How to overcome risk of lodging in oilseed rape

• Study gives hope for oilseed rape

• Yield potential despite flea beetle

• New spray reduces crop lodging

New research suggests oilseed rape can still be profitable – despite the challenge of lodging and cabbage stem flea beetle.

Growers can reduce the lodging risk even when flea beetle numbers are high, suggests newly published ADAS research. It found that flea beetle tunnelling damage – rather than larvae number – is a better indicator of lodging risk.

The finding could pave the way for an oilseed rape revival in UK rotations. The four-year study was conducted by ADAS consultant Thomas Wilkinson, a senior researcher in crop physiology.

Dr Wilkinson presented his findings at a recent roundtable discussion alongside BASF agronomy manager Matt Keane and Norfolk grower Chris Eglington. Even when high, crop canopy management is as important as ever, said Dr Wilkinson.

“From measuring various stem lodging risk parameters and flea beetle damage in over 350 plants, we’ve gained a number of insights that can help growers lower the risk of lodging.”

“Larvae counts have often been used as the primary indicator for flea beetle damage, but interestingly, in this study, we noted the percentage of the stem damaged by flea beetle tunnelling.

When rape has reached its maximum height and is therefore most at risk of lodging, the flea beetle larvae will – in most situations – have left the stem.

Even where there are a lot of larvae, the severity of the damage we see compared to the overall structure of the crop can range from high to low, particularly in those plants with thicker stems.

Risk model

“During the study we used a lodging risk model rather than actual lodging events because it enabled us to take multiple measurements and meant we weren’t relying on the vagaries of the British weather.”

Dr Wilkinson described important new observations in the understanding of flea beetle impact from the work. He found crops with a high proportion of damage from flea beetle larvae were related to reduced stem diameters.

“This is a key factor in determining stem strength. Yet the flea beetle tunnelling damage we saw, weakened stems more than could be accounted for by the reduced stem diameter alone.

“Whether this tunnelling causes thinner stems, or thicker stems dilute flea beetle tunnelling damage, we are not wholly sure yet, but what we do know is that crops with large robust stems tend to yield well and be more tolerant to larval pressure.”

Growth regulator

Dr Wilkinson also highlighted the importance of using BASF plant growth regulator Caryx. It halved the lodging risk by increasing stem strength, in addition to its already understood height reduction.

“Where plants had up to 25% of the stem area damaged by CSFB, Caryx increased the modelled windspeed threshold from 11.1 to 12.4 metres per second, or 24.8mph – 27.7mph, showing that it reduces lodging risk in typical UK conditions.”

BASF agronomy manager Matt Keane said the results mirrored his observations in the field: “It’s not unusual to find rape crops with robust stems yielding well, even when carrying high numbers of larvae,” he added.

Past ADAS research has highlighted the importance of reduced leaning or lodging of the crop. Lodging at flowering, for example, reduces yield by 46% (lodged flat) or by 20% (lodged to 45°), with an oil reduction of up to 4%.

“It’s important to understand how flea beetle might affect this too. Upright crops maintain all-important light penetration to the lower canopy which minimizes pod abortion, and so maintain yield potential.”

‘Early establishment vital for success’

Norfolk grower Chris Eglington continues to achieve high oilseed rape yields despite high levels of larvae in his crops.

Mr Eglington farms at Letton, near Shipdham. He sows early and aims to achieve 35 plants per square metre, double rolling to retain moisture and encourage even germination.

ADAS researchers recorded a record high of 56 flea beetle larvae per plant on one of Mr Eglington’s oilseed rape crops. But the field in question still went on to produce 4.6 t/ha, he says.

“It’s vital to get oilseed rape established quickly and ensure you are selecting the right seeds – I will throw away 70% of the seeds I have saved and just keep the big ones to ensure I give my crop the best chance.

“My low plant populations are still yielding well, and I’m convinced it’s because the flea beetle does not like the big thick stems. I joke that it takes a chainsaw rather than a penknife to cut them open and see what’s inside but it’s essential to find out.”

Mr Keane reinforces the value of rape as a break crop. It helps spread the drilling and harvest workloads – and a decent market price means it has been a profitable option for growers who have stuck with it over recent years.

“I, like many, would like to see improved confidence in it and it is through ADAS’s enlightening research and their practical advice to growers that will help reduce the crop’s recent variability.”

How to combat lodging in rape crops

Oilseed rape stem damage can be relatively easily assessed simply by looking at the crop, says ADAS consultant Thomas Wilkinson.

“A crop may appear to be developing normally but can still contain internal stem damage, so opening up the stem and having a check can be a useful way of observing what’s going on inside,” he says.

“This might help you decide how you plan your future crops. If you are in an early drilling situation, which is more likely to see higher larval pressure, for example, there might be a way to increase the robustness of your stems to dilute any potential damage.”

Management

Canopy management is important in plants which continue to develop normally despite larval damage, since they will have weak stems. This could require a plant growth regulator coming out of winter.

BASF agronomy manager Matt Keane recommends monitoring crops at the end of February or early March. Any with a green area index (GAI) above 0.8 – which is about 50% ground cover – should be earmarked for a Caryx application. “Wait for stem extension to ensure the crop is developing normally and apply Caryx at 0.7-1 litres/ha from green bud. Big crops, with a GAI of 2 or more will warrant a higher rate of 1-1.4 litres,” he says..

“Where flea beetle larvae are a concern, growers can hold back applications until yellow bud to look for any serious damage. If the crop is still growing well, it will still only be about 50% of expected final height so Caryx will still be beneficial.

“Because Caryx is fast-acting, it is still very effective at this later timing. While there is a bit less height reduction than at green bud applications, there is a greater effect on the canopy and secondary branching.”