Glyphosate-resistant (GR) horseweed has become a major problem in many row crop production systems in the United States. Horseweed is a winter annual weed common in no-till production systems. Fall-applied herbicides were compared with spring-applied treatments for the control of horseweed. In cotton, fall-applied trifloxysulfuron provided similar or greater control of horseweed when compared to spring-applied treatments of glyphosate + dicamba. Cotton yields with fall-applied trifloxysulfuron, clomazone, and flumioxazin were comparable to or better than spring-applied glyphosate + dicamba both years. Fall-applied cloransulam-methyl, flumetsulam, sulfentrazone, and the combination of chlorimuron-ethyl + metribuzin resulted in horseweed control and soybean yields comparable to spring-applied glyphosate + 2,4-D both years. Multiple-resistance to glyphosate and paraquat exists in a horseweed population from Mississippi. Herbicide rates of 0.066 kg ae/ha glyphosate and 0.078 kg ai/ha paraquat were required to reduce susceptible horseweed biomass 50%; whereas, rates of 0.78 kg/ha glyphosate and 0.67 kg/ha paraquat were required to reduce biomass of resistant horseweed to a similar intent. This is the first broadleaf weed species reported as exhibiting multiple-resistance to glyphosate and paraquat. The addition of metribuzin to paraquat improved control of paraquat-resistant horseweed. Paraquat at 0.84 kg/ha plus all rates of metribuzin controlled 15-cm tall horseweed at least 90% both years compared to 73% with 0.84 kg/ha paraquat alone. The addition of 1 and 2% methylated seed oil (MSO) to saflufenacil controlled horseweed 91 and 93%, respectively compared to 78% control with saflufenacil alone. The addition of saflufenacil to glyphosate improved control of GR horseweed from 50% to 100% at 21 d after treatment; control of horseweed with the combination of saflufenacil + glyphosate was additive. Saflufenacil did not affect absorption of glyphosate in glyphosate-susceptible horseweed; however, absorption increased in GR horseweed from 36 to 44% at 48 h after treatment with the addition of saflufenacil when compared to glyphosate alone treatments. Overall, the addition of saflufenacil reduced glyphosate translocation in horseweed at least 6%; however, due to the exceptional efficacy of saflufenacil on horseweed these reductions did not reduce control of GR horseweed.

Italian ryegrass (Lolium perenne) is one of the most troublesome weeds of rice in Mississippi. Its resistance to multiple modes of action has made it more difficult to control in recent years. The most effective and economical management strategy to control Italian ryegrass is fall-applied residual herbicides; however, the most effective products are not labeled for use in the fall prior to rice seeding. Therefore, research was conducted in Stoneville, MS, to evaluate the effect of fall-applied residual herbicides on rice growth and development. Acetochlor should not be applied in fall targeting Italian ryegrass in fields where rice is scheduled for seeding the following spring. Clomazone remains the only viable treatment as a fall-applied residual herbicide in rice areas.