Weed Seed Wizard case studies

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Don't stop doing harvest weed management just because its a dry year

Photograph of a combine harvester parked in a paddock and blowing material into a chaff cart. A lot of dust and dry plant material is covering the chaff cart.

Work done by Australian Herbicide Resistance Initiative (AHRI) has shown that annual ryegrass seed numbers can still be relatively high in poor seasons (see Table 1). In 2012, 29 annual ryegrass plants per square metre produced 7000 seeds. These seeds can carry over to the following year to reduce yields.

Table 1 Wheat yield and annual ryegrass seeds produced in 2011 (a good year) and 2012 (a dry year)
Year Wheat yield (t/ha) Annual ryegrass (plants/m2) Annual ryegrass (seeds/m2)
2011 4.0 19 12 000
2012 0.6 29 7000

(Note the comparative ryegrass seed yields for the two seasons. There was not much of a yield penalty for ryegrass in the drought year, 2012, when compared with wheat).

Cut your crop lower in a dry year to catch more ryegrass seeds

In a good year, with a big crop, there will be less light penetration and the annual ryegrass tillers will be upright and easier to catch. In a low-yielding year with a light crop and an open canopy, the ryegrass tillers will be also shorter. The work done by AHRI showed that a 40cm harvest height in 2011 (high yielding crop) collected about 60% ryegrass seed at crop maturity compared to about 2% in 2012 (low yielding crop).

The more seeds dropped in one year, the less yield in the following year

To illustrate the difference in cutting height in a dry year, we simulated the wheat yields in 2013 after 7000 annual ryegrass seeds/m2 were set in 2012 using the Weed Seed Wizard. If the crop was cut at 10cm, only 1250 ryegrass seeds/m2 are returned to the seedbank with a resulting 400 kilograms per hectare (kg/ha) of wheat yield loss the next season. This compares to a yield loss of 1.4 tonnes per hectare (t/ha) when the crop is cut at 40cm and most of the ryegrass seeds are dropped (Figure 1).

The simulated yield loss in 2013 after harvest in 2012 at three harvest heights; 10 cm, 20 and 40 cm.
Figure 1 Simulated yield loss in 2013 after harvest in 2012 at three harvest heights; 10, 20 and 40cm

Also in dry years where wheat yield is low, it is possible to burn narrow windrows in wheat. For wheat crops of 2-2.5t/ha or less it is possible to burn just the windrows. Cutting low is imperative to keep the fire in the windrow and to optimise the burn.

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Page last updated: Thursday, 19 August 2021 - 11:07am