Integrating New Technology and Strategies for Management of Glyphosate-resistant Palmer amaranth (Amaranthus palmeri) in Cotton (Gossypium hirsutum)

The commercialization of glyphosate-resistant (GR) crops revolutionized weed management. Following widespread adoption of this technology and extensive reliance on glyphosate, GR Palmer amaranth evolved. To control GR Palmer amaranth, glufosinate and the protoporphyrinogen oxidase (PPO) inhibiting herbicides are largely relied upon and resistance to these herbicides is now a concern. Experiments were conducted to evaluate new technology and strategies for controlling GR Palmer amaranth in cotton and avoid glufosinate- and PPO-resistance.;In no-till cotton production, use of flumioxazin preplant followed by fomesafen preemergence (PRE) to control Palmer amaranth is standard. These herbicides are PPOinhibitors. To reduce selection pressure on the PPO-inhibitors, field experiments were conducted to determine the potential to substitute diuron for one PPO herbicide. Applied preplant, flumioxazin was more effective than diuron. Programs with flumioxazin preplant followed by diuron PRE were as effective as flumioxazin preplant followed by fomesafen PRE. Therefore, it is feasible to reduce PPO selection pressure without sacrificing Palmer amaranth control.;Pyroxasulfone and acetochlor, similar herbicides to S-metolachlor, effectively control Palmer amaranth. Field experiments were conducted to determine cotton tolerance and Palmer amaranth control with pyroxasulfone, acetochlor, and S-metolachlor applied PRE and postemergence (POST). Pyroxasulfone and acetochlor applied PRE were effective for control of Palmer amaranth. All herbicides applied POST in combination with glyphosate did not increase Palmer amaranth compared to glyphosate alone. Cotton was less tolerant to pyroxasulfone applied PRE or POST than acetochlor applied PRE or POST or S-metolachlor applied POST. Another experiment determined cotton was tolerant to acetochlor combinations applied PRE and acetochlor effectively controlled Palmer amaranth.;Cotton cultivars with tolerance to both glyphosate and glufosinate are currently popular. Field research evaluated Palmer amaranth and annual grass control with glyphosate and glufosinate co-applied or sequentially applied. Greater GR Palmer amaranth control was achieved following sequential applications of glufosinate compared with glyphosate. In general, co-application of glufosinate plus glyphosate was more effective than glyphosate alone but never more effective than glufosinate alone. Glyphosate controlled goosegrass greater than glufosinate and a similar response was observed for large crabgrass. Co-applied glufosinate and glyphosate were always less effective than glyphosate alone on annual grasses, but not more effective than glufosinate alone. Glufosinate followed by glyphosate and glyphosate followed by glufosinate was the preferred order for control of Palmer amaranth and annual grasses, respectively.;Cotton tolerant to dicamba has recently been deregulated and will be commercialized during 2015. Field research was implemented to determine the appropriate timing of dicamba applications to control GR Palmer amaranth in glyphosate- and glufosinate-based systems. Dicamba increased Palmer amaranth control when added PRE and at all POST timings when added to glyphosate. However, in the glufosinate-system, dicamba only improved Palmer amaranth control when the weed was large at time of application.;Fluridone inhibits the biosynthesis of carotenoids. In previous research, cotton tolerance and residual control of Palmer amaranth by fluridone was excellent. However, crops often grown in rotation with cotton are thought to be sensitive to fluridone. Field research was conducted to determine the carryover potential of fluridone to corn, peanut, grain sorghum, and soybean following applications to cotton the previous year. Minimal visual injury of rotational crops was observed. More importantly, yield of non-treated rotational crops was equivalent to crops treated with fluridone the year prior.;Field experiments were conducted to determine if pyrithiobac enhances necrosis caused by acetochlor and S-metolachlor on cotton. Pyrithiobac slightly increased necrosis caused by S-metolachlor. However, pyrithiobac did not increase cotton necrosis caused by acetochlor. For control of Palmer amaranth, pyrithiobac seemed more important than Smetolachlor and acetochlor to programs utilizing glyphosate. However, in glufosinate-based systems, S-metolachlor and acetochlor were more important than pyrithiobac.