Archive Article: 1997/09/06
EVERY decision counts in oilseed rape – not just for the current crops welfare but also for the following crops in the rotation.
The opportunity to use rape as a cleaning-up crop to reduce grass weeds before cereals go in has long been exploited by growers. This year should be no exception; rapeseed prices may be down, but the cost of weed control is partly offset by lower prices for many herbicides used on rape.
These are predominantly older products but the strength of the pound is responsible for cheap imports of herbicides to the UK this year.
Experts agree that it is best to tackle weeds early while they are still at their smallest vulnerable stages. Of course, if time allowed, a stale seedbed approach after set-aside would represent an excellent opportunity to tackle grass weeds and volunteer cereals.
John Garstang, of ADAS Huntingdon, points out there has been a high percentage of tailings after combines this year, with small barley screenings, in particular, returned to the field. "These could come up in thick swathes," he warns.
"However, a lot of people around here have gone through with discs after harvest, and the thunderstorms we have had will mean the shed seed will be chitting." A total herbicide, such as glyphosate, before drilling will take out these volunteers.
Pre-emergence herbicides such as Treflan (trifluralin) will work well in the right conditions but the weather can easily stay dry after they are applied and the products are less effective than expected.
If this happens, growers need to consider something like Butisan (metazachlor) or Benazalox (benazolin + clopyralid) applied early post-emergence. Mr Garstang warns that they may still not get the control they want from a more expensive product if weeds are slow to emerge or if their spray timing is wrong.
"If you dont get in when weeds are at the cotyledon stage, then they get too big."
He agrees that oilseed rape can be a very competitive crop able to suppress weeds in the right conditions – but not cleavers or heavy stands of volunteer cereals. He suggests Galtak (benazolin-ethyl) can be used against cleavers but growers should expect to return in the spring with a cleaver product.
Bridget Carroll, company agronomist with Aubourn Farming in Lincolnshire, agrees that Galtak is a useful contact product against cleavers but more likely to be used in the spring. "We have found very good results with Galtak on cleavers at 0.5 litre/ha – costing about £14.80/ha," she says.
Because of the downward pressure on herbicide prices Aubourn is now able to buy Galtak at well below £30/litre and Butisan at around 16% less than last year – both well down on last autumns recommended retail prices.
"Trifluralin is still the out-and-out winner in my book – once growers try it they never look back, even on farms with specific weed problems. Damage is often seen but really is limited."
Even on heavy clay soils, Aubourn Farming has achieved good results from trifluralin in dry conditions.
Some growers have had success using trifluralin post-emergence against poppies but only when the poppies are less than 2.5cm (1in) across.
A pre-emergence mix being adopted widely by growers, but without manufacturer approval, is trifluralin plus Butisan which is very effective for around £21/ha (£8.50/acre). Different combinations of rates for both components are used but Ms Carroll suggests cutting back the rate of Butisan too much can weaken cleaver control.
"This year crops are likely to emerge very quickly so timing for pre-em Butisan will be critical, getting it on before chitting could prove difficult."
Choosing a graminicide will depend not just on cost but on the particular weeds to be tackled. "Early blackgrass control is probably best with Falcon – costing around £4.50/acre – while early volunteer barley control is probably most cost efficient with Pilot at 50ml/ha or less, costing around £3.20 plus a wetter.
"Obviously, with the huge drop in profitability, every decision counts once again," says Ms Carroll. "The key is to make those decisions early. People prepared to go to the most trouble at drilling will, I feel, get the best results."
Early action is particularly relevant where growers experienced difficulty last year in blackgrass control. This may simply have been the result of bad timing on large weeds or unsuitable conditions, as well as increasing tolerance of a farms blackgrass population to the fop and dim herbicides.
Where resistant blackgrass is not yet proven, Ms Carroll says good blackgrass control can still be obtained by careful application of either Laser (cycloxydim) or Falcon (propaquizafop) on small weeds. Her personal preference is Falcon for blackgrass control and Laser if wild oats is the main target.
• Zeneca have just introduced a purpose built adjuvant, Partna, to improve grass weed control with Fusilade 250EW.
As returns from oilseed rape are squeezed, David Millar asks how growers tackle autumn weed control cost effectively.