To determine the long term impact of narrow row spacing and residue burning or retention on annual ryegrass seed production.
Average crop yield was greater in the unburnt plots (1.64t/ha) compared to the plots where residue was burnt each autumn (1.53t/ha). Average yield also increased at narrower row spacing, with 1.66, 1.64, 1.55 and 1.49t/ha in the 90, 180, 270 and 360cm row spacing treatments.
Average annual ryegrass seed production at harvest was lower in the burnt plots, with 56 and 296 seeds/m2 in the burnt plots and unburnt plots. Seed density was greater in the wide row spacing treatments, with 57, 77, 222 and 332 seeds/m2 in the 9, 18, 27 and 36cm row spacing treatments.
Burning stubble was a highly effective method of weed control, reducing annual ryegrass seed production to close to zero by the end of the 11 year management period. However, burning over the entire plot area caused a yield reduction (possibly due to reduced soil moisture retention). Alternative methods of harvest weed seed destruction (like narrow windrow burning, chaff carts etcetera) will destroy annual ryegrass seed without causing a reduction to yield.
Narrow row spacing reliably reduced annual ryegrass seed set and also increased crop yield. This is partially due to reduced weed competition, but narrow row spacing increases crop yield in the absence of weeds, due to canopy closure at an earlier stage, increased light interception, reduced evaporation and reduced intra-species competition for resources. Very narrow row spacing is not viable in high yielding areas, where the resulting crop residue is difficult to manage during the subsequent seeding operation. While growers may not want to use 9cm spacing, any reduction in their current spacing will increase crop yield and reduce weed seed production.