Roadside mowing spreads yellow bristle grass (Setaria pumila) seeds further than by natural dispersal

Yellow bristle grass is a highly invasive pasture weed that has spread rapidly through many dairying regions. Seven trials were conducted on roadsides infested with yellow bristle grass to evaluate natural and assisted dispersal. To trap seeds, yellow sticky traps were laid out at various intervals both perpendicular to and parallel to the road. Traps were in place for 24 h in the four natural dispersal trials but only for the event in the mowing trials. Seeds on the retrieved traps were counted and the seeds caught in the mower estimated. For natural dispersal, 90% of seeds fell within 0.5 m. When mown, 90% of the seeds fell within 2 metres in the direction of mowing and 80% within 20 cm in the perpendicular direction. Additionally, hundreds of seeds were caught in the mower and presumably could fall off anywhere. Mowing mature yellow bristle grass on the roadside will result in the seeds being spread along the roadside for many metres and potentially many kilometres.