A High Yielding Crop Of Winter Oilseed Rape Biology Essay

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To grow a high-yielding crop of winter oilseed rape, a number of factors must be considered before even choosing your seed. The best variety must be chosen for the situation of the field, rather than simply selecting a high yielder.

The HGCA produce a Recommended List for winter oilseed rape, ranking varieties by yield corrected for percentage oil content, and agronomic data including resistance to lodging, club root, light leaf spot and stem canker, as well as stem shortness and stiffness, earliness of flowering and maturity. Some growers want a low input regime, or organic system, so high disease resistance is important for these.

Geographic aspects must be taken under consideration also, such as phoma infections being sighted in west registered under 10%, even to 0% being noted, in east it may be 80%+, or light leaf spot is seen much more in the north and Scotland than southern England. (Crop Monitor, 2009.) Also, weather conditions rather than the date should be main factor for choosing a drilling date. This will allow good conditions to be created prior to sowing to give the seed the best chance at establishment and to encourage good health of the crop from the beginning. OSR has a small seed, with limited seed reserves and intial rooting ability, therefore soil moisture retention and seed to soil contact are the critical factors in determining the proportion of seeds that will germinate. The autumn vigour shown by most hybrids is an important consideration governing plant survival over winter, alternatively, selection of large high protein seeds more capable of growing in dry autumn conditions may be a better option.

Different aspects of the seedbed also need to be considered prior to planting, such as pH, soil compaction, organic matter content and the seed rate needed for the variety. Slug trapping to find out rates should be done when the previous crop is in to know if treatment is needed when the seed is planted, before damage can be done to emerging seedlings. It is better for future management to create good conditions for the seed rather than to rely on increased seed rates.

Rotational position:

An effective rotational sequence is important for crop health management. Continuous production of a single crop, or short sequences of crops with similar management practices promote the increase of weeds adapted to those conditions. The resulting weeds can be highly competitive and difficult to control. (Froud-Williams, 1988.) In contrast, using a diverse rotation employing crops with different planting and harvest dates, different growth habits and different management practices will allow weeds to be challenged with a wide range of stresses and mortality risks. (Liebman and Dyck, 1993; Derksen, 1997; Liebman and Ohno, 1998.)

Sieling's 2003 study showed that OSR grown after peas and barley yielded the most (3.9 t haˉ¹ and 3.8 t haˉ¹), but with wheat in the preceding season, the yield was reduced to 3.6 t haˉ¹. The crop grown in the year previous affected seed yield, but so did the crop grown two years previous, though a smaller effect was seen than the directly preceding year. OSR clearly responds to a favourable preceding crop. As it is often grown as a break crop with wheat, this may be a reason for decreased yield even if other factors are ideal.

Establishment:

To till or not to till:

Seed yield (t haˉ¹ 91% dry matter) of oilseed rape in different tillage systems 1991-2000

Tillage System

Experimental Year Relative 1991 1994 1997 2000

Plough (25cm)

4,42

4,44

4,17

3,96

Cultivator (25cm)

4,68

4,43

3,73

4,28

Plough (15cm)

4,57

4,36

3,70

3,94

Cultivator (15cm)

4,30

4,18

3,86

4,14

Rotary harrow (10cm)

4,52

4,05

4,61

4,24

Zero tillage

4,61

4,05

4,61

15,5

Adapted from Christen et al. 2003.

From this table it is demonstrated that conservation tillage can yield respectable seed yields compared to conventional tillage. This 2003 study by Christen et al. suggested the following amendments to most conservation tillage cultivations:

A diverse crop rotation is needed, the more spring sown crops and legumes the easier it is for weed and volunteer management.

Correct harvest technology: the combine harvester in previous cereal harvest needs to have excellent straw chopper and chaff spreader qualities, especially as climatic conditions which only allow a short decomposition of crop residues after harvest

Depending on straw distribution it may be necessary to use a comb harrow once or twice to improve spatial straw distribution in the field.

These improvements to current minimum tillage practise, combined with a variety suited to minimum tillage, such as the ideal timing for sowing after the previous crop, root systems and vigour to get away from the chaff should yield respectable amounts compared to traditional cultivation methods.

Minimum tillage under dry conditions may also prevent excessive moisture loss, improving the prospects of early germination and growth. However, under very wet conditions, and where soil structure has been damaged, ploughing may still be the most appropriate strategy, to accelerate drainage, facilitate soil drying and remove compaction. Drilling seed into uncultivated soil under dry conditions can prevent excessive moisture loss from the soil, as over 90% of rape is sown in late August or early September when the preceding cereal crop has normally dried the soil profile.(Lunn et al. 2001)

Soil type is an important factor for choosing cultivation practices, for example, clay soils when cultivated at the extremes of wetness and dryness, tend to produce cloddy seedbeds unsuitable for the small OSR seeds, makes it difficult to achieving uniform plant establishment, and creates a good habitat for slugs to move in and landing areas for wood pigeons to attack the crop. Sandy light soils may be over dried by ploughing.

Timing of tillage or cultivation determines how effective it is for weed management. Cultivation of soil is more effective in hot weather since under those conditions uprooted weeds dessicate quickly without rerooting. (Terpstra and Kouwenhoven, 1981) Weeds are adapted to exploit the brief pulse of nutrient availability that follows disturbance (Qasem, 1992) and therefore allowing them to grow, then spraying with a broad-spectrum herbicide before emergence of the crop allows the crop a chance to grow before the weeds begin competition again.

Seed Rates:

Sowing more seed does not overcome patchiness usually only few extra plants establish where the soil conditions are poorest. However, in the parts of the field where the seedbed is better, higher seed rates result in too many plants, leading to thin stems and severe lodging (McWilliam et al.1998). Established seed company seed rates for each variety should be followed.

When to plant:

Weather conditions for establishment play an important role. In wet conditions, straw acts as a mulch and increases the soil water content, leading to anaerobic conditions and the production of

acidic leachates during its decomposition. These are damaging to rape plants young seedlings. However, waiting for the ideal planting weather can pose other problems, winter OSR yield has been found to decline with later autumn planting (Lutman and Dixon, 1986). Planting too early may cause a forward crop, but this can be modified with growth regulators. Although OSR has a good capactiy to compensate for damage, an optimal stand establishment efore the onset of winter is a prerequisite for both high yield and high yield stability. (Sierts et al.1987) The current guidelines are to plant in the UK are to sow prior to the 1st of September, to allow the OSR to have good establishment before winter.

How to plant:

Broadcasting seed will give a more spread out growth pattern, but early inter-row hoeing is impossible, so a sterile seed bed is needed, thus broadcasting is unsuitable for organic systems.

Organic systems could transplant seedlings rather than sow seed as oilseed rape is a small seed and establishes slowly and remains non-competitive for an extended period when growing from seed. If some herbicides are lost due to resistance, or due to lack of re-registration, conventional growers may begin to transplant as well. (Libeman et al. 1991)

Weed management:

The presence of weeds change the cultural conditions that result in maximum crop yield. Optimal choice for crop density, cultivatar and planting date often depends on whether or not the grower can expect signifcant weed competion with the crop. Crop yield in the presence of weeds increases with the difference in the height, biomass and leaf area of the crop relative to that of the weeds on the onset of competition. The crop needs a headstart compared to the weeds, factors such as increased planting density, selecting for larger seeds, transplating, larger or more rapidly growing cultivars and planting dates that maximise early season growth of the crop need to be considered.

Desired action on weeds need proper timing relative to season and weather. A given weed species will be more susceptible to a certain type of disturbance at some stages in its development than at others. The stage of the crop's development will determine what disturbance it can take.

Binding weeds such as cleavers damage harvesting equipment, and contamination of weed seeds effect oil content especially if they are similar species such as runch, charlock or other brassicas. Weeds can be hosts to a number of diseases and pests, such as other brassicas being hosts for white-leaf spot, downy mildew and powdery mildew. Volunteer rape plants may be hosts to sclerotinia.

To maintain the effectiveness of herbicides as weed management tools, weeds should be exposed to them as infrequently as possible. Also, certain herbicides can jeopardise enivronmental quality and human health. Herbicides also constitute a rising proportion of crop value. To minimise these factors, effective weed management systems that are less reliant on herbicides are needed. (Liberman et al. 2001)

Residual herbicides are reliant on moisture for activity, as well as having a good, even seedbed, so cloddy soil reduce their activity, on the other hand, light soils allow better activity so dosages may be reduced in early sprays. Their effectiveness is also reduced if the soil has a high organic matter content. (SAC 2005)

Pest management:

Slugs:

Seedbed preparation and quality are at least and potentially more important than chemical control of slugs. In addition, slug damaged winter rape crops are more susceptible to damage from insect pests such as pollen beetle, as the vigour of the crop is reduced, and its ability to cope with further pest damage is diminished. Cloddy seed beds are easier for slugs to move on, so seedbeds need to be rolled to reduce clods after sowing, espeically if planting in clay soil which is prone to clodding, and most OSR is grown on it due to it's water retention properties.

Trapping can help to assess the risk of slug activity. It may be worth trapping in the previous cereal crop up to 10 days before harvest as weather conditions during the brief period between harvesting the previous cereal crop and drilling winter oilseed rape may not suit trapping. If the risk is high, broadcast slug pellets immediately after drilling, do not wait for emergence. OSR seeds won't be eaten, but it will be attacked from cotyledone stage up to 4 true leaves, then monitoring is needed after that stage. Future treatment can be applied if crop growth is slow due to slugs.

Pollen beetles:

HCGA thresholds for pollen beetles on winter OSR are quite high compared to spring thresholds, 15 pollen beetles per plant in well grown crops, 5 in backward, compared to only 3 beetles per plant in the spring. While the well grown winter crop can compensate for damage by producing more and larger seeds in lower pods. This threshold

It has been estimated that bringing flowering forward by 1 week, reducing flower cover by 0.25 and increasing leaf area then the increase in photo-assimilate during the period of seed detemination is estimated to increase by 39%. (Berry and Spink, 2006)

Others:

A dense uniform crop is needed to prevent pigeons landing in the crop, therefore management of other pests to prevent crop death and therefore patchiness is important. Rabbits will eat OSR in severe winter weather due to hunger.

Trap cropping:

Trap crops have been defined as "plant stands grown to attract insects or other organisms like nematodes to protect target crops from pest attack, preventing the pests from reaching the crop or concentrating them in a certain part of the field where they can be economically destroyed" (Hokkanen, 1991) Cárcamo et al.'s 2006 study shows that the seed weevils clearly preferred the trap crop Brassica rapa to the main crop Brassica napus with a factor of 3-5 times more weevils present in the trap than the main crop. Cabbage seed weevils are well known (Dosdall et al. 2006) for invasion from the outside of fields into the field, then distributing throughout the crop, and a field strip of trap crops on the outside of the field is a successful strategy here. There would be additional savings of time and potentially other pests with the same invasion strategy such as flea beetles would be sprayed off also in this field trap strip.

For other pests and Brassica species, evidence for successful trap cropping is based on the trap crop physically taking the role of the host for a crucial part of the pest's life cycle. Cabbage root fly (Delia radicum) is a pest of economic significance for all members of the genus Brassica in Europe, and the most significant pest of Brassica species for Scotland (Evans, 2003). In a research article by Kostal and Finch (1994) it was shown that normal female cabbage root fly flight behavior is interrupted by the non-host plants surrounding the hosts, when crucial host-plant stimulus preceding oviposition is lost, thus ovipositioning on hosts is reduced by at least 50%. This experiment was carried out on a mixed Brassica cropping, with bare soil, peas, clover, grass and four non-living materials. The results proved their hypothesis, and showed that visual disruptions are much more important for pest reduction that chemical repulsion, which could mean the cheapest and easiest to grow of local cover crops or mulches could be used for pest reduction, which would be an economic benefit to farmers.

Other alternatives:

An alternative method, though not one licensed in the EU is that of modifying OSR with the Bt toxin. The Bt toxin is expressed within the plant tissue: cabbage stem flea beetle larvae, weevil larvae and pod midge larvae all feed within the plant and are difficult to control with a surface pesticide spray.

Another alternative is that of biocontrol. According to data from pitfall traps in Goltermann's 1994 study, the predator community in OSR field is dominated by ground beetles and rove beetles in spring and early summer, the time when many of the larvae of OSR pests have dropped to the ground from the flower stands. Spiders dominate in the autumn and early winter when aphid virus vectors and adults and larvae of cabbage stem flea beetles and the turnip sawflies are present. Establishment of elements such as hedges and beetle banks will aid their season-to-season survival (Sotherton, 1984). However feeding habits outside the OSR pests are not well known, Krooß's 1996 study is one of the few comphensive studies on rove beetles, this could mean imported populations to control pests may die in the seasons without pests available. Insects that are extremely useful in other crops for aphid control, such as hover flies and lady birds are of no use in OSR due to their populations being highest in the summer, long after the aphid colonies have become established. (Büchner, 1995) Pollen beetles are also attacked by 3 main groups of parasitioids, with estimites that 25-50% of larvae are killed by parasitoids on unsprayed crops in the UK, these could be introduced to a crop for natural control, espeically with the rise of pyrethroid insecticide resistance. (HCGA 2008)

In the near future these methods, and others based on the same principles of chemical input reduction will become standard as our use of pesticides are reduced for financial, health and environmental reasons, but also due to new legislation in the EU due to come into effect in 2011 further reducing the list of allowed agricultural chemicals (European Parliament Press Release, 2009). The guidelines for this legislation specifically mentions integrated pest management strategies to be used whenever possible instead of pesticides. Sustainability is the key factor, these methods are unlikely to cause any build up of pest resistance, specific trap crops will not interfere with beneficals, or honey bees, and the reduction of chemicals will reduce financial outlays throughout the cropping seasons as well as reducing negative human and environmental effects.

Disease management:

Phoma stem canker is the most damaging disease of winter OSR in the UK (BASF, 2008). Cultural controls are important, early sowing led to smaller stem cankers, whereas high N availability during the vegetative stage led to higher stem canker development "cultural practices greatly influence phoma stem canker development and that the adaptation of cropping systems may be an effective way of controlling the disease." (Aubertot et al., 2004) Infected stubble can only produce primary incolum at soil surface so burying OSR residue can limit the production of ascospores, it may slow the spread of disease within a cropping area, or between fields. Phoma spot stage seen in autumn needs sprayed as spraying is ineffective once pathogen has spread to the stem.

Light leaf spot needs to be treated in the autum too, suspected infection can be revealed by placing leaves in polythene bags at 10-15°C for three days to induce sporolation.

Guidelines for effective resistance management, adapted from BASF

Apply the active ingredient in combination with one or more fungicides of a different type or alternatively as one component in a program of successive fungicide treatments.

Restrict the number of treatments applied per season, and apply only when necessary. Maintain manufacturer's recommended dose.

Avoid eradicant use.

Crop nutrition:

Fertlisers:

Increasing nitrogen dose consistently reduced seed oil content. Altering application timing, by applying more in early spring, or delaying some until April, gave no advantage.In most well-established crops, nitrogen applications can be delayed to March. However, growth of poorly established crops may be boosted by late winter/early spring N dressing if soil conditions allow and regulations permit. Usually no more than 190kg N/ha should be applied. (HGCA, 2006.)

At a deficient site, not applying a sulphur-containing fertiliser substantially reduced both seed yield and oil content. Application of sulphur increased yield and sometimes also oil content at sulphur-deficient sites. The excess or definiciency of one or both the N and S elements may distrub protein synthesis (Anderson, 1990)

Crop size and canopy management:

Many oilseed rape crops are too thick. Trials carried out jointly by ADAS and Nottingham University, and funded by HGCA, BASF, Bayer (and in kind by CPB Twyford) aimed to identify such crops in early spring and test effects of several treatments to reduce canopy size. An application of tebuconazole or metconazole, applied in March or April, can reduce canopy size and disease incidence.This often leads to increased yield.(HGCA Topic sheet 82)

A novel method is that of digital canopy analysis, designed by Harket et al. 1993, using digital photos of the area every week and a C++ program in Windows to anylayse, a grower can quantify effective crop canopy closure and in conjunction with other agronomic indices this can be a quick, accurate and useful tool to predict crop/weed competition, and to determined the influence of agronomic and environmental factors on the growth and vigour.

Proposed management plan for maximum yield:

Favourable preceeding crop.

Well prepared seedbed.

Good timing for planting.

Paying attention to forecasting of pests and diseases and implenting controls if required.

Application of tebuconazole or metconazole, applied in March or April, to reduce canopy size and disease incidence.

Pest, weed, disease, crop canopy and nutrient application thresholds must be established for the particular needs of a particular crop.

Control weeds: need strongly competitive crop, with a vigorous, uniform plant population.

Decision frameworks based on specificed treatments for specific pests should be utlisied rather than general spraying, such as the one below.

Decision framework for future OSR pest management

Decision

Influenced by

Variety and sowing date

Previous crop

Previous pest history

Trap catches

Pest forecasting

Soil type

Weather

Need for insecticide

Trap catches

Pest forecasting

Crop assessment

Economic factors

Insecticide choice

Pest identity

Economic factors

Repeat insecticide treatment

Trap catches

Pest forecasting

Crop assessment

Pest identity

Economic

Modified from Evans and Scarisbrick, 1994.

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