Seed set control in annual ryegrass (Lolium rigidum) is a major driver of long-term seedbank reduction in cropping systems

Long term weed seedbank depletion underpins sustainable weed management yet end of season seed set control is often a lower priority compared with early season herbicidal control efforts. Here we quantified the cumulative effects of weed seed control, rotational diversity and length of pasture phase on weed biomass, weed seed production and associated seedbank. Firstly, a 13-year phased crop rotation experiment located at Cunderdin (Western Australia) (2007–2019), compared the effects of different rotations (wheat monoculture with cereal, diverse and farmer style rotations) and two residue managements (full residue retention vs windrow burning as a form of harvest weed seed control (HWSC)) on weed biomass and seedbank. An additional pasture-crop study was located at two sites (Wyening and Brookton 2019–2022) and compared wheat monoculture, chemical fallow, a diverse crop sequence and different durations of serradella ( Ornithopus sativus ) pasture phases, that varied in the frequency of late season weed seed set control, on weed pressure. At the Cunderdin crop rotation trial, residue management through the HWSC windrow burning technique was the dominant driver of weed outcomes, reducing weed biomass and substantially lowering the annual ryegrass ( Lolium rigidum Gaud.) seedbank. In the pasture–crop trials, wheat monoculture maintained the initial weed seedbank numbers across the four years at both sites. At Wyening, which had lower initial weed numbers, seedbanks declined at a constant rate of about 54% year −1 for all other rotations. At, Brookton, with higher initial weed numbers, chemical fallow had a similar seedbank decline rate to Wyening (56% year −1 ), whereas there were slower rates of decline for 3-yr pasture and killed pasture (42% year −1 and 34% year −1 , respectively). The other rotations initially produced similar reductions in seedbank density but showed substantial seedbank recovery in the final year. all treatments. The current research demonstrates that sustained, highly effective, tactics to prevent weed seed return for 3–4 years are required under high weed pressure situations. By contrast, under low initial weed pressure a wider range of tactics was shown to successfully reduce annual ryegrass seedbanks to low levels.