Challenging season for desiccating oilseed rape

Spring 2012 was a season of extremes. An arid, hot March was followed by record-breaking rainfall in April. Cool wet weather continued through most of May and some late frosts were recorded, before a mini-heatwave hit at the end of the month.

This unpredictable weather pattern gave rise to one of the longest flowering periods for oilseed rape in recent years, says John Taylor, United Oilseeds’ area manager for East Anglia.

“Flowering got off to an early start before crop development slowed. Higher temperatures at the end of May then extended the flowering period. Pod production was extended by about a fortnight, occurring over eight to 10 weeks.”

As a result, crops are exhibiting a much wider variation in maturity than normal, making it difficult to judge when to apply pre-harvest glyphosate.

“Timing will be critical this year and accurate crop assessment is a must to identify the three-to four-day application window that will produce the best results,” Mr Taylor advises.

Applying glyphosate too early will stop immature pods developing, leading to a high percentage of small seeds and affecting yield. Oil content can also be reduced, as levels build during the latter half of the seed filling period.

“Oil crushers will reject crops that contain more than 4% immature seed,” he says. “Seeds should have a bright yellow, oil-rich inner – too green indicates excess chlorophyll which can contaminate the oil during crushing.”

Spraying too late risks excessive yield loss through pod shatter as the sprayer passes through the crop. Equally important, efficacy may be affected.

“Crops still need to be actively growing to take up glyphosate and translocate it effectively,” says Mr Taylor. “Otherwise stems remain sappy much longer, delaying harvest and/or slowing combining.”

Glyphosate should be applied when two-thirds of seeds in pods halfway up the main raceme are turning from green to brown. Samples of 15-20 pods should be taken from representative areas across the field.

“Start looking once the crop’s dark green colour starts to pale, and go back every two or three days. Don’t examine headlands as they receive less nitrogen and mature earlier – walk into the field. Once seeds are at the right stage, spray within three to four days.”


A pod sticker such as Pod Stik or Arrest will minimise the risk of seed loss during the two- to three-week period before the crop becomes fit to combine.

“Don’t be tempted to go earlier. The crop will still be sappy, making combining more difficult, increasing losses and contaminating the sample. With the crop worth £345/t for harvest plus 10% oil premiums giving a total value of £379.50/t, it is well worth investing a little extra time and care to protect those returns,” says Mr Taylor.

Effective desiccation puts growers in control of harvest, says David Ellerton, Hutchinsons’ technical director. “It helps even up maturity, especially important in a season like this, and if the weather stays wet desiccation will be vital to stop crops growing.”

Many crops were lush and thick from the start and received comprehensive PGR applications, so stems are shorter and squatter than usual, he adds. “Strobilurins were also widely used, keeping leaves greener. Crops will require optimum desiccation efficacy to minimise harvesting difficulties and to ensure they are ready in good time.”

Glyphosate also gives effective pre-harvest control of weeds, notably problem grass weeds like couch and blackgrass through direct kill and by reducing seed viability.

“The overall effect is to dry up all the green material in a field, easing combining and reducing contamination. This helps keeps combine sieves clean, reducing seed loss over the back, and can also lower drying costs,” says Dr Ellerton.

Crops should be regularly inspected from the start of senescence. “You need to get a feel for each individual field,” he advises. His advice differs slightly from Mr Taylor’s – he suggests taking 20 middle pods per raceme and if two-thirds of seeds in 15 or more of those pods are turning from green to brown then crops should be sprayed within four days, or a week in cooler conditions.

All recommended glyphosate products will work, he says. “However, don’t assume they are all the same. Some formulations work better than others.”

Roundup Flex, a 480g/litre formulation available commercially for the first time this season, is one. Hutchinson plot and commercial trials last year confirmed its exceptional rainfastness, rapid penetration and reduced drift characteristics, says Dr Ellerton.

Whatever glyphosate is chosen it should be applied at 1080g of active ingredient/ha, or 1480g where couch and thistles are a problem. Water volumes of 100-150 litres of water will ensure the active gets well down in to the crop and onto weeds hidden under the canopy, he advises.

“Crops tend to shut down later in the day, so start early in the morning and finish late afternoon to get the best results.”

Pod sealers

Pod sealers such as Pod Stik or Zip Pod, a new, more flexible formulation, are pretty much a standard recommendation. “They ensure more mature pods don’t shed early, either due to inclement weather or during combining. In extreme circumstances you can lose 50% or more of the crop through shedding.”

Some growers apply sealers a few days before glyphosate as there is some evidence that they work better on earlier maturing pods, Dr Ellerton notes. The practice also reduces pod shatter when the sprayer runs through with glyphosate a few days later. “But most growers don’t like going through the crop twice so generally they combine the two at the glyphosate timing.”

A place for diquat

Diquat (Reglone and others) has largely been superseded for pre-harvest desiccation. But it is still useful in particularly weedy areas, usually around headlands or in more open patches of crop, where rapid burn-down and a long drying period are needed, both men agree.

“Crops are at a much higher risk of shedding because diquat makes everything brittle, so an earlier pass with a pod sticker should be considered,” says Mr Taylor.

Crops should be assessed as for glyphosate, but applications should begin when two-thirds of seeds are turning from brown to black, he advises.

Dr Ellerton favours diquat on crops that have kinked stems after lodging. “Kinks prevent glyphosate being translocated, so it won’t kill lower stems. Apply diquat in 200-250ltres/ha to ensure adequate coverage.”

Desiccation pointers

  • Variable maturity

  • Thick, lush crops

  • Glyphosate favoured

  • Timing critical

  • Assess crops carefully

  • Be patient – allow crop to dry before cutting

‘Avoid green stems’

Growers are being urged by one combine manufacturer to do everything possible to avoid harvesting green oilseed rape stems to minimise combining problems..

“Big, thick stems with ready access to soil moisture are a major challenge for any desiccant,” says Claas’ Tom Pine. Add serious leaning and uneven ripening and it promises to be even greater this season. So, as well as effective desiccation, setting-up and operating the combine correctly will be crucial to the most efficient harvest.

“Even with the wider throat and accelerated pre-separation (APS) technology of our combines, ensuring an even flow into the main drum will require particular care and attention. Which means constant adjustment of combining speed to crop condition to minimise jams.”

For the greatest combining efficiency, Mr Pine advises a wide concave setting, together with a slow secondary separation rotor speed to avoid too much trash going through the machine.

To prevent table auger blockages becoming increasingly common, he suggests regularly adjusting the slip clutch to maintain the recommended pressure which tends to decline with each successive blockage. He also recommends cleaning out the preparation pans under the drum every day to avoid the gumming and rubbish adherence that both compromises grain samples and increases seed losses.

“With crops as prone to shedding as many are likely to be this season, it’s very important to check the ground under them before combining to establish the level of pre-harvest losses too,” says Mr Pine. “Otherwise you could easily slow down unnecessarily to try to limit the amount of the seed you see behind the combine when it actually has little, if anything, to do with your harvesting.