Adaptation of grassweeds to spring cropping through changes in germination, flowering time and fecundity.

The ability of weed populations to adapt is pivotal for their success in overcoming adverse conditions, including control measures and climate change. Here, we show evidence for the local adaptation of blackgrass (Alopecurus myosuroides) and Italian ryegrass (Lolium multiflorum) to the distinct seasonal windows for crop establishment. We assessed germination, flowering, and fecundity traits in 16 parental populations (eight each of A. myosuroides and L. multiflorum) sourced from fields with repeated autumn- or spring-cropping backgrounds. We find evidence of contrasting maternal and genetic influences on the response of A. myosuroides germination to light exposure. Alopecurus myosuroides seeds collected from fields with a spring-cropping background demonstrated higher germination rates (86%) in darkness than seeds collected from fields with an autumn-cropping system (46%), evidence of maternal effects. Conversely, when parent plants from those same autumn and spring background populations were grown under identical conditions, the offspring seeds (one generation removed from the field) from an autumn-cropping linage germinated more in darkness (81%) than those from a spring-cropping linage (42%), evidence of a contrasting genetic effect. Parental biotypes of both species from fields with a spring-cropping history flowered earlier under spring-like vernalisation treatments, including no vernalisation, compared to their autumn counterparts. Additionally, parental seeds from spring-cropping fields of both species had higher seed production under treatments reflecting the cropping history of their parent populations. These findings provide phenotypic evidence for the potential adaptation of A. myosuroides and L. multiflorum to spring cropping systems. Diversifying cropping practices, such as integrating seasonal rotations within crop establishment windows, is therefore essential for disrupting weed adaptation and promoting sustainable weed management strategies.