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Simple crop rotations like maize and soyabean grown in succession often coincide with simple weed management tactics, resulting in weed infestations that are highly adapted to the conditions maintained for the selected crops. Amaranthus tuberculatus is a prominent weed species which possesses biological characteristics that allow it to persist in maize–soyabean rotations. Spring wheat and oats emerge earlier than A. tuberculatus and are grown in narrower rows than maize and soyabean, which may increase their competitive capability. A field study which measured morphological and phenological characteristics of three A. tuberculatus cohorts that emerged under fallow, maize, soyabean, spring wheat and oats was conducted to compare their competitive ability. Early emerged A. tuberculatus grew similarly in fallow, maize and soyabeans and grew to heights of >150 cm but was significantly reduced in spring wheat and oats, never exceeding 40 cm. Heights of the last emerged cohorts were more influenced by crop canopies never reaching 15 cm in spring wheat and oats compared to >70 cm in fallow. Early emerged A. tuberculatus canopy diameter and branches plant−1 were reduced by >80% in spring wheat and oats compared to fallow. Flowering was delayed in spring wheat and oats compared to fallow and seed production and biomass were reduced by 99% across cohorts. Growth suppression of A. tuberculatus, based on the findings, can be achieved under dense, competitive crop canopies such as in spring wheat and oats and these crops should be considered as additional crops in maize–soyabean rotations. [ABSTRACT FROM AUTHOR]