While oilseed rape crops which survived establishment have, for the most part, been growing well in benign autumn conditions, issues of dry soils and cabbage stem flea beetle damage were still top of mind for iOSR growers at their latest meeting.
Held on the site of the dedicated Syngenta iOSR Focus Site, at Rougham in Suffolk, the consensus was that pest pressure had been somewhat worse than normal, but the damage caused had been made significantly greater by the dry soils and slow establishment.
This season’s experiences of cabbage stem flea beetle (CSFB) have highlighted the challenges of adapting sowing dates in an attempt to avoid damage, witnessed on the iOSR Focus Site and confirmed by growers.
In Lincolnshire, iOSR grower Andrew Ward has previously largely escaped the worst attention of CSFB by delaying drilling into September, but this year, even with spray treatments, areas have already been written off and successfully re-drilled with wheat.
“Even if damaged OSR does struggle on, the indications are that there will be high larvae numbers in the remaining plants, which will severely reduce its ability to yield,” he said.
Norfolk iOSR grower Chris Eglington reported CSFB damage was worse than he has normally experienced and, unusually for his farm, did require spray treatment this season.
Coupled with dry weather and establishment issues, for the first time he has concerns that up to 15% of his OSR area may not make it through winter.
The iOSR Focus Site’s trials to look at establishment and drilling techniques – including seed rates, sowing dates and row widths, along with companion cropping and other innovative approaches – will provide practical ideas for growers looking to outgrow CSFB.
Syngenta technical manager Georgina Wood added that, interestingly, where surface trash had been spread at the iOSR Focus Site, the crop had established far more strongly and evenly than where straw and trash was removed.
“It may be that the straw shielded the crop from pest pressure, or that in the dry conditions it served to retain any available moisture and helped the crop to grow away,” she said.
Establishing oilseed rape at very high seed rates could prove counterproductive in reducing the impact of cabbage stem flea beetle.
Crops established at high plant stands appear to be infected by correspondingly high numbers of larvae, according to studies. That not only has a detrimental effect on the growing plant, but has a legacy of perpetuating large numbers of beetles at harvest.
Speaking to iOSR growers at the Syngenta iOSR Focus Site, insecticides technical manager Dr Max Newbert reported research showing crops planted at 10, 20 or 40 seeds/sq.m suffered only marginally greater pest feeding damage, compared to crops at 80 or 120 seeds/ sq.m, even in higher damage situations.
When that translates to the numbers of larvae, the infestation per plant is similar meaning crops at higher density could have up 10 times the populations – potentially hitting 500/sq.m or more.
Where intense larvae numbers lead to side branches being damaged or lost in spring, crops at high plant density have limited ability to compensate, compared to more open crops, he pointed out.
Companion crops sown with oilseed rape have potential to protect soil quality and claimed enhancement in establishment. But in the dry autumn, competition from companions, such as clovers, would appear to have checked crop growth, reported Georgina Wood.
“The effect has been most severe where the companion cropping has been sown in the rows with the OSR,” she said. “However, where it was sown between the OSR rows, crop establishment has been better.
“Now that some of the cover has been killed off by frosts, we can see and assess how well the crop can recover.”
Focus Site trial plots cut back in November to remove growth containing CSFB larvae.
This season the Syngenta iOSR Focus Site is undertaking a series of trials looking at chopping back autumn growth over winter to physically remove green material with cabbage stem flea beetle larvae inside and break the lifecycle.
Some plots have already been subjected to different severity of a single or double chop, with others set to be cut down in February.
Georgina Wood said, with the relatively mild autumn conditions, this season a high proportion of CSFB eggs would have already reached the 200-day degree temperatures required to hatch and infest plants.
“We have certainly seen a lot of the petioles cut off contain larvae. With the early chop the larvae should, hopefully, be too small to survive and re-infest the plants, which may be the case with bigger larvae at the later timing,” she added. “However, the early spring cut may catch more of the later hatching larvae.”
Norfolk iOSR grower Chris Eglington said that he plans to take part in an ADAS trial to assess the practicality and implication of cutting back the crop to target CSFB populations.
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