Antagonistic Interaction of Propanil and Pyribenzoxim on Barnyardgrass Control

The interaction of two postemergence herbicides, propanil (3ʹ4ʹ-dichloropropioanilide) and pyribenzoxim-{benzophenone O-[2,6-bis[(4,6-dimethoxy-2-pyrimidinyl)oxy]benzoyl]oxime, code name; LGC-40863}, was investigated using barnyardgrass [Echinochloa crusgalli (L.) Beauv.] at the four-leaf stage. Propanil at rates from 0.5 to 4 kg ai ha-1 was antagonistic to pyribenzoxim, particularly at higher doses of propanil. Propanil treatment during the period of 1 day before through 5 days after pyribenzoxim treatment was antagonistic. This antagonism was evident only when the two herbicides were applied to the same leaf. No antagonism occurred when the two herbicides were applied to separate leaves. Foliar absorption of [14C] pyribenzoxim was about 32% of the total applied at 48 h after treatment and was not affected by propanil. Translocation of radiolabel out of the treated leaf was about 11% of the total absorbed 48 h after treatment, but translocation was reduced to 3.5% when propanil was applied to the same leaf. Translocation of radiolabel increased to 15% when propanil was applied to an adjacent leaf. Chlorophyll fluorescence analysis showed that propanil decreased the quantum yield of the treated leaf, but did not affect the untreated leaves of the same plant. When the pyribenzoxim-treated leaf was excised, within 5 days after treatment, plants were not completely controlled. Thus, translocation within 5 days of treatment is essential for pyribenzoxim activity. These data suggest that propanil decreases whole plant activity of pyribenzoxim in barnyardgrass by reducing translocation to adjacent tissues.