Control of glyphosate-resistant (GR) horseweed in no-till cotton has become a serious challenge for midsouth producers. Control of GR horseweed prior to no-till cotton planting (burndown) was examined with glufosinate applied at a standard rate (0.47 kg ai/ha) 30, 21, and 14 d before planting (DBP) and at 0.35 and 0.58 kg ai/ha 14 DBP. Glufosinate at 0.47 kg ai/ha was also tank-mixed with 2,4-D, dicamba, or flumioxazin 30 DBP, and with diuron, pendimethalin, prometryn, or glyphosate at 14 DBP. Glufosinate applied at the standard rate 21 or 30 DBP did not control GR horseweed (<55%), but did provide better control (85%) when applied 14 DBP. The high rate of glufosinate (0.58 kg ai/ha) applied 14 DBP controlled GR horseweed. Tank mixtures of glufosinate with dicamba or 2,4-D at 30 DBP controlled GR horseweed. Tank mixtures of glufosinate with either prometryn or diuron provided initial burndown and residual weed control. Tank mixtures of glufosinate pendimethalin or flumioxazin did not increase control of existing horseweed, but did provide residual control 56 days after application.
Nomenclature: Horseweed, Conyza canadensis (L.) Cronq. #3 ERICA, cotton, Gossypium hirsutum L. ‘FiberMax 966’.
Additional index words: Burndown, herbicide application timing.
Abbreviations: DAA, days after application.
