Assess the winter survial of your organic winter oilseed rape

Plant population, damaged main shoots, cabbage stem flea beetles, and sclerotinia steam rot are the key factors to assess in your organic oilseed rape field.

By Sven Hermansen

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Below you will find some useful advice on how, in early spring, to check whether conditions in your oilseed rape field are good enough for the crop to survive. If the crop is too weak, it may be more worthwhile to plough it down as a cover crop and establish a new crop instead.

Late summer and autumn 2024 generally provided good conditions for establishing winter oilseed rape. Depending on the variety and field conditions, this may have resulted in vigorous crop development with soft stems and elevated main shoots.

Therefore, fields should be inspected now. It can be a difficult decision, so consider whether you need a “second opinion” from your organic adviser, who sees many winter oilseed rape fields.

Plant size and distribution, weed pressure, and overall field condition should all be included in the assessment of your winter oilseed rape at this early spring stage.

Below, you can read about four key points for assessing your organic winter oilseed rape field in spring.

1: Count the number of oilseed rape plants in the field

The first thing to check is the plant population in your oilseed rape field. Ideally, there should be more than 30 evenly distributed plants per m² to ensure a robust and competitive crop. In fields with a thin plant stand, weeds will easily become too dominant, resulting in yield losses.

The optimal plant population is in the range of 30–60 plants per m² when growth resumes in spring. However, we know that a winter oilseed rape plant in good condition has an excellent ability to compensate for low plant density by producing side shoots. Therefore, all recommendations require an overall assessment of the field.

You can estimate the plant population by counting the number of plants per running metre in a row and multiplying this by the number of rows per metre in the field. If your row spacing is 50 cm, there are 2 metres of row per m², meaning there should be at least 15 plants per metre of row to reach 30 plants per m². It is important to carry out counts in several different areas of the field to get a reliable impression of overall field conditions.

At 12.5 cm row spacing: 4 plants per running metre correspond to 30 plants per m²
At 25 cm row spacing: 8 plants per running metre correspond to 30 plants per m²
At 50 cm row spacing: 15 plants per running metre correspond to 30 plants per m²

A thin oilseed rape field that should be assessed to determine whether the plant population is adequate..

2: What does the oilseed rape plant look like?

It is also important to examine the condition of the individual plants, particularly whether the main shoot is intact. If the main shoot has been damaged by frost or pests, it is necessary to assess the quality of the side shoots and the level of weed pressure, as it will take longer for the crop to provide the necessary competition against weeds.

Oilseed rape field with varying numbers of main shoots. Assess the entire field.

3: Check for cabbage steam flea beetle infestation

A third factor you should assess is how severely your winter oilseed rape plants are infested with cabbage stem flea beetles. You can do this by making a diagonal cut just above the growing point in the stem of a plant and counting the number of flea beetle larvae. Again, it is important to walk the field and carry out the test in several different locations.

The larvae of the cabbage stem flea beetle can cause significant damage by feeding inside the stem. However, vigorous plants in good growth can tolerate damage from up to 3–5 larvae. In contrast, as few as two larvae in a small plant can be critical.

As a rule of thumb, a stem diameter of more than 8 mm indicates a strong plant. Weaker stems are generally more vulnerable. Therefore, it is important to assess plants across the entire field. The number of larvae that plants can tolerate while still producing good yields also depends on plant population, row spacing, and weed pressure.

Decision-support tools have been developed for advisory use, incorporating the key parameters for field assessment. If you are unsure, consult your adviser.

Larvae of cabbage stem flea beetle in a split oilseed rape leaf petiole.

4: Remember to consider sclerotinia stem rot

Later in the spring, after flowering, symptoms of sclerotinia stem rot may appear. It is important to assess any infections, as this has implications for how oilseed rape can be managed within the crop rotation in the future.