, ‘Cocodrie’, ‘Cypress’, ‘Drew’, ‘Jefferson’.
Additional index words: Crop injury, herbicide drift, off-target movement, rice germination, rice vigor.
Abbreviations: DAP, days after plating; DAT, days after treatment.