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Munch, Munch, Munch: OSR Weed Control – CPM – Simon Wilcox, Ed Stevens

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Simon WilcoxIt's become impossible to consider oilseed rape without mentioning cabbage stem flea beetle in the same breath. CPM finds out how herbicide strategies are shaping up this autumn.

Cabbage stem flea beetle (CSFB) is shaping the way UK farmers grow oilseed rape and has single-handedly been responsible for a wholesale shift away from using pre-emergence herbicides. OSR establishment this autumn has been a mixed bag, depending on the time of drilling, says Cambs farmer and Hutchinsons agronomist, Simon Wilcox.

"Wet weather during the first half of August brought a catchy start to harvest with two major rains of over 10mm on Aug 9 and 14 here in the East. Since then, high pressure brought warm days and nights until 24 Sept, when low pressure moved in bringing rain with it."

Generally crops that went into the ground before mid-Aug were safely through the ground before there was any sign of CSFB migration. Whereas later drillings have suffered badly from dry conditions and CSFB damage, he explains.

"Growers that chose to take advantage of the unsettled start to August have generally got OSR crops with between 6-10 true leaves. These crops quickly got to 3-4 true leaves before there was any sign of CSFB damage.

"Any placed fertiliser has been available in the moist conditions and long day length has aided crop growth," he adds.

Fast-moving crops

Regular crop inspection has been the key to managing the early planted, fast-moving crops. "Decisions about early volunteer cereals, grass and broadleaf weeds were made early as many herbicides can't be used later without damaging the crop."

Notts-based AICC member and Arable Alliance agronomist Andy Wells is making good use of new herbicide approval Belkar (halauxifen-methyl+ picloram) because of the flexible approach it offers to broadleaf weed control.

"My weed control strategy has been to make sure of a clean start by making use of glyphosate pre-planting and I've tried not to use any residual herbicides. The uncertainty over establishing the crop means we don't want to incur herbicide costs so it's a wait and see approach," he adds.

Last season gave many agronomists the opportunity to use some Belkar, even though supplies were very limited. Andy is adopting the split dose approach and has applied 0.25 l/ha when weeds are small and then will make a decision on how to follow it. He believes in many cases the one split will do enough before following with propyzamide or Astrokerb (aminopyralid+ propyzamide) later on.

Belkar has done a cracking job where he's used it this season, says Norfolk agronomist Ed Stevens of Hutchinsons. "I've already gone in with 0.25 l/ha in some crops and I doubt I'll need to follow it up. Where l do, it'll be a mix and match depending on the weed spectrum," he says.

Simon has also found Belkar to be a godsend in the new era where pre-ems are no longer relied upon. "In warm early conditions, I've been impressed with the speed and spectrum of broadleaf weeds that Belkar is suppressing, including small thistles."

But there is a cautionary note among the praise for Belkar. "In some instances last year we saw crop effects which were quite severe, causing the petioles to stick together to produce a rhubarb-like plant," says Will.

According to Corteva, reports of such symptoms are rare. "We estimate they account for less than 1% of crops treated with Belkar. In all cases crops have recovered and there has been no negative yield impact. We believe the cause may be too high a rate being applied too early, to stressed crops," says product manager John Sellars.

Crops under stress

"Applications at 0.25 I/ha should be made from when the crop is at BBCH12-14 and for 0.5 I/ha applications the crop must be from BBCH16. Sprayer operators should be careful to avoid boom overlaps and Belkar shouldn't be applied when crops are under stress," he adds.

Ed StevensFor Ed the main grassweed problems on his farms are ryegrass and bromes but in many other regions, black-grass is the priority.

"Earlier sowings have received an early broad spectrum graminicide to reduce crop competition during establishment from cereal volunteers. Where early germinating black-grass established during Sept, these crops have also received a well-timed Centurion Max (clethodim) application," says Simon.

Following the stewardship guidelines for clethodim is important, particularly after crop damage was noted to be worse last season in crops which were badly infested with CSFB, highlights Andy. He witnessed this in some of his crops last year where clethodim was applied towards the tail of the treatment window, so he intends to apply well before the 15 Oct stewardship cut-off this time around.

As crops go into Oct, residual herbicides will be planned, particularly for forward crops, says Simon. "As we enter the main grassweed germination, these crops will be assessed for an early carbetamide or propyzamide application in order to inhibit emerging weeds while soil conditions are moist, rather than try to combat deep-rooted large black-grass later," he comments.

Agrii agronomist Will Foss says he'll be keeping a close eye on black-grass in the earlier drilled crops. "Any black-grass that has emerged since clethodim was applied earlier in the autumn may need an interim application of carbetamide in advance of the main propyzamide application period," he says.

Propyzamide usually takes the last curtain call before the spring, with a tricky spot in terms of timing. The soil temperature needs to be 10°C or less at a depth of 30cm to prolong the half-life of the active ingredient and these conditions are usually met around 10 Nov, in an average season.

Any earlier carbetamide will be sequenced with Astrokerb, depending on soil temperatures and soil moisture, says Simon.

"On mineral based soils, early Nov applications often achieve the best results from propyzamide and protect the water courses from applications later in waterlogged or frozen soils," he says.

The fortunes of OSR crops drilled after mid-Aug are more mixed, with all four agronomists citing crop failures.

"These crops vary in success and size depending on their proximity to the Aug rains. For my money the later Aug-planted crops have proved to be the most problematic to manage," comments Simon.

Moisture management and drilling technique have been the key to success during this window, he believes. "I've encouraged growers to chop the straw and leave longer stubbles in a bid to maintain moisture and cooler soil temperatures whilst building beneficiaries.

Low disturbance

"On the whole precision disc-based drills or very low disturbance tine drills have been more successful during this dry spell as they've helped conserve any moisture and have produced a more uniform and full emerging plant strike. Even crop emergence has been key to diluting the CSFB feeding pressure," he says.

Ed also highlights a strong correlation between cultivation technique and crop viability."Where plough and combination drill have been used when conditions were dry, this has produced cloddy seedbeds and resulted in uneven emergence and more CSFB damage, particularly where it wasn't rolled.

“It's not necessarily about the machinery, it's attention to detail that matters. What's important is to achieve a consistently right drilling depth that will result in even emergence of the crop," he comments.

By Sept there were already some re-drilling situations and whether by intention or necessity, it's this later window which looks most hopeful for OSR crops, believes Simon.

"These drillings are mainly a result of previous failures and were re-drilled around the 20 Sept. They have received over 20mm of rain and are now at emergence to expanded cotyledon. They will need favourable growing conditions this back end before the winter shut down to get to five true leaves before winter," he comments.

Best average yields

On the plus side these crops are emerging without any CSFB present." If this remains the case, they will escape any direct feeding and the egg laying period. This was also observed last season and produced some of the best farm average yields. Whilst less competitive in the autumn and at considerable risk to pigeon and other pest issues, they're generally lower cost to produce, requiring only one graminicide and at less risk from TuYV."

But some unanswered questions do remain, he points out. "Will the CSFB that were active during the previous failed drilling impact on these newly emerged crops? And will eggs laid during that phase impact the current crop? Only time will tell," says Simon.

Belkar is likely to feature heavily in herbicide programmes for these later drilled crops, according to all the agronomists.

"Later sowings will be approaching this application in the next 14 days (by mid-Oct), while the very late drillings are likely to be treated with a single application of Belkar and Astrokerb together at the appropriate timing later in the season," says Simon.

"Where carbetamide hasn't been applied to later sowings, this will be available to be applied in Jan, if required. Later sowings often have a smaller more open canopy at this time of the season. As day length begins to increase and soils warm, often a spring flush of black-grass can be triggered.

"Carbetamide at this timing can help reduced black-grass filling in the gaps and compromising the following rotational decisions, but don't apply if soils are very wet and crops are very small in order to avoid any crop damage," he concludes.

Beating adult CSFB is only half the story

The bigger canopies on early planted crops leave them in a good position to compete with weeds, but Simon warns that all may not be as rosy as it seems.

"In my experience these crops always flatter to deceive when it comes to their final performance. They often cost more to produce due to higher disease, pest and weed pressure and at the point of harvest, they can succumb to verticillium wilt and produce a small seed sample."

And that's not where the potential problems end. "Even though direct CSFB damage may have been avoided, these plants have been playing host to the CSFB egg-laying phase. Larval infestation may result in these crops going backward in the spring as they're eaten from the inside out.

“Inspection during late autumn will be required if any flailing or grazing techniques are to be adopted in order to stop stem invasion from infestations in the petioles occurring in the spring," he adds.

Earlier drillings are also exposed to a higher risk from aphid and subsequent TuYV infection. "The main aphid vector for TuYV is the peach potato aphid (Myzus persicae). A useful service to give a handle on aphid pressure is the Rothamsted/SASA suction trap network which is communicated through the AHDB," says Simon.

“These models are best used in conjunction with regular crop inspections. Peach potato aphid shouldn't be confused with mealy cabbage aphid (Brevicoryne brassicae) which are conversely a poor vector for TuYV, but if present in significant numbers can cause direct feeding damage.

"Many harvest trials this summer have highlighted the yield penalties from TuYV infections, with resistant varieties climbing to the top of the AHDB Recommended List this season following last year's mild winter," he says.

Andy has been advising his growers to adopt OSR varieties with resistance to TuVY now there's plenty of choice available, such as Aspire (conventional) or there's a wide range of hybrids with the trait.

“TuYV has become a serious issue, with 75-100% infection in Dec last season in Notts. I'm not sure foliar insecticides are a very effective way of controlling the virus because it has a wide infection window and plenty of local virus sources. It's better to use genetics to solve the problem rather than spraying," he comments.

Biscaya (thiacloprid) has an approval to control the aphid vector of TuYV in OSR but Will believes the insecticide may have an effect on CSFB larvae, especially when mixed with the pyrethroid, lambda-cyhalothrin.

"CSFB adults arrive in the crop and spend two weeks feeding before starting to lay eggs. It then takes 240-day degrees before the larvae hatch, so approx. a further three weeks. This season the main CSFB migration occurred over the Aug bank holiday weekend, so larvae should start to hatch and move into plants at the end of Sept," he explains.

"The first instar larvae are the most susceptible to insecticide treatment according to Rothamsted so we're evaluating using Biscaya to target them this autumn. It's only one instar of the larvae but the first is probably the most important one," he adds.

Winter stem weevil is just as significant as CSFB in Andy's area so he's looking to target these as well. “The stem weevil larvae can be found in the stems of plants over the winter, whereas the CSFB larvae are usually in the petioles. Although the weevil larva has a white body like the CSFB larva it can be distinguished by its pinkish brown head and curled shape in the stem," he adds.