Hitting the target with pre-emergent herbicides

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How you manage stubble at harvest affects the efficacy of pre-emergent herbicides applied next season.

Stubble can prevent pre-emergent herbicides from reaching the soil surface, leading to uneven coverage and reduced herbicide efficacy. While this varies depending on the herbicide, its solubility and soil moisture, stubble generally starts to be a problem when stubble loads exceed 45 to 50 per cent of ground cover, which is between 1.7 and 2.5 tonnes per hectare of cereal stubble.

Graphic showing the increasing spray coverage achieved by an increasing water rate
Figure 1 Herbicide soil coverage is reduced as cereal stubble height increases.

The impact of stubble on the efficacy of pre-emergent herbicides was investigated by several farming systems groups as part of the GRDC’s Stubble Initiative. Each group has developed local guidelines to help growers use pre-emergent herbicides effectively in stubble-retained systems.

Stubble management begins at harvest. Stubble not left standing will break down more rapidly if spread evenly, rather than left in the header row, and when choppers are used at harvest to mulch and pulverise stubble into smaller pieces.

It is more effective to spray standing stubble but the Lower Eyre Agricultural Development Association (LEADA), working with the South Australian Research and Development Institute (SARDI), has found that getting the herbicide down into the stubble can be challenging. As part of the GRDC’s Stubble Initiative they demonstrated that spray coverage of soil decreases with increasing stubble height (Figure 1). Setting the spray boom height so that the double overlap occurs at the height of the stubble rather than the ground enables a more even application and maximises efficacy; however, this can also increase the potential for spray drift.

Scatter plot showing herbicide soil coverage is reduced as cereal stubble height increases
Figure 2 Spray cards set up by LEADA in 2016 show the increasing spray coverage achieved in stubble with increasing water rate: (from left) 60L/ha (11 per cent coverage), 100L/ha (16.5 per cent coverage) and 140L/ha (26.5 per cent coverage).
SOURCE: Blake Gontar, SARDI

Higher water rates allow greater penetration of the stubble and better weed control (Table 1). Research during the initiative has confirmed water rates greater than 70 to 80 litres per hectare in medium stubble loads and 80 to 100L/ha in high stubble loads are necessary for adequate herbicide application. A wind blowing across the stubble rows enables more herbicide to hit the soil or target weeds.

Speeds slower than 16 kilometres per hour allow more spray to reach the soil, improving herbicide efficacy. FarmLink Research has shown that only 10 per cent of herbicide reached the soil when spraying at 30km/h and less than 20 per cent at 20km/h. Where possible, spray in the direction of the stubble rows to maximise the herbicide that reaches the target or soil.

Match nozzle spacing to row spacing (when using guidance) so that nozzles can be positioned between stubble rows to minimise stubble interception. Ideally, go no wider than 25-centimetre nozzle spacing to minimise misses. Nozzles that produce a larger droplet size have a better chance of penetrating stubble. Air-induction nozzles can increase droplet size at lower rates, but the trade-off is that droplets are more likely to be retained on stubble.

When burning stubbles, ash will bind to herbicides reducing efficacy. Aim for a hotter burn weeks ahead of sowing to reduce the amount of ash retained in the paddock.

Table 1. Pre-emergent herbicides provided more effective control of ryegrass when higher water rates were used at Ungarra, South Australia, in 2015.

Water rate
Reduction of ryegrass plants
compared with control (%)
50 21 a 52
100 12 b 73
150 11 b 75

GRDC Research Codes BWD00024, CSP00174, CWF00018, EPF00001, LEA00002, MFM00006, MSF00003, UNF00002, YCR00003

More information:

Amanda Cook, SARDI
08 8680 6200