Hamish Mulcock

With spring around the corner and soil temperatures rising many will be starting to think about sowing brassica crops. These crops typically provide high-quality feed when pasture production and quality declines through summer or a bank of winter feed for stock over the winter months when pasture growth is at its slowest.

An additional benefit which is often under-valued is utilising brassicas as a break crop in pasture renovation programmes to break weed and pest cycles before returning the paddocks to improved permanent pasture.

Common forage brassicas used to fill these feed gaps include forage rape, leafy turnips, bulb turnips, kale, swedes and more recently Palleton Raphnobrassica.
Regardless of the brassica species used there are some key principals required to produce a successful crop.

PLANNING
Planning for spring-sown brassica crops should ideally start in the previous autumn, by identifying where crops will be sown and allows an opportunity to correct underlying fertility, drainage or compaction issues prior to the crop and subsequent pasture being sown.

The more planning that occurs up front prior to the crop being sown the better the outcome will be in growing a high yielding, healthy brassica crop.
When selecting paddocks for winter grazing (eg: kale or swedes) consideration should be given to environmental impacts. Slope, soil type, stock class, drainage and proximity to water sources can all have a major environmental impact. In general, flat, well-drained paddocks with deep soil profiles will have less risk of both overland flow and N leaching (Source: Beef & Lamb; Fact Sheet September 2018).

  • In locations and catchments where N leaching reductions are sought, try to ensure crop paddocks are not located on light soil types that carry an increased risk of N leaching.
  • In locations where P and sediment runoff are of greatest concern, try to ensure that crops are not located on poorly drained soils and/or sloping land.
    Soil testing should be completed six to eight months before sowing allowing time for correction of any fertility issues identified. Of importance is soil pH, ideally, we would like this to be at 5.8 or higher, and correction by lime can take at least six months to increase. Olsen P levels over 15-20 and pH levels between 5.8 – 6.2 are generally recommended for maximising brassica yields (Table 1).

PADDOCK PREPARATION
Identification of critical source areas (CSAs) is also important when preparing the paddocks. CSAs are low-lying parts of the paddock including gullies and swales where runoff can carry sediment, nutrients and pathogens to waterways. Ideally buffer zones around CSAs should be left unsprayed and uncultivated to act as buffers, filtering and slowing overland flow.

The more planning that occurs up front prior to the crop being sown the better the outcome will be.

Cultivated paddocks should be sprayed out in spring with glyphosate and an appropriate knock-down insecticide for pests such as springtails and nysius fly. If problem weeds are present a spike (additional herbicide) can be partnered with glyphosate to broaden the weed spectrum and enhance knockdown. The most appropriate spike should be based on weed spectrum and discussed with your local seed or retail representative.

Best practice for direct drilling is a double spray programme incorporating an initial glyphosate and a spike at least six weeks prior to sowing followed by a second glyphosate and insecticide immediately before drilling. Direct-drilled crops can be at risk from soil-dwelling pests such as grass grub and slugs so using the appropriate insecticide/slug bait is advised.

In contrast paddocks that have received full tillage (eg: exposed soil), need to be worked to a fine, firm, moist seedbed. Having such a seed bed allows more control on depth of sowing, increases soil/seed contact and enhances efficacy of pre-emergent herbicides.

Brassica crops have high nitrogen and phosphate requirements and are critical for early shoot and root development. Other major nutrients such as potassium are only applied if a deficiency is identified with the soil test. At sowing or prior to the final cultivation a starter fertiliser is typically applied that includes nitrogen and phosphate. For bulb crops especially phosphate sown down the drill is highly recommended with additional applications placed on top.

Generally, a DAP type fertiliser is applied at this stage with the quantity applied based on the expected crop requirement and current soil fertility status.

Boron is an essential micronutrient for brassica growth and is required at the initial stages of plant establishment. Deficiencies of boron can eventually lead to brown heart in swedes/turnips and stem disorders in kale/rape. It is almost always applied with the initial applications of phosphate and further applications may be required post-establishment to meet plant requirements.

DRILLING
Ensure the correct seed is chosen and sown at the right rate. Seed selection is based on a variety of factors including both crop rotation and what the crop is going to be fed to and importantly use the right seed treatment to give some protection against insect pests and seedling diseases.

At drilling, calibrate the drill to the correct sowing rate and depth which is approximately 10mm. Recheck the drill settings for both after sowing the initial few rounds of the paddock. With drilling, it is important not to do so when the ground is excessively wet, allow it to dry out first and sow when soil temperatures are about 10C and rising. In environments that are exposed to very cold conditions in early spring (late frosts) swedes and kale should be sown no earlier than November 20.

Rolling is recommended immediately after sowing especially with coulter and direct drills. Rolling promotes good seed, soil contact which leads to a more uniform establishment and in the case of direct drilling closes the slot which also helps prevent slug damage. Even with air-seeders that have rollers, a secondary rolling can help if the seed bed is still soft post-sowing.

Pre-emergent herbicides are best practice as brassicas are especially susceptible to weed competition during establishment. The most appropriate herbicide should be chosen based on expected weed spectrum, moisture requirement and required plant back into subsequent crops or pasture.

POST-SOWING MONITORING
Following establishment, crops should be monitored one or two times a week. Most crop failures due to insects occur during establishment including slugs, Nysius fly, greasy cutworm and numerous others so early detection and control is key to preventing crop losses.

Post-emergent broadleaf and/or grass weed herbicide and insecticide maybe required post sowing. It is important to get advice on the best options, to ensure a targeted approach to control and ensures no subsequent flow-on affects for feeding to animals and post pasture/crop plant back.

Post-emergent applications of nitrogen may be required as the crop approaches canopy closure to maximise growth. Timing and application rates will depend on several factors related to both plant growth, climate, environmental requirements and the grazing animal.

Overall there is plenty to consider when planning and growing brassicas however attention to detail will be rewarded with high quality crops well suited to filling key feed deficits.

  • Hamish Mulcock is PGG Wrightson Seeds senior agronomist.