DPIRD looks at strategic tillage and its impact on weeds

Recent research by Department of Primary Industries and Regional Development (DPIRD) research scientist Catherine Borger has shown how well a weed species recovers after soil amelioration is dependent on both seed size and burial depth.

Burial depth is hard to keep consistent, as it widely varies between soil types, soil conditions and implement setup, even when using the same soil tillage implements at a consistent speed.

Soil amelioration through the use of deep tillage is commonly used in Western Australia, with Dr Borger believing full soil inversion using a mouldboard plough is a great weed control technique.

However, there is very little information on the impact of burial via amelioration on weed seeds of different sizes.

"There is little information on the impact of soil loosening or mixing on topsoil and weed seed burial or subsequent weed growth," Dr Borger said.

"However, it is assumed these techniques may stimulate weed emergence, similar to an autumn tickle."

The DPIRD and Grains Research and Development Corporation (GRDC)-funded study used three sites where contrasting soil was ameliorated by a full inversion, loosening or mixing.

It aimed to determine how weed seeds were buried by amelioration and how weed density was affected in the crop in a three-year rotation after amelioration.

"Field trials were established at Yerecoin, Darkan and Williams, with amelioration treatments applied to a moist soil at the beginning of the growing season, after the opening rain," Dr Borger said.

The three sites had different soil types and the Williams site had very dense soil.

While strategic tillage reached depths of more than 30 centimetres at Yerecoin and Darkan, Williams had a maximum working depth of 24cm.

"It is a general rule-of-thumb that after a full soil inversion, most weed seeds are at roughly half the total depth of amelioration and this result was confirmed at all three sites," Dr Borger said.

As predicted, seed burial varied widely between sites due to the different soil conditions.

A full soil inversion buried about 90 per cent of weed seeds at 10-20cm at Yerecoin and Darkan, whereas at Williams the seeds were up to 10cm.

This highlights how important it is to test if seed burial measures are working on your farm's soil type.

Deep ripping proved to be the ultimate tillage technique, as it was the only method that reliably put a proportion of seed at the maximum working depth.

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The percentage of seed buried by loosening or soil mixing was highly variable between sites.

Loosening or soil mixing had very little impact on weed density and it did not consistently stimulate weed emergence.

Annual ryegrass has a small seed and emergence is substantially reduced at a depth greater than 5cm, whereas great brome will emerge from up to 15cm depth.

"After soil inversion in 2019, annual ryegrass and great brome density were close to zero but while annual ryegrass remained at low density, great brome recovered over the following three years," Dr Borger said.