Background: Hairy fleabane (Conyza bonariensis L.) is a difficult weed to control in soybean production systems as it presents resistance to some herbicides; thus, leading to substantial grain yield losses. Objective: The objective of this study was to evaluate the efficacy and selectivity of herbicides applied before and after soybean planting on glyphosate-resistant C. bonariensis. Methods: Field experiments were conducted over two soybean growing seasons in 2021/22 and 2022/23 near Erechim, Rio Grande do Sul, Brazil. Early weed management treatments included terbuthilazine (1,500 g ha-1), mesotrione+atrazine (120+1,200 g ha-1), metsulfuron-methyl (6 g ha-1), dicamba (720 g ha-1), 2,4-D amine (1,612 g ha-1), flumioxazin (50gha-1), diclosulam (35.28gha-1)andatrazine+simazine(1,500+1,500gha-1) applied 60 days before soybean planting. For pre-plant burndown, diquat + saflufenacil (600+70 g ha-1) were applied three days before soybean planting. For post-emergence weed control, 2,4-D choline+glyphosate (971.28+1232.65 g ha-1) were applied at the V3 to V4 soybean growth stage. The C. bonariensis plants present in the experimental area were resistant to the herbicide glyphosate. Results: The best early hairy fleabane control was achieved with mesotrione+atrazine, dicamba, and 2,4-D amine, providing more than 90% efficacy. Diquat+saflufenacil applied pre-plant burndown provided excellent level of control, particularly following dicamba in early management, reaching 100% control. Post-emergence application of 2,4-D choline+glyphosate after early treatments with mesotrione+atrazine or dicamba resulted in 100% control. Conclusions: The combination of atrazine + simazine for early management, diquat + saflufenacil for pre-plant burndown, and 2,4-D choline+glyphosate post-emergence effectively controlled hairy fleabane and contributed to higher grain yield.
Background: Paspalum urvillei Steud. (vaseygrass) is an important Poaceae species in several countries. This species is native to Brazil; however, it has become a troublesome weed, especially in pasture areas in Florida, USA. Vaseygrass is botanically similar to forage species in Brazil, including Paspalum notatum, Paspalum dilatatum, and Urochloa brizantha. Therefore, herbicide options to control vaseygrass are limited. Also, polyploid grasses often possess enhanced herbicide detoxification capacity, which can affect control of this species. Objective: Determine the ploidy level and evaluate chemical control of two populations of vaseygrass historically exposed to herbicide selection pressure. Methods: The ploidy level was evaluated using flow cytometry, and the chemical control was assessed using five herbicide treatments in two experiments: Control, imazapic, nicosulfuron, glyphosate, and nicosulfuron + glyphosate. Visible injury, plant height, culm diameter, number of leaves, total relative chlorophyll content, number of tillers, and dry matter were evaluated. Results: The ploidy analysis revealed that both populations had similar DNA content and were classified as tetraploids. The average DNA content in accessions of vaseygrass was 2.31 pg for Fence and 2.25 pg for Pasture. Considering visible injury, glyphosate and nicosulfuron + glyphosate caused 65% at 7 days after treatment. Conclusions: The two populations P. urvillei are tetraploids. Glyphosate alone or mixed with nicosulfuron are the best herbicide options to control P. urvillei.
