- Heavy stubbles can present numerous difficulties but can be managed
- Canola needs shallow seed placement as it has little energy reserves to push through stubble
- No option to manage heavy stubbles should be ignored
Establishing irrigated canola in heavy stubbles
Establishing canola in heavy stubbles under irrigated conditions can be achieved, but thorough planning and suitable equipment are required.
Canola is a small seed with small energy reserves for germination, so anything that hinders germination and emergence can result in poor establishment. Poor seed/soil contact, variable seeding depth, mats of stubble covering the seed, hair pinning, tall stubble blocking the light, variable stubble cover affecting soil moisture, harbouring of pests and nutrient tie-up are some of the factors that need to be addressed for success.
While retaining stubble has its benefits as well as its difficulties (which most can be overcome), ultimately if the stubble is going to impact on the canola establishment, no option that is available for stubble management should be ruled out.
The advantage of irrigated break crops such as canola is that the risk associated with growing them in a dryland situation is eliminated. Time of sowing and spring moisture are in the hands of the irrigator. This allows for far more certainty in the rotation compared with a dryland system.
Canola is generally regarded as a high nitrogen (N) input crop, and so often follows faba beans or other legumes in an irrigated cropping rotation. The advantage of following a legume is that the legume stubble loads at sowing are generally lower than that of the cereals and handled relatively easily by most sowing equipment.
If canola is to follow a cereal, harvesting the cereal lower than usual (approximately 250 mm high) can allow more light down onto the emerging plants.
Ensuring an even spread of the chaff and straw is essential. If an even spread is not possible, consider windrow burning (although it can be difficult to limit the burn to the windrow in a thick stubble). With the right equipment and correct set-up, canola can and has been established in maize stubble. To achieve this the stubble was flattened and the canola sown with a disc seeder and then watered up.
An alternative strategy is to ensure cereal stubble broken down to the point of not interfering with the sowing equipment. Field trials conducted by the ICC have shown incorporating the cereal stubbles with a multi-disc, plus the addition of N and P fertiliser, followed by irrigation a month before sowing results in sufficient stubble breakdown. The rate of N applied was between 35 and 70 kg N/ha. The crops sown after 35 kg N/ha were N deficient shortly after emergence, while those sown after 70 kg N/ha were not. Rapid stubble breakdown is also expected to show improvements in soil carbon and thus soil structure.
Given the right soils (surface tilth to allow seed/soil contact once watered, no surface crusting) and layout (water on and off in less than 12 hours,) canola sown dry and watered up is highly successful. Stubbles must be evenly spread prior to irrigation and then not move too much during irrigation, so as not to smother the emerging seedlings by creating impenetrable mats of stubble.
Disc seeders are ideal in this situation as they generally can better handle the stubble loads and have accurate shallow seed placement. However watering up negates the use of pre-emergent herbicides (dilution of the herbicides by the volume of water applied) so there is no opportunity for a pre-sowing knockdown.
Alternatively, if growers pre irrigate, a coulter and tyne set-up is best. In this instance, pre-emergent herbicides and a knockdown are possible, and the sowing depth is a little less critical. It should be noted that uneven stubble cover can also result in variable soil moisture, which can impact on trafficability and seed to soil contact if the point smears rather than cultivates the seeding groove.
Canola can be sown on wider rows compared with cereals. ICC trial results revealed that there was no yield penalty when spacings were widened from 18cm to 36cm. However wider spacings did see a yield reduction (Table 1).
Irrigation helps to remove one of the issues of wider rows – excessive fertiliser placed with the seed down the tube. An ICC canola establishment trial compared canola sown with rates of up to 40 kg P/ha and 30 kg N/ha placed with the seed and watered up using approximately 1 Ml/ha (100 mm) and found there was no effect on seedling establishment (Table 1).
An alternative to using a seeder to sow canola is to broadcast the seed and water up. This works best on cultivated (eg prickle chained) or self-mulching soils so as the seed has niches to fall into. Irrigation should follow. Success, however, does depend on the ability of the spreader to evenly distribute the seed. To help with an even spread, the seeding rate can be halved and the spreading runs overlapped.
Once emerged, canola seedlings are relatively weak and can succumb to pests and diseases. Stubbles offer pests such as slugs, earwigs and snails habitat to survive over the summer. Pre-emptive bating may be necessary, or close monitoring to ensure pests do not cause excessive damage.
High cereal stubble loads can also create N deficiency through tie-up as the soil microbes break down the stubble.
Irrigated canola crops should be sown in April to maximise yield potential. Early sowing also favours the soil microbes that feed on stubbles as the soil temperatures are still quite warm, allowing the microbes to multiply rapidly.
Canola sown into heavy stubble may require topdressing sooner than normal to avoid N deficiency.
Even if stubbles are retained on the surface and little breakdown occurs, it is likely the N budget will have to be increased to factor in the lower soil N due to following a cereal.
This research is being conducted by ICC as part of the GRDC Maintaining profitable farming systems with retained stubble initiative (project BWD00024 ‘Maintaining profitable farming systems with retained stubble in Victoria and Tasmania’).