Four critical months to minimise OSR yield losses

Oilseed rape yields can vary by more than 1t/ha between seasons and researchers have identified ways that farmers can manage their crops to minimise seasonal effects.

While the yield trend has been upwards, the UK average has seen fluctuations, especially in recent years because of the greater extremes in weather.

To identify the effects of weather on yields, Basf funded work carried out by Adas which analysed Defra yield data going back to 1979 in parallel with Met Office weather data.

See also: A round-up of enhanced urea fertilisers and additives

Analysis revealed four key weather factors that lead to high yields: a warm October, a dry December, a sunny, dry April, and a wet/cool/dull May.

These four factors account for 37% of the yield differences seen. Here we take a closer look at how farmers can maximise yields.

1. Warm October

An increased temperature of 2C is associated with 0.17t/ha extra yield.

Why?

Adas researcher Christina Clarke explains that a higher temperature in October leads to better crop establishment and rooting, which in turn means more vigorous plants going into the winter.

“They are then less affected by winter conditions than a low biomass crop,” she explains. Plants are also more able to withstand pest pressure.

In the following spring, having a larger canopy means plants have more resources (nitrogen) to draw on when growth kicks off.

How to mitigate?

In a cold October, look to improve crop vigour by:

  • Maximising seed-soil contact to improve establishment.
  • Timing drilling to maximise chances of having sufficient moisture.
  • Increasing the water holding capacity of soil to avoid slow establishment because of drought, as happened in autumn 2018.
  • Using varieties with high early vigour.

2. Dry December

A 50mm reduction in rainfall is associated with a yield increase of 0.11t/ha.

Why?

This yield increase is down to less waterlogging, which inhibits root growth.

Christina Clarke points out that unlike cereals, oilseed rape roots don’t have aerenchyma. These air spaces or channels help the movement of air from the shoots to the roots, which is why OSR plants cannot cope as well as cereals with waterlogged soils.

Stunted root growth also means plants are less able to scavenge nutrients.

How to mitigate?

In a wet December, look to reduce waterlogging by:

  • Tackling any field drainage problems.
  • Avoiding soil compaction.
  • Maximising autumn plant vigour.

3. Dry sunny April

A 50mm reduction in rainfall is associated with 0.2t/ha extra yield.

Why?

This increase in yield with less rainfall is down to better light penetration of canopies, thus increasing the number of seeds/sq m.

Adas researcher Christina Clarke says drier conditions help to avoid canopies becoming over-large. A large canopy at flowering will reflect a lot of sunlight and is less efficient at harvesting light, she says.

Interception of light post flowering is important, as this is the period when the crop is setting the number of seeds. The target is 100,000 seeds/sq m.

Dr Clarke highlights that drier conditions also reduce the sclerotinia risk, as soil moisture is key the development of the disease.

How to mitigate?

In a wet April, look to increase light penetration for maximum seed numbers by:

  • Avoiding too large a canopy at flowering – aim for a green area index of 4-4.5 at flowering. This can be achieved by delaying nitrogen and using growth regulators.
  • Carefully timing flowering fungicides to minimise sclerotinia.

4. Wet/cool/dull May

Increased rain of 15mm/1C cooler/15% fewer sun hours is associated with a yield increase of 0.12t/ha

Why?

This is all about reducing water stress and prolonging the duration of a green canopy by delaying senescence.

Survival of leaves is important, as they are much more effective at converting light into energy than pods, says Dr Clarke.

This is the key seed-filling period and crops need adequate water supply to avoid losing pods.

Last year, some Yield Enhancement Network (Yen) farmers got to the target 100,000 seeds/sq m, but still failed to hit 5t/ha because of a lack of water at this critical stage.

Looking at the top and bottom yields, there was a 1t/ha difference and one key factor was that the top yielding crops had an extra 10 days between flowering and desiccation.

Basf business development manager Clare Tucker says using Filan (boscalid) to manage sclerotinia can also help delay senescence in some conditions.

This is because the fungicide counteracts the effects of ethylene, the stress hormone that regulates leaf senescence.

How to mitigate?

In a hot/dry May, look to reduce seasonal effects by:

  • Maximising photosynthesis and canopy duration using fungicides and late foliar nitrogen.
  • Using growth regulators to stimulate lower order branching to increase the number of pods and seeds.
  • Keeping on top of disease – light lead spot can lead to early leaf drop and phoma can affect water uptake.