Late nitrogen in oilseed rape

Nitrogen fertiliser is usually the most important input for oilseed rape. Pete Berry, from ADAS High Mowthorpe, finds out how to get the most out of this valuable input.

Nitrogen fertiliser is usually the most important input for oilseed rape, since without it yields would be approximately halved.

High fertiliser and seed prices as well as environmental considerations mean it is more important than ever to match fertiliser rate with crop requirement and time applications so crops use fertiliser as efficiently as possible.

Building yield

In oilseed rape it is essential to set as many seeds as possible because this determines the yield potential. Seed number is determined by the amount of photosynthesis carried out during the 2-3-week period after mid-flowering.

Late nitrogen can be an effective way of getting the optimum oilseed rape canopy

The efficiency with which the crop uses sunlight to produce biomass at flowering can be maximised by achieving an optimum-sized canopy. If the canopy is too small then it will not intercept all sunlight. If it is too large then the thick flower layer intercepts and reflects more than 50% of the sunlight and early lodging is more likely, both of which reduce photosynthesis. The timing and rate of nitrogen are critical decisions for achieving the optimum sized canopy.

Once a high seed number has been achieved, the crop must be managed to ensure that all seeds are fully filled. It is vital to keep the canopy green and able to photosynthesise until the end of seed filling by avoiding lodging, ensuring sufficient nitrogen is available and diseases are controlled. This is doubly important because the majority of oil is formed during the second half of seed filling.

The optimum osr canopy should have a GAI of 3.5 at flowering

N rate

The target is to achieve an optimum-sized canopy at flowering and then prolong canopy greenness as long as necessary to fill all the seeds. The optimum-sized canopy at flowering has a green area index (GAI) of 3.5. This means the crop has 3.5m2 of green leaf, stem and pod per square metre of ground. The crop must take up 50kg N/ha to build each unit of GAI. This means that the crop must take up 175kg N/ha by flowering to achieve the optimum GAI.

To calculate how much fertiliser to apply, first estimate how much nitrogen has already been taken up by the crop and how much remains in the soil. Crop N can be estimated by measuring the GAI and multiplying by 50, or by recording the fresh weight of crop in kg taken from a 1x1m area and multiplying by 40.

Soil N can be measured by laboratory analysis, or by using the tables in DEFRA's RB209 Fertiliser Recommendation booklet.

Fertiliser N must make up the difference between the N in the crop and soil and the target uptake of 175kg/ha, taking account of the fact that the crop takes up the equivalent of 60% of the fertiliser N applied. A worked example of how to do this calculation is shown in Table 1.

Applying enough fertiliser to achieve a GAI of 3.5 is sufficient to achieve a yield potential of about 3.5t/ha. However, if a greater yield potential is predicted then more N must be applied. Recent research funded by Growhow UK Ltd and HGCA has shown that a crop with a yield potential of 4t/ha requires an additional 30kg N/ha, a 4.5t/ha crop requires an additional 60kg N/ha and a 5t/ha crop requires an additional 90kg N/ha. This additional N must be applied as late as possible to avoid producing an over-large canopy at flowering.

N timing

Oilseed rape can take up N from the soil very quickly up to flowering, after which the rate of N uptake slows down significantly. It is therefore necessary to apply solid N early enough to allow enough time for the crop to take most of the N up by flowering, but not too early otherwise the crop will over-shoot the optimum canopy.

Crops coming out of the winter with a GAI of 1 or more are at risk of producing an over-large canopy and therefore benefit from delayed N. Work funded by Growhow has shown that delaying N into April for a crop with GAI of 1.4 increased yield by 0.4t/ha. Delayed N also reduces stem growth which reduces lodging risk. Large canopies often require both delayed N and a PGR to manage them sufficiently.

By contrast, crops with a small canopy and a low supply of N in the soil often require a large amount of fertiliser and this must be applied early to give the crop sufficient time to take the majority of it up by flowering.

It has been shown that the crop can take up N at a rate of 3kg/ha per day during stem extension. This information, together with the likely flowering date, can be used to estimate the latest safe date for applying N depending on how much must be applied.

Figure 1 illustrates how this might work based on a mid-flowering date of 15 April. This illustrates that if only 100 kg N/ha must be applied then this can be delayed until the end of March.

Flowering date varies so it is important to adjust the dates in light of when flowering is likely to fall, e.g. If mid-flowering is expected to occur in early May then application dates must be delayed by two weeks.

Delaying N for large crops into late March or April poses a relatively small risk of slow uptake due to dry weather. Only a few mm of rain is required to dissolve solid N and make it available for crop uptake. Also large canopies already contain a lot of N which the plant is able to use to produce pods and seeds. Even in the dry spring of 2007, when little rain fell in April, delayed N strategies produced good yields.

If there is a high risk of poor solid N uptake or the crop is too tall to apply solid N, then foliar N is an option. This must be applied after the end of petal fall and has been shown to increase yield by 0.3 t/ha. The efficiency with which oilseed rape uses foliar N compared with earlier applied N is not fully understood. For this reason it is not certain whether all of the additional N required by crops with a high yield potential could be delivered in this way.