Herbicide resistance status of sourgrass

Sourgrass [Digitaria insularis (L.) Mez ex Ekman] is one of the most challenging herbicide-resistant weeds in the grain-producing areas of Southeast, Central-west, and Northeast of Brazil. This species is a perennial grass that is highly competitive with the C4 photosynthetic pathway. It easily adapts to different environments and reproduces through both seed and rhizomes. The objective of this review was to compile what is known about sourgrass biology, the state of herbicide resistance and its associated mechanisms, and main weed management strategies. The high seed viability, ease of dispersal and the strong propensity to develop resistance to herbicides like glyphosate and acetyl coenzyme A carboxylase (ACCase) inhibitors in the prevalent no-tillage system in Brazil make the sourgrass one of the most difficult weeds to control. Due to the great genetic variability, the resistance mechanisms conferring glyphosate-resistance among sourgrass populations range from reduced absorption, altered translocation, enhanced metabolism, target-site mutations, and 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) overexpression. In addition, Trp2027Cys mutation was found to confer cross-resistance to ACCase inhibitors. Sourgrass management strategies include herbicide rotation, herbicide tank mix or sequential applications, as well as the use of mulch to reduce infestations. These measures must be implemented before crop sowing because the range of management options is greater than in after crop sowing. Additionally, the best control of sourgrass is achieved when management is carried out during the early growth stages, before the plants develop rhizomes and form clumps.