Managing oilseed canopies in early spring

With oilseed rape crops at different growth stages, ranging from some very forward crops to those that are struggling, optimising nutrition is important to build the optimum green canopy for each crop.  The forward crops will have already built good root systems and taken up a good amount of nutrition so for these crops, it is important to keep the momentum moving whilst making sure the nitrogen rate is not too high to avoid building too large a canopy.  Whereas the backward crops are likely to have developed poorer root systems so need a bit more help to get going and will respond well to fresh applications of phosphate.

Nitrogen rate is crucial to achieving an optimum oilseed rape canopy

Oilseed rape trials have shown that optimising green canopy area is crucial to achieving the maximum yield. If the canopy is too sparse this is inefficient since not all light is being intercepted and if the canopy is too dense then light penetration to lower leaves and pods is reduced and can lead to the abortion of seeds within developing pods.   Trials have consistently shown that the optimum canopy has a green area index (GAI) of between 3 and 4, however, the canopy structure of this crop is important for optimising photosynthetic rates. Different components of the plant have different photosynthetic abilities, with the leaves being the most efficient dry matter producers and the stems the least. It is therefore important to ensure that the GAI is between 3 and 4 and that there is the correct leaf to stem ratio in order to make the most efficient use of the light that is intercepted. The construction of the oilseed rape canopy and the development of the green area index is greatly influenced by nutrition, particularly nitrogen, and there is a close relationship between plant nitrogen content and the green area index.

Photo analysis accurately predicts oilseed crop N requirement

Yara has refined this relationship based on many years of trials to develop a photo analysis algorithm that is available within the Atfarm app which accurately predict the nitrogen requirement of individual oilseed crops.  When using the Atfarm app, growers simply take photographs of the oilseed crop, then based on these the app calculates the amount of nitrogen taken up already and the crop biomass and gives a field-specific nitrogen recommendation. This same calculation is performed by the Yara N-Sensor when using absolute mode which allows real-time variable rate nitrogen applications based on crop sensing.

So, determining crop nitrogen requirement is straightforward if using the Atfarm app or the N-Sensor, however, whilst nitrogen rate drives the crop canopy all the other nutrient requirements need to be met for the nitrogen to be used efficiently. As can be seen below oilseed crops also require significant phosphate, potash and sulphur.

Phosphate availability often restricts oilseed crops growing in cold wet soils

One thing that could slow growth is the availability of phosphate (P) from the soil. When soils are below 8^oC phosphate availability is less than 10%, which means that when the crop starts to regrow it can’t access the soil-P straight away. P is important for shoot growth and root growth, which can also be compromised after winter if the roots have sat in waterlogged, anaerobic conditions. These factors can slow the development of the crop early in the season until soil temperatures increase.

Sulphur is also important as the availability of sulphur is also limited, yet sulphur is needed in direct proportion to the rate of nitrogen, as shown above, which means sulphur needs to be applied together with the nitrogen.

A compound NPKS fertiliser should be applied as the first spring dressing

An application of a compound fertiliser, containing N, P, K and S will supply fresh P for the crop to take up immediately, whilst soil-P is unavailable. Potash (K) is also key for root systems, particularly those smaller secondary roots, increasing their number and density. Therefore a product such as YaraMila Actyva S (16-15-15 + 6.5% SO₃) supplies fresh P and K, along with nitrogen and sulphur, all of which are critical to early regrowth in the spring.

Alternatively, where a crop is really struggling in cold water-logged soil with low phosphate availability, foliar phosphate can help to get the crop moving until conditions improve and soil phosphate becomes available again. An application of foliar phosphate such as YaraVita Magphos K at 5 l/ha during stem extension, will give a much needed 'energy boost' to get crops moving again. This then should be followed up with a solid NPKS fertiliser such as YaraMila Actyva S very soon afterwards so that the crop doesn’t run out of momentum once it's has started.

Finally, it is worth remembering micronutrients as any deficiency will also limit nitrogen use efficiency and prevent the crop from developing the most efficient canopy.  YaraVita Brassitrel Pro contains Mg, Mn, B, Mo which are the key micronutrients required by oilseed crops so is a simple and efficient means of preventing deficiency.