MISSISSIPPI
Extension Rice Specialist
bgolden@drec.msstate.edu
Early season rice injury due to off-target preemergence soybean herbicides has been a continual issue in the Mississippi Delta over the past several years. In the past, most of the early season issues centered on glyphosate drift, but with the evolution of resistant palmer amaranth, a shift has been observed to paraquat mixtures with residual herbicides.
Off-target drift occurs from numerous soybean residual herbicide chemistries, and proper identification is needed to determine if the rice crop can be kept or should be replanted. Since 2015, graduate student Ben Lawrence has been working with Jason Bond to determine how numerous soybean herbicides influence the rice crop’s growth and develop. In a two-part study, Ben has evaluated paraquat applied alone or in combination with residual chemistries at multiple rice growth stages.
To date, Ben’s research has shown that rice injury from paraquat was greater than 40 percent regardless of application time (spiking rice to panicle differentiation) 14 days after application. His research shows injury remained up to 28 days after application from treatments that contained only paraquat.
Across all treatment timings, delays in rice maturity were greater than six days but ranged up to two weeks when applications were made during reproductive growth stages. This data is important because delays in maturity result in added expense in rice inputs to finish off the crop.
The greatest yield loss occurred with rice damaged during reproductive growth (12 bushels per acre). Yield losses to rice exposed to paraquat between spiking and one leaf were 7 bushels per acre. In the tankmixture study, metribuzin or fomesafen applied alone resulted in less than 10 percent rice injury.
However, when these herbicides were tankmixed with paraquat, injury increased to 58 and 68 percent for fomesafen and metribuzin, respectively, at two weeks after application. In these trials, yield was reduced up to 28 percent compared to untreated rice, with mixtures resulting in similar yield loss.
Keeping a crop or replanting after a drift event is always a judgment call, depending on crop growth stage, how much of the field is affected and stand loss. However, research like Ben’s, gives us data to help make that decision by allowing us to have an indication on rice performance after the event occurs.
Delays in rice maturity in the early season can result in added herbicide, fertilizer and irrigation expense that must be taken into account with planting date data to determine the best solution moving forward.