When it comes to tillage, timing matters and drives weed communities

Tillage is a foundational management practice in many cropping systems to manage weed seedbanks and reduce reliance on herbicides for weed management. Tillage alters established weed communities but can also stimulate weed seed germination and emergence. Hence, viewed within a community assembly framework tillage is a strong assembly filter that can either constrain or advance the membership of species within the subsequent weed community. At four locations in the Northeastern United States, twelve treatments of a single primary tillage (10-cm depth) event at 2-week intervals over the course of the growing season were compared using a randomized complete block design with four replications. The emergent weed community in each treatment replicate was quantified 6 weeks after the tillage operation. We identified three main periods of tillage timing that resulted in similar communities. Across all sites, total weed density tended to be greatest and weed evenness tended to be lowest when soils were tilled early in the growing season. The timing of tillage explained approximately 50% of the weed community variability. Weather conditions, mainly growing degree days, but also precipitation occurring before tillage, were important factors and could improve our ability to predict the impact of tillage timing on weed community assemblages. No single trait or combination of traits were consistently associated with species-by-tillage time groupings across locations; however, within each location several traits were associated with particular groups of species, including seed length, seed weight, cotyledon type, life span, ploidy level and photosynthetic pathway.