Ground Cover Supplement : GC Supplement - Weeds
3 Issue 128 | May – June 2017 | GRDC GROUNDCOVERTM SUPPLEMENT: WEEDS GROUNDCOVER ROW SPACING PHOTO:DAVENICHOLSON NARROW ROWS AID THE BATTLE FOR LIVING SPACE An 11-year trial has shown how annual ryegrass weed seedbanks can be driven down with narrow row spacings and harvest weed seed control By Dr Catherine Borger, Glen Riethmuller and Mario D’Antuono n While narrow row spacings are generally accepted as effective in increasing a crop’s competitive advantage, this has been difficult to demonstrate in single year studies. However, in 2003 we had the opportunity to start measuring annual ryegrass seed production in a long- term crop rotation trial that had been running since 1987 at the Department of Agriculture and Food, Western Australia (DAFWA) Merredin Research Station. The trial was established to look at row spacing and stubble retention versus burning in wheat, barley, canola and pulses. Herbicides were applied to the trial each year to reflect regional practice, but by 2003 there were dense stands of annual ryegrass in the trial, particularly in the wide-row unburnt plots. By measuring ryegrass seed production over the 11 years from 2003 to 2013 we found that narrow row spacings had greater crop yield and reduced annual ryegrass seed production (see Table 1). The impact of row spacing on weed seed production compounded over time leading to a substantially higher weed seedbank in the wide-row plots compared to the narrow row spacing. Admittedly, it is not practical for growers to go to nine-centimetre spacing, especially in high-yielding areas, where high stubble loads are difficult to manage at seeding. However, modern machinery has improved the ability of growers to achieve narrow rows, and there is now the opportunity for growers to reassess their chosen crop spacing. Any reduction in row spacing will increase yield and reduce weed seed set. Narrow row spacing has been shown to increase crop yield even in the absence of weeds. This is due to canopy closure at an earlier stage, increased light interception, reduced evaporation and reduced competition within the crop for resources. HARVEST CONTROL In the long-ter m trial, residue burning reduced average crop yield, but also substantially reduced ryegrass seed at harvest. The reduced yield may be at least partially explained by the loss of stored soil moisture in burnt plots. Over the 11-year trial, burning reduced the annual ryegrass seed production to levels closer to zero in the narrow row spacings. While burning is an unlikely first choice for growers today, it is simply a for m of harvest weed seed destruction. Alternative methods are now available that will destroy annual ryegrass seed more effectively than burning the entire field and without causing a reduction in yield. These include the harvest seed destructor, chaff cart or narrow windrow burning. Finding the right method of harvest weed seed management for your far ming system is an important way to prevent input into the weed seedbank. Research aimed at perfecting these methods continues and is now being trialled in the eastern states (SFS00032). o More information: Dr Catherine Borger, DAFWA, 08 9690 2220, firstname.lastname@example.org FAST FACTS n Any reduction in row spacing will reduce the seed production of annual ryegrass in crops. n When combined with weed seed harvest, narrow row spacings can reduce annual ryegrass to levels close to zero. TABLE 1 To tal annual ryegrass seed yield at harvest was reduced by narrow row spacing and residue burning. Year Crop Annual ryegrass seeds at harvest (seeds/m2) Burnt Unburnt 9cm 18cm 27cm 36cm 9cm 18cm 27cm 36cm 2003 Wheat 120 117 170 141 324 296 702 382 2004 Wheat 42 117 213 313 318 312 757 1001 2005 Field peas 147 221 354 1101 375 558 1930 1581 2006 Wheat 5 5 22 13 14 18 29 27 2007 Barley 6 23 28 105 25 54 424 789 2008 Chemical fallow 0 0 0 0 0 0 0 0 2009 Canola 55 152 159 622 140 319 3056 3468 2010 Wheat 3 1 6 17 17 24 36 173 2011 Wheat 2 0 0 17 159 162 334 552 2012 Chickpeas 3 0 4 10 60 50 135 287 2013 Wheat 0 5 0 0 2 1 51 171 SOURCE: CATHERINE BORGER DAFWA researcher Dr Catherine Borger during a summer weeds survey in 2016.
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