Controlling invasive annual grasses in grazed pastures: population dynamics and critical gap sizes.
Vulpia (Vulpia bromoides and V. myuros) and barley grass Hordeum murinum are prevalent annual grass weeds of native grasslands and grazed pastures in temperate climates, for which more effective control strategies are needed. Annual grass weeds can negatively impact natural grassland ecosystem function, in addition to causing productivity loss in agricultural systems. We investigated the effects of gap size, time of sowing, grazing method (continuous or rotational grazing) and species sown in the gap (vulpia and/or barley grass) on vulpia and barley grass panicle production and intrinsic rate of population growth (λ) in southern Australian pastures. From these data, we estimated the critical gap size below which vulpia and barley grass populations would decline (λ <1). Panicle production declined rapidly with decreasing gap size, particularly in rotationally grazed pastures. Barley grass produced more panicles than vulpia in some treatments, while time of sowing, and sowing these annual species in mixture had little effect on panicle production. The rate of population growth (λ) increased with gap size, but at the same rate in rotationally and continuously grazed pastures. There was no effect of annual species treatment (vulpia or barley grass) or time of sowing on the rate of population growth. The critical gap diameter was 0.04 cm and 2.31 cm for continuously and rotationally grazed pastures, respectively. A sensitivity analysis showed that reducing plant fecundity (seeds plant-1) and propagule survival prior to seedling establishment by 60% could increase the critical gap diameter by 1.8 cm. Synthesis and applications. The results of this study highlight the importance of minimizing bare ground throughout autumn and winter to suppress annual grass weed population growth, as λ was unaffected by time of sowing. Estimates of λ and the critical gap diameter show that rotational grazing will better control weedy annual grasses than continuous grazing. Further, similar sensitivities of the critical gap diameter to fecundity and propagule survival prior to establishment lend support to weed management strategies that focus on reducing both fecundity and propagule survival.