Row spacings a tactical weed suppressor

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Sowing crops so they run in rows from east to west, sowing crop rows closer together and choosing a bulkier variety are three ways to suppress weeds and improve the effectiveness of herbicides.

A tractor sowing a crop

Weed control can begin with both the width and orientation of sowing rows.

The rise in herbicide-resistant weeds in northern cropping systems has meant that in some situations weeds are dictating more and more how growers can farm.

With multiple resistance occurring in some weed species, such as ryegrass and wild oats, there is an increasing need for growers to use some non-chemical measures to improve the effectiveness of their herbicides.

Varying row orientation and row spacing, as well as choosing a competitive crop, can all help suppress weed growth says Greg Brooke, R&D agronomist with the New South Wales Department of Primary Industries. Mr Brooke presented the results of several non-chemical weed-control trials conducted in northern NSW at the recent GRDC Research Update at Coonabarabran, NSW.

“All of these strategies work on the principle of increasing crop competition so that the emerging weeds are disadvantaged,” Mr Brooke says. “The extra competition for light, moisture and nutrients places extra pressure on the weed population.”

Mr Brooke says in situations where weeds have been sprayed with in-crop herbicides the extra competition makes it harder for the weeds to recover.

“Competitive strategies enable additional suppression of weeds over what herbicides can do alone.”

Sow east to west

Orienting crop rows at 90 degrees to the sunlight direction, so they run east–west, works on the principle that the crop will intercept more sunlight than crops in rows running north–south. Mr Brooke says this gives weeds less chance to develop between the crop rows.

In Western Australia, researchers have measured approximately 30 per cent more light being intercepted by east–west-sown crops compared with north–south. This has been tested at both windy and non-windy sites and the effects in weed suppression are most likely from row-direction factors. Researchers found that surface soil moisture was also better in east–west rows during the growing season.

In 2012, Matt Gardner and his team established a trial at Bithramere, NSW, to test this theory, using two barley varieties – Hindmarsh and Skipper Australia – into which he sowed 44Y84 canola as a substitute weed. Almost 40 per cent less weed biomass grew between the crop rows that ran east–west compared with crops sown north–south. Similar results have also been found at similar latitudes in WA.

“WA has been running this kind of trial since 2002 and has consistently found that an east–west crop orientation decreases weed biomass by 30 to 50 per cent,” Mr Brooke says.

“These trials in NSW were the first of their kind to validate these results on a similar latitude in an eastern state, and show that row orientation at 90 degrees to the sun’s rays can drastically reduce weed bulk in eastern growing regions.”

Sow closer rows 

As part of the Bithramere 2012 trial, Mr Gardner and his team also used 30-centimetre and 50cm row spacings to see if the extra space between crops might allow more weeds to grow within the crop rows. They found in this trial that row spacing had less effect in reducing weeds than row orientation, although the wider row spacing did reduce crop yield by 11 per cent.

However, a trial at the Trangie Agricultural Research Centre in 2011 produced quite different results to those at Bithramere. In this trial fleabane was sown in 33cm and 66cm row spacings of the wheat variety LongReach Crusader. Researchers measured 120 per cent more fleabane in the stubble immediately after harvest on the wider rows.

“Obviously 66cm is an excessively wide row space for winter cereals, but the trial showed that the effect of row space is real and measurable, and can add significantly to other weed-control practices,” Mr Brooke says.

Based on these and past trial results in WA, Mr Brooke agrees with the findings that narrower row spacings give better weed suppression. In WA, the ideal set-up is about 25cm for disc seeders and about 30cm for tyne seeders, and is potentially narrower for eastern regions.

Choose competitive varieties

Weed competition trials with barley show there is as much difference between barley varieties as there can be between crop types.

The Bithramere trial showed the more vigorous barley variety Skipper Australia reduced weed (canola) biomass by 30 to 40 per cent more than Hindmarsh.

Skipper Australia also out-yielded Hindmarsh with both weeds present and not present and at both 30cm and 50cm row spacings.

Mr Brooke says bulkier barley varieties such as Granger, Scope and Commander will give the greatest reduction in weeds but will also need more water to grow.

More information:

Greg Brooke
0437 140 577


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