Horseweed Control with Preplant Herbicides after Mechanical Injury from Small Grain Harvest

Abstract

Horseweed [Conyza canadensis (L.) Cronquist] plants present during small grains harvest are variable in size, but tall plants are mechanically cut (“topped”) by the combine. Horseweed is best controlled prior to planting double‐cropped soybeans, which must be planted quickly after harvest to preserve yield potential. This urgency eliminates an opportunity for horseweed regrowth, leaving topped plants with reduced leaf area for herbicide spray interception, likely impacting herbicidal control, but little information has been reported in this regard. Horseweed control is further complicated by the prevalence of multiple herbicide‐resistant biotypes. Therefore, field research was conducted to evaluate herbicidal control of glyphosate‐susceptible horseweed that was topped (cut to 46 cm tall) prior to treatment. Additional greenhouse experiments evaluated glyphosate‐resistant horseweed at the rosette and bolt growth stages as well as topped plants prior to treatment. Paraquat alone or in combination with 2,4‐D or dicamba, resulted in 80 to 100% control of topped horseweed across field and greenhouse experiments. In greenhouse experiments, the rosette growth stage was most effectively controlled, followed by the bolt growth stage, which was similar or more effectively controlled, compared with topped horseweed. The authors recommend paraquat with 2,4‐D or dicamba for control of topped horseweed plants, due to efficacy and mixing of multiple effective herbicides. Core Ideas Small grains harvest equipment can cut horseweed, reducing herbicide efficacy. Paraquat containing treatments controlled rosette, bolt, and topped horseweed ≥80%. Dicamba and 2,4‐D controlled rosette stage glyphosate‐resistant horseweed >90%. Authors recommend paraquat with 2,4‐D or dicamba to control topped horseweed.

Conyza sumatrensis Resistant to Paraquat, Glyphosate and Chlorimuron: Confirmation and Monitoring the First Case of Multiple Resistance in Paraguay

“…This reinforces the need to use different herbicides to control weeds, focusing not only on management, but also on preventing the selection of new resistant biotypes. Other studies demonstrated the effectiveness of these herbicides in the control of species of the genus Conyza [25][26][27][28][29]-in most situations, in combination with other herbicides, including products with confirmed resistance.…”

Herbicide alternative for Conyza sumatrensis control in pre-planting in no-till soybeans

“…Thus, the use of herbicides with different mechanisms of action, the combination of herbicides, and the adoption of other tools, in addition to chemical control, are essential in preventing the selection of resistant weed biotypes, as well as in their management (Gage et al, 2019). Studies show the effectiveness of dicamba (Flessner, Pittman, 2019), halauxifen (Krenchinski et al, 2019), glufosinate (Tahmasebi et al, 2018;Zobiole et al, 2018), saflufenacil (Budd et al, 2017), diclosulam (Braz et al, 2017, Krenchinski et al, 2019, especially in combinations, in the control of Conyza spp. and that can be used as control options in the cases of resistance.…”

Agronomic implications of paraquat ban in Brazil

“…Paraquat is widely used to control species such as Conyza spp., in Brazil and other countries, during the off-season and in pre-sowing, after the application of glyphosate plus other herbicides (Santos et al, 2015;Zobiole et al, 2018;Cesco et al, 2019;Flessner, Pittman, 2019;Soltani et al, 2020). In recent years, there has been a reduction in the effectiveness of paraquat in controlling Conyza spp.…”