Control of Amaranthus spp. using the allelopathic potential of Brassica extracts

In response to the increasing resistance of various Amaranthus biotypes to herbicides, this study evaluated the allelopathic potential of aqueous extracts from canola (Brassica napus), mustard (Brassica juncea), and forage radish (Raphanus sativus) against purple amaranth (Amaranthus hybridus), creeping amaranth (Amaranthus deflexus), and common amaranth (Amaranthus viridis). Germination and early growth bioassays were conducted in a completely randomized design with four replicates, using five extract concentrations (0%, 12%, 25%, 50%, and 100%). A preliminary phytochemical screening, based on qualitative reactions of each crude extract, was also performed. Extract concentrations of 50% and 100% completely inhibited both germination and seedling development across all Amaranthus species, with A. deflexus exhibiting the highest sensitivity. In contrast, the 12% concentration unexpectedly stimulated growth in all tested species. All extracts negatively affected the germination speed index, with a 100% reduction observed at the highest concentration. Although hypocotyl length did not differ significantly among treatments, root system development was severely impaired, leading to poor shoot growth. Seedlings exposed to increasing extract doses displayed clear phytotoxic symptoms, culminating in necrosis that precluded fresh weight measurements. Phytochemical analysis revealed the presence of seven allelochemically active compound classes: coumarins, steroids, flavonoids, phenolics, glycosides, tannins, and terpenes. These findings indicate that brassicaceous extracts possess strong allelopathic activity and may offer a viable alternative for managing Amaranthus species.