Autumn nutrition is important to get the oilseed crop off to the best possible start. Getting OSR established and away rapidly is the best way to try to counteract the various pests that are rife at that time of year.
Oilseed rape establishment is influenced by autumn nutrition
The seed yield of oilseed is a function of producing an optimal number of seeds/m2. This can be derived from developing a canopy that has the optimal number of pods / m2 and seeds / pod. AHDB funded research has concluded that this number should be 6 – 8000 pods/m2. At this density pods will typically contain 10-15 seeds giving around 100,000 seeds/m2, the amount required for a yield of 5 t/ha. The target canopy size to achieve this can be described by the Green Area Index (GAI - the ratio of green plant material relative to a square metre of soil). The target is to grow a canopy of a GAI of 3.5 during flowering and seed fill.
Establishment planning should be tailored towards this target of 3.5. The spring plant population that will result in this 3.5 is between 25 – 40 plants / m2. It is important to know what your typical field losses are to enable an accurate seed rate calculation. Harvest is an ideal time to do some stubble counts and relate this to the starting seed rate and plant counts done through the season. From this data a better farm % establishment can be derived. To ensure good consistent seed germination then the aim is to drill into moisture at a consistent depth of 2 – 3 cm. If the depth goes greater than 5 cm, emergence is greatly reduced. The seedbed quality is also critical, with an ‘aggregate’ size of 5.5 mm to 6.7mm being ideal. This will give good seed to soil contact.
Seedbed nitrogen and phosphate increase oilseed rape emergence and biomass
Autumn fertiliser, particularly nitrogen and phosphate significantly increase both emergence and establishment of the oilseed crop. Early root growth is key to rapid emergence and this is stimulated by readily available phosphate and nitrogen which in turn allows the plant to develop a bigger more efficient root system.
NVZ rules allow up to 30kg/ha of nitrogen to be applied to the crop in the autumn however due to losses each year from cabbage stem flea beetle, pigeons or slugs, growers are reluctant to make that initial investment.
Phosphate is also very important for the crop at this crucial establishment phase. Phosphate is important for protein synthesis, root development and energy transfer within the plant. It has very poor soil mobility, moving less than 1mm, so placement at drilling makes it immediately accessible to the seed. As well as assisting tap root growth it also aids secondary root formation, assisting root structure development and water and nutrient uptake.
Autumn applied potassium and phosphate increases overwinter survival
Potassium is particularly important since when present in leaves it helps to lower the cell sap’s freezing point (an ‘anti-freeze’) preventing frost damage, increasing winter survival. Good water regulation is important in dry autumns or in freezing conditions. Potassium helps limit water loss when the plant struggles to access sufficient from the roots.
Potassium is also beneficial as sufficient levels help improve the crop’s tolerance to stress (as well as disease) meaning it would be better equipped to cope with colder temperatures heading into winter. An all-round healthy plant is better able to endure abiotic stresses therefore making sure all nutrients are sufficient in the autumn during active growth is vital.
Phosphate also helps overwinter survival since this promotes a bigger more efficient root system.
Bigger early spring canopies require less nitrogen top dressing in the spring
Autumn applied PK fertiliser leads to an incease in nitrogen use efficiency so more nitrogen can be taken up in the autumn which means less nitrogen will be lost through leaching and less nitrogen will need to be applied in the spring top dressing.
Larger canopies are able to scavenge more nitrogen so less nitrogen needs to be applied in the spring. The raverage relationship is almost 2:1 so for every extra kilo of N in the canopy the application rate can be reduced by 2 kg.