Can I retain stubble and keep weeds under control?

Tony Swan, Senior Experimental Scientist with CSIRO Agriculture and Food says growers can manage stubble without compromising weed, disease and pest management or the timeliness of the seeding operation.

Tony Swan, Senior Experimental Scientist with CSIRO Agriculture and Food says crop sequence, including a double-break, is an effective way to manage weeds, improve profit and manage stubble residues in southern grain farming systems, with or without livestock.

“Decisions at harvest, post-harvest/pre-sowing and at sowing influence the success of a seeding operation into stubble,” he says. “With a flexible approach, growers can manage stubble loads to suit their seeding operations, weed control tactics, herbicide choice and profit.”

“As a rule of thumb, the stubble load after harvest is 1.5 to 2 times the grain yield for wheat and 2 to 3 times the grain yield for canola,” says Mr Swan. “A high stubble load can create issues for all types of seeding systems by restricting herbicide choice, effectiveness, contact on the soil or weed target and even reduce crop emergence.”

In 2014 an experiment was established at Temora, NSW in a paddock with high levels of Group B resistance in annual ryegrass. The trial compared the yield, profit and annual ryegrass (ARG) population status in three management strategies in a no-till (tine opener) or zero tilled (disc opener) farming system.

Introducing more crop and herbicide diversity into the farming system generated a higher average net margin over three years while reducing the seedbank from 1864 plants per m² to 351 plants per m² in just 24 months. Following the wet 2016 season, with a soft late finish, three years of the diverse strategy (which included a double break) reduced the ryegrass seed bank by 70% compared to the aggressive (high input) strategy (145 cf 573 seeds/m²), while the conservative (low input) strategy increased the seedbank by 600% to above 4000 seeds/m².

“Crop sequence is an effective way to manage weeds, improve profit and manage stubble residues in southern grain farming systems, with or without livestock,” says Mr Swan.

Mr Swan will be presenting his research into double-break cropping at the WeedSmart Forum on 21 August, in Wagga Wagga. Go to the website to register.

Does choice of seeder type influence weed control?

Short answer: In our experiment, after 3 years ARG seed numbers were generally lower when crops were sown with a tine seeder.

Longer answer: Averaged across the three management strategies, ARG seedbank populations were lower in tine seeded plots in 2016 (650 seeds/m² in tine cf 1080 seeds/m² in disc) and 2017 (384 seeds/m² in tine cf 944 seeds/m² in disc).

However, the use of expensive pre-emergent herbicides eliminated any significant difference in ARG seedbank populations in crops sown with a disc or tined seeder. In the conservative strategy where trifluralin can not be used with a disc seeder (not on label), the result was a higher ARG seedbank population (4045 seeds/m²) with a disc seeder compared to 1840 seeds/m² with a tine seeder.

By February 2017, following the 2016 decile 9 season the lowest average ARG seedbank population was found in the diverse cropping strategy sown with a tine seeder (82 seeds/m²). In the conservative strategy weed populations continued to increase when sown with a tine seeder (2322 seeds/m²) and with a disc seeder (7631 seeds/m²).

Tony Swan inspecting the stubble management trial plots, looking at the ‘Conservative (lower input)’ second year wheat treatment sown with a tine seeder (left) compared to the ‘Conservative’ second year wheat treatment sown with a disc seeder where trifluralin could not be applied (not on label for disc seeders).

What options do I have if stubble is too thick to sow through?

Short answer: Reduce stubble load by grazing, baling, mulching, incorporating with nutrients, or use a strategic late burn.

Longer answer: Tine seeding systems will struggle to establish crops into large stubble loads >6 t/ha. While disc seeders may penetrate the stubble more easily and plant crop seeds into high stubble loads, patchy crop establishment and ineffective herbicide application can result in any seeding system sown into high stubble loads. Retaining stubble, with all its benefits, should not be allowed to compromise effective weed control. Mulching, incorporation and grazing all potentially retain more nutrients in situ however mulching or incorporation are likely to lead to nutrient tie-up in the soil unless additional nutrients are added.

Grazing and late burning both improve in-crop nitrogen availability, and often the yield of the following crop. It is not recommended to grow wheat after wheat unless the stubble from the first wheat crop is burnt or grazed, or supplementary nitrogen is applied to offset N immobilisation. To try and avoid burning at all costs, the best option is to sow a diverse crop sequence and use the legume crop to reduce the cereal stubble.