The potential distribution of the herbaceous weed Seneeio madagaseariensis Poir.

Sonchus oleraceus (common sowthistle) is a dominant weed and has increased in prevalence in conservation cropping systems of the subtropical grain region of Australia. Four experiments were undertaken to define the environmental factors that favor its germination, emergence, and seed persistence. Seeds were germinated at constant temperatures between 5 and 35°C and water potentials between 0 and -1.4 MPa. The maximum germination rate of 86-100% occurred at 0 and -0.2 MPa, irrespective of the temperature when exposed to light (12 h photoperiod light/dark), but the germination rate was reduced by 72% without light.At water potentials of -0.6 to -0.8 MPa, the germination rate was reduced substantially by higher temperatures; no seed germinated at a water potential >-1.0 MPa. Emergence and seed persistence were measured over 30 months following seed burial at 0 (surface), 1, 2, 5, and 10 cm depths in large pots that were buried in a south-eastern Queensland field. Seedlings emerged readily from the surface and 1 cm depth, with no emergence from below the 2 cm depth. The seedlings emerged during any season following rain but, predominantly, within 6 months of planting. Seed persistence was short-term on the soil surface, with 2% of seeds remaining after 6 months, but it increased with the burial depth, with 12% remaining after 30 months at 10 cm.Thus, a minimal seed burial depth with reduced tillage and increased surface soil water with stubble retention has favored the proliferation of this weed in any season in a subtropical environment. However, diligent management without seed replenishment will greatly reduce this weed problem within a short period.

The adoption of no-till farming and the desire to maintain stubble cover when sowing legumes in northern New South Wales and southern Queensland have resulted in an increase in commercial row spacing for chickpea (Cicer arietinum L.). This paper examines the effects of increasing crop row widths on weed competition in chickpea crops. Weed densities of 0, 2, 4, 8, 16, and 32 plants/m2 of wild oats (Avena sterilis ssp. ludoviciana ) and turnip weed (Rapistrum rugosum) were established with chickpea crops planted with either 32�or 64�cm row configurations in northern New South Wales during 1996 and 1997. A rectangular hyperbolic model adequately represented the loss in chickpea yield with increasing density of either weed. Even low densities of <10 plants/m2 caused large (approx. 50%) reductions in yield, particularly with turnip weed. In these experiments, weed-free yields were higher when chickpea was sown in 32�cm rows compared with 64�cm rows, but weeds caused no greater loss in crop yield with the wider row spacing. The results of this work show that the use of wide rows has minimal impact on weed competion in northern chickpea crops.