Producers commonly apply two or more herbicides in a single application to improve the weed control spectrum, reduce production costs, and/or prevent the development of herbicide resistance in weed populations. Studies were established at the Louisiana State University AgCenter Rice Research Station and the Mississippi State University Delta Research and Extension Center to evaluate several herbicide mixtures and their impact on several weed species. Previous research indicates a potential for synergism between a pre-packaged mix of propanil plus thiobencarb and imazethapyr when mixed for the control of red rice (Oryza sativa L.) and barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.].

Two studies were established to evaluate the interactions between the pre-packaged mix of propanil plus thiobencarb and the individual components when mixed with imazethapyr. The experimental design was a randomized complete block with four replications in a two-factor factorial arrangement of treatments. In the first study, factor A consisted of imazethapyr at 0 and 70 g ai/ha. Factor B consisted of no mixture herbicide, propanil plus thiobencarb at 1680 and 3360 g ai/ha, propanil at 840 and 1680 g ai/ha, and thiobencarb at 840 and 1680 g ai/ha. Propanil and thiobencarb rates are equivalent to those found in the pre-packaged mixture of propanil plus thiobencarb. In the second study imazamox at 0 and 44 g ai/ha was substituted for imazethapyr. A modified Colby’s equation using a non-linear mixed model analysis was used to determine if a synergistic, antagonistic, or additive response occurred.

Red rice control at 14 days after treatment (DAT) resulted in a synergistic response when imazethapyr was mixed with propanil plus thiobencarb at both rates evaluated and propanil at 1680 g/ha. An additive response was observed for red rice control when imazethapyr was mixed with propanil at 840 g/ha and both rates of thiobencarb. An additive response was also observed with all mixtures evaluated for barnyardgrass control.

At 21 DAT, imazethapyr mixed with both rates of propanil plus thiobencarb resulted in a synergistic response for red rice control, while all other mixtures evaluated resulted in an additive response. A mixture of imazethapyr with propanil plus thiobencarb increased barnyardgrass control and was also deemed synergistic. An additive response was shown with propanil at 840 and 1680 g/ha and thiobencarb at 840 and 1680 g/ha when mixed with imazethapyr for barnyardgrass.

At 35 DAT, imazethapyr mixed with propanil plus thiobencarb at 3360 g/ha increased control of red rice and resulted in a synergistic response. All other mixtures displayed an additive response.  Imazethapyr mixed with propanil plus thiobencarb at both 1680 and 3360 g/ha was shown to be synergistic for the control of barnyardgrass. An additive response was observed with imazethapyr mixed with propanil or thiobencarb.

In general, yield increased as herbicide rates increased. Rice treated with imazethapyr mixed with propanil plus thiobencarb resulted in a yield of 6850 kg/ha, which further supports the synergistic response observed for control of red rice and barnyardgrass.

In conclusion, imazethapyr with propanil plus thiobencarb was shown to be synergistic for control of red rice for all evaluations and barnyardgrass at 21 and 35 DAT. This increased weed control translated into higher yields. This mixture can also be beneficial by helping prevent or slow the development of imazethapyr resistant red rice and barnyardgrass.