Oilseed rape establishment for good weed control

There are many factors affecting establishment of winter oilseed rape including sowing date, sowing depth and sowing method.

Sowing method can play a large role in the need for weed control. The timing, depth, method and frequency of cultivations influence the composition and density of the weed population. 

In conjunction with cultural control options, chemical methods of weed control remain an integral tool. It is therefore essential that effective and available actives are used correctly.

Propyzamide is one such active which, when used as part of an integrated programme including non-chemical methods, can deliver effective control of problematic weeds such as blackgrass and Italian ryegrass benefitting the rotation.

An example of an integrated approach is using min till cultivations to keep weeds seeds in the top 5cm of the soil; this helps to optimise control from propyzamide.

Consequently, oilseed rape is a crop in the rotation that can be used to help manage blackgrass.

Sowing date

Drilling into moist soils is key. Traditionally, the sowing date for winter oilseed rape is from mid-August to early September.

Drilling after the end of the second week of September is possible but is more of a gamble and can lead to small, poorly established and patchy crops.

Alternatively, earlier drilling in late July is one strategy being employed on farms in a bid to use the warmer temperatures and moisture to aid rapid establishment.

This is to enable crops to grow away from pests such as cabbage stem flea beetle (CSFB) and slugs and having a deep root helps make them be more resilient against the challenges of a cold, wet winter.

Drill timing is increasingly driven by the biology around CSFB infestation.

Early sowing into moist soils (before mid-August) increases the likelihood that the crop will emerge and establish sufficiently before CSFB migration begins and thus be able to withstand attack.

Late sowing (second half of September) may result in the crop emerging after the migration peak, reducing the threat from adult beetle feeding – such crops are also associated with reduced larval pressures.

This is because egg laying and development from the tail end of an infestation is slowed by cooler conditions.

Crops drilled between mid-August and mid-September tend to be at greatest risk from adult CSFB, especially those drilled from the end of August to early September, due to alignment between beetle migration peak and the most susceptible crop growth stages.

Pyllotreta atra (cabbage flea beetle) damage on OSR © Corteva Agriscience

Sowing rate

Seed rates should be geared to achieving about 25-35 plants/m² for both hybrid and conventional varieties.

The seed rate necessary to achieve this target will depend on the percentage plant establishment – which in turn depends on the conditions and slug pressure.

Predicting plant establishment is difficult, but rates are altered to target establishment in the range from 50-80%, with 70-80% being typical in most seasons.

In general, conventional varieties are sown at 50-120 seeds/m² (4-6kg/ha), while hybrids go in at 50-70 seeds/m² (2.5-5kg/ha).

Where 70-80% establishment is being achieved, the optimum seed rate is 35-40 seeds/m².

Sowing depth

Ideally, oilseed rape seeds need to be sown at about 15 to 20mm and covered with well consolidated soil so seed to soil contact is good – any deeper and emergence can be significantly affected.

Seeds sown below 60mm are too deep: at depths up to 100mm they may not emerge at all. Seeds close to or on the surface may be in dry soil for too long and not germinate.

Moisture

Sufficient soil moisture levels during crop emergence are particularly critical. To help optimise moisture levels, timing of sowing can be planned when there is sufficient moisture present in the soil, or to coincide when rain is imminent.

Good seed-to-soil contact is important for best results as it allows moisture to get through the seed coat and give good germination.

Sowing method

There are a number of different approaches to sowing winter oilseed rape with the aim of creating optimum conditions for root growth, general growth and herbicide efficacy.

The different sowing approaches include broadcast into standing crop/stubble (‘Autocast’), direct drill and roll, sub-cast (e.g. drill with a subsoiler, roll), non-inversion tillage (e.g. disc, combi-drill, roll) and ploughing systems (e.g. plough combi-drill, roll).

In oilseed rape, subcasting is often used to establish crops with minimum soil disturbance, a subsoiler leg is fitted with a seedbox and the oilseed rape is dropped into the gap before it is closed.

Another method is strip tilling where a narrow band of soil is cultivated in which to establish the crop and the majority of the field is left uncultivated.

Strip tilling combines the benefits of minimising soil moisture loss and preserving crop residue on the soil surface.

Each cultivation and establishment method can impact weed populations in different ways.

Where blackgrass is a problem, min till cultivations are advised so that grass weeds are not distributed in the soil profile and remain in the top 5cm where they can be controlled with propyzamide.

Ploughing can bring old seeds to the surface whilst burying new seeds below the shallow cultivation area of the soil. In general ploughing reduces weed populations, particularly grass weeds.

The use of non-inversion tillage is variable in terms of weed population impact as there are many types of cultivations from light tines to heavy discs. 

But in general, the use of non-inversion tillage will lead to lower levels of broad-leaved weeds and an increase in the level of grass weeds, particularly bromes, ryegrass and blackgrass.

No till/direct drilling has also led to an increase in grass weeds. In oilseed rape, a subsoiler leg is sometimes used which reduces soil movement and helps to prevent weed seed germination.

Good seed to soil contact is essential for oilseed rape, particularly where the tilth is drying out.  Where there is good soil moisture, this can be less of an issue, although factors such as slugs can increase.