Research Authors:

Imma Farre, DPIRD South Perth

Optimum Sowing Window

According to the APSIM-Canola model and local climate records, the Optimum Sowing Window (OSW) to achieve maximum yields, for a mid-season canola cultivar (i.e. ATR Bonito) in Kojonup is on average from early-April to mid-May (Figure 1). The optimum sowing window was defined as the sowing period that achieves on average more than 95% of the maximum yield for each cultivar type. Simulation of yield penalties due to frost and/or heat stress is still under development.

Importance of Early Sowing

We used the APSIM-Canola model and long-term climate records since 1976 to simulate yields for sowing dates from mid-March to end-June in Kojonup (Figure 1). The canola cultivars simulated were long (i.e. Hyola 725), mid (i.e. ATR Bonito) and short (i.e. ATR Stingray) maturity types.

The yield versus time of sowing relationship for Kojonup has a bell shape with a flat top (Figure 1), with long maturity cultivars outyielding medium and short maturity cultivars for early sowings. Differences in yield between cultivars disapeared for late sowings. Even though there is a lack of experimental data for very early sowings, the simulation results suggest that very early sowing such as March, would have a yield penalty compared to the optimum sowing period.

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Figure 1. Simulated canola yields for Kojonup for sowings from 15-March to 30-June for short, mid and long maturity type cultivars. Lines are average yields for 43 years (period 1976-2018).

Chances Of Having A Break Of The Season

Looking at the Kojonup rainfall data for the last 43 years, the probability of having a sowing opportunity (defined as at least 15 mm rain in 3 days) within the optimum sowing window (OSW is shaded in Table 1) is 65 %.

With late sowing opportunities, it is important to assess the chances of achieving a certain yield by certain sowing date (Figure 2). For example, in Kojonup, there is over 80 % chance of getting 2.0 t/ha canola yield when sowing in April, but this is reduced to around 50 % for sowing in mid-June (Figure 2). Sowing by the end-May still has about 80% chances of achieving 2 t/ha.

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Figure 2. Percentage of years (%) with yield above certain thresholds for the period 1976-2018 for Kojonup, sowing ATR Bonito (mid maturity cultivar) for different sowing dates.

Table 1. Cumulative chance (%) of having a first sowing opportunity, by week (w) and month in Kojonup. The Optimum Sowing Window to obtain maximum yield is shaded.

Importance Of Stored Soil Water At Sowing

Starting soil water, defined as plant-available water at sowing, produced on average no yield advantage in Kojonup (Figure 3), based on the last 43 years of climate data. Simulations were done with sowing at the first sowing opportunity each year, with either soil dry at sowing or with 40 mm soil moisture at sowing. Stored soil moisture has a bigger contribution to yield in low and medium rainfall locations.

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Figure 3. Probability distribution of canola yields for the period 1976-2018, assuming soil is dry at sowing (dry) (dashed line) or has 40 mm of soil moisture at sowing (40 mm) (solid line) for Kojonup.

  • Early sowing is the key to maximise canola yield. However, in your area, profitable yields can be achieved with sowings after the optimum sowing window, depending on season.