Crop nutrition and fertiliser requirements for the double low oilseed rape crop
The purpose of this review is to draw together the available information from the U.K. and internationally on the major and trace element requirements of oilseed rape, with particular reference to double low varieties. Topics which need further research are identified and appropriate studies are recommended.
Double low varieties of oilseed rape (Brassica napus), with low seed contents of both erucic acid and glucosinolates, are now grown almost exclusively in the U.K. Most information previously obtained on the fertiliser needs of single low varieties can also be applied to double low varieties, however there is recent evidence of different responses between the two types to certain nutrients. Although heavier textured soil types predominate, winter oilseed rape is now grown on a wide range of soils, including sandy soils which are more prone to give trace element deficiencies in arable and other crops. Apart from S, deficiencies of major nutrients are of less concern because of fertiliser inputs over many years, also organic manures where applied, which have raised the p and K reserves of most arable soils to a satisfactory status.
The area of spring-sown oilseed rape has to date been very small and mainly confined to those areas with soil types and climatic conditions which are more suited to spring cropping, particularly where the amount of pigeon damage during the winter would be unacceptable. Interest had revived in the spring crop because those double low varieties can consistently achieve glucosinolate contents below 20 µmol/gram, when EEC premium payments for harvested seed was based on seed glucosinolate content. The change in the EEC price structure including support payment on an area basis for oilseed rape, and resultant lower prices for rapeseed, has again focused attention on the spring-sown crop because of its lower input requirements including N and a gross margin comparable with the winter crop despite lower yields.
Each of the major and trace elements are discussed in respect of
- their role in plant nutrition
- deficiency symptoms and occurrence in oilseed rape
- crop requirement and effects of nutrient application on seed yield and quality
- type, rate and timing of fertiliser application for optimum yield or to either prevent or correct deficiencies.
Methods of predicting or diagnosing nutrient deficiencies, both from a knowledge of soil type characteristics and by using foliage or soil analysis, are also discussed for particular elements. Oilseed rape will tolerate a wide range of soil pHs, 6.5 to 7.0 is considered ideal for most mineral soils but pH 6.2 is recommended for light textured soils in Scotland.
The application of nutrients influences biomass production, but not the development stages of the plant, and N has the most important role. The amount of vegetative growth and subsequent branching of the oilseed rape plant increases with N application and greater biomass production during the growth period pre-flowering results in more pods per plant. This is the main component of yield which is responsible for yield responses to increasing amounts of N fertiliser, effects of N on seed size and seed weight are less consistent. Around 90% of the yield can however be obtained with only 60% of the optimum N rate, because of the very shallow but still economic yield responses obtained at the larger N rates. The application of N does not seem to influence the sequence of flower development but delays and prolongs the flowering period.
The effects of rate and timing of autumn and spring applications of N fertiliser on seed yield, oil and glucosinolate contents are discussed. Experimental evidence indicates that double low varieties have similar N requirements to previous, single low varieties of oilseed rape. The economics of applying autumn N are often marginal and possible factors which will reliably predict the chance of a unique yield response to autumn N are still poorly understood.
Recent experiments have however identified straw disposal as one such factor. Prior to the change in the EEC price structure towards the end of 1991, autumn N at 50 kg N/ha had been recommended for winter oilseed rape in most situations. Similarly, spring N applications of 200 to 280 kg/ha for winter oilseed rape and 125 to 150 kg/ha for spring-sown crops were recommended as optimum rates after cereal cropping. Recommended rates of autumn and spring N have now been reduced as a result of the major drop in the price of rapeseed. At present, prediction of N requirement is usually based on empirical site factors, as direct measurement of the soil N supply is only more beneficial where the fertility is greater than usual. Timing of spring N fertiliser depends on the start of crop growth in the spring and the risk of leaching loss in different soil types. The appreciable residues of N left after oilseed rape and the need to take account of them when fertilising the following crop are discussed, also the use of urea as a suitable alternative to ammonium nitrate for spring top dressings on winter oilseed rape.
Nitrification or urease inhibitors, used in conjunction with urea, have not shown any advantage at normal timing of spring applications. Effects of plant growth regulators and irrigation, relative to N use, are also discussed.
Experimental evidence on the yield responses to P and K is summarised. Deficiencies of either nutrient are rare in the U.K., as most arable soils are well supplied with these nutrients, and yield responses to freshly applied have only been obtained in experiments where soil P or K reserves are very low. Appropriate fertiliser strategies for P and K are discussed, in most situations only maintenance dressings are required to balance crop offtake of these nutrients in the seed at harvest. Seed quality is usually unaffected by P and K application.
The situations in which Mg deficiency may occur, and the possible need for application of Mg fertiliser to prevent or correct a deficiency are discussed. Oilseed rape is not very susceptible to Mg deficiency, which is most likely on sandy soils. The deficiency can however be induced on a wide range of soils if the soil or growing conditions put the crop under stress. The symptoms are often transient in such situations and treatment with a foliar spray of Mg is unlikely to increase yield. Maintenance of adequate Mg status in the soil for the overall arable rotation is advocated, to prevent a deficiency of Mg which could limit yield.
With the introduction of double low varieties and the decline in atmospheric S levels, S deficiency in oilseed rape is becoming more widespread in the U.K. Yield can be as much as trebled under conditions of severe S shortage but the seasonal expression of S deficiency varies according to the interaction between soil S supply and crop uptake. Plant S supply differs widely according to soil type and location and has a primary influence on seed glucosinolate concentration. This influence is still poorly understood and modified to a greater or lesser extent by other factors, notably the effect of N supply and season on crop structure. The efficacy with which the different forms of S fertilisers improve crop S status is known in principle but the optimum amounts and timings of these forms to maximise yield yet minimise glucosinolate content of rapeseed have not been precisely quantified.
There is very limited information on the trace element (B, Cu, Fe, Mn, Mo, Zn) status of oilseed rape grown in the U.K. Advisory experience indicates that deficiencies of trace elements generally are not limiting U.K. production of oilseed rape. However, deficiencies of boron and, to a lesser extent, manganese occur infrequently on sandy soils of alkaline pH and peaty soils in some seasons. Yield responses of up to 0.5 t/ha have been obtained from application of boron fertiliser but prediction of such yield responses has not been identified satisfactorily.
Recommendations are made for further studies on the efficiency of N recovery by the new varieties of oilseed rape with shorter growth habit under different conditions of crop structure as a result of emergence date and N application. Such studies, in conjunction with measurement of N residues left by the crop after different rates of N fertiliser application, would enable the development of an N balance model to determine N requirement of the crop more reliably. Studies are also proposed on examining the S status of U.K. crops to determine possible links with seed glucosinolate content at harvest, and the influence of site, soil, season and crop factors on glucosinolate content. Such studies would also identify the extent to which crops are becoming deficient in S. Further work is needed on both the behaviour of S in soil, the prediction of S deficiency and also the best strategy for applying S fertiliser.
Further studies on the trace element contents of oilseed rape on various soil types and the procedure for predicting boron deficiency, are recommended, also a more comprehensive review of the physiology of oilseed rape.
Related research projects
- Review of AHDB-funded research on phosphorus management in arable crops
- Offtake values for phosphate and potash in crop materials
- Use of autumn nitrogen in no-till farming systems
- A review of the function, efficacy and value of biostimulant products available for UK cereals and oilseeds
- Routes to improving the efficiency of phosphorus use in arable crop production
- Nutritional value of oilseed rape and its co-products for pig and poultry feed: potential improvements and implications for plant breeders
- A review of the non-NPKS nutrient requirements of UK cereals and oilseed rape
- Response of cereals to soil and fertilizer phosphorus
- Better estimation of soil nitrogen use efficiency by cereals and oilseed rape
- Research needs on nitrogen and phosphate management in cereals and oilseeds