Summary

Barley grass is a major challenge in broadacre farming systems in South Australia, particularly in the low rainfall zones such as the upper Eyre Peninsula (EP). A three-year research project investigated the impact of seed dormancy and herbicide resistance on control of barley grass in upper EP farming systems and evaluated new herbicide options.

Background

Biological traits of barley grass, such as seed dormancy and delayed emergence, can reduce the efficacy of knockdown herbicides and the early onset of seed production can reduce effectiveness of crop-topping or spray-topping in pastures. Barley grass also sheds seeds well before crop harvest, which can make harvest weed seed strategies less effective compared to other weeds like ryegrass. Resistance is also increasing to the Group 1 herbicides that are commonly used to control barley grass in a pasture or break crop phase.

Research Aims

The core objectives of the project were to:

• Assess the impact of new herbicides and management options in cereals and break crops for improving barley grass control.
• Assess current barley grass genotypes on upper EP for length of seed dormancy and germination patterns.
• Monitor five farmer paddocks per season where barley grass escapes or suspected resistance is occurring to identify environmental factors and management strategies that affect the efficacy of current herbicides.

In The Field

Four rotational systems trials were conducted annually for three seasons at Mt Cooper to investigate barley grass control.

• Trial 1: control in the cereal phase of a wheat/field pea/wheat rotation.
• Trial 2: control during the pulse break crop phase of a break/wheat/wheat rotation.
• Trial 3: control in a two-year break in a break/break/wheat rotation.
• Trial 4: control in IMI tolerant cereals in a cereal/cereal/cereal rotation.

Two replicated cereal herbicide management trials were also conducted at SARDI Minnipa Agricultural Centre evaluating new herbicides Luximax® (Group 30), Mateno Complete® (Groups 32, 15 and 12) and Overwatch® (Group 13) for control of barley grass.

A total of 47 barley grass seed populations were collected from growers’ cropping paddocks to investigate seed dormancy in seed trays under ambient conditions.

Barley grass patches that escaped after application of Group 1 herbicides were identified in five grower’s paddocks and samples sent for resistance testing. Soil samples were also collected to assess if soil chemistry, primarily salinity, was impacting plant growth and water movement or reducing herbicide efficacy.

Results

In the rotational systems trials, the lowest barley grass numbers and highest grain yields were achieved in Trial 1 (wheat/field pea/wheat rotation). In Trial 3, the medic/medic, canola/canola and medic/vetch brown manure treatments recorded the highest grain yield and lowest barley grass numbers compared to other treatments. Trial 2 and Trial 4 achieved slightly lower grain yields than Trial 1 and 3.

In the herbicide trials, all herbicide treatments reduced barley grass compared to the nil control. Trials showed herbicide and seed separation is important when using Luximax® and a sowing depth of more than 3cm is needed to ensure adequate germination and crop safety. Overwatch® caused visual bleaching for three weeks after emergence but did not impact establishment, early dry matter or grain yield.

Early to mid-June was the peak of barley grass weed seed germination under ambient environmental conditions. Seed bank populations recorded approximately 100 per cent germination over the two seasons assessed, with 50 per cent germinating in each year.

Varying levels of resistance to Group 1 herbicides were detected in paddocks on upper EP. Reduced herbicide efficacy in some paddocks was related to management, including not removing stock early enough for barley grass to be actively growing, cold or frosty conditions before or after applications, poor water quality or inappropriate nozzle selection.