OSR: To grow – or not to grow?

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The decision to grow, or not to grow, WOSR this autumn will largely revolve around the question of “can I control Cabbage Stem Flea Beetle”(CSFB), both at the adult feeding stage and the larval stage of the pest, says Hutchinsons technical manager, Dick Neale.

Like black grass, one of the primary issues to understand is what the pest does at specific times and any parameters that may cause a certain reaction from the pest, he says.
“We know we have significant numbers of larvae within the crop and these will be pupating in the soil during June. This will lead to adult emergence during July and feeding on weeds, lower leaves in WOSR and field boundaries.

“Monitoring numbers of adult beetles present in the harvested crop will give a clear indication of the potential problem in the 2016 autumn crop.”

During August, the adults feed slowly and prepare for the mating season. In early September the adults move to new crops and mate, this is when the aggressive feeding phase of the life cycle starts and the new crop is placed under most pressure.

Mr Neale believes that a clear counter to this is to establish crops as early in August as possible, so that plants are sufficiently robust to cope with some feeding in early September. “Adult feeding continues until egg laying commences, at the base of established plants, during late September and October. Predation and the onset of winter cause a rapid decline in adult beetle survival after October.”

“The eggs develop in the soil driven by temperature and eggs hatch from October to February. This is a key phase to monitor - the treatment threshold is 2 larvae per plant, or 50% of leaf petioles damaged.”

Where high numbers of adult beetles are evident during the harvesting process, the question of “should I grow WOSR at all?” has to be asked.
As with black grass and disease control in many crops, the agronomic optimums for yield have to be compromised in order to facilitate reliable control of the overriding issues, he says.

“In WOSR the optimum plant population, drilling date and avoidance of verticilium wilt are now being overruled by the need to get the crop past CSFB in the ‘hot spot’ areas of the country.”

 “Frequently over looked when the mind is set on gross output, variety choice is vitally important for verticilium control and soon to be a parameter on the AHDB Recommended Lists, and currently it should be a question for breeders to provide guidance.”

Adjusting seed rates

The optimum plant population for WOSR is, around 25-30 plants/m2 established. This has been facilitated by seed rates from 45 seeds/m2 and for a 5g TSW seed lot, this would equate to 2.25kg/ha.

However, Mr Neale believes that there is an increasing need to calculate seed rates much along the lines we have developed now in cereals for black grass control and late sowing.

Target plants established pre winter 50/m2 = 2.50kg/ha
Seedbed loss 10% - 0.25
Loss to slugs 10%  - 0.25
Loss to adult CSFB 50% - 1.25
Total seeding rate - 4.25kg/ha (85 seeds/m2 for a 5g seed lot)

The agronomic parameters have changed and for hybrid seed (sold in 50 seed/m2 packs) more packs would be required to meet the above criteria, he adds.

“Conventional seeds are sold in 4 million seed packs (100 seed/m2) so there is still room to adjust here to 5 – 6 ha’s, bearing in mind the weight calculation above.”

“The above calculation is a logical approach to the issues we now face; one thing to avoid is applying significantly higher seed rates than these, because having far too many plants results in thin, weak stems that will allow easier entry for CSFB larvae, compared to fewer, larger more lignified stems.”

“WOSR is very capable of regulating its own plant population over winter. Any population below 85plants/m2  pre-winter will self-regulate to a spring population of around 45/m2 by the spring, so if losses to CSFB and slugs amount to only 40%, the resulting higher plant count will not be agronomically detrimental.”

Establishment cultivations

The next parameter to address is establishment cultivation. A minimal amount of soil movement is vital to allow adequate control of black grass via the herbicide propyzamide. Any amount of subsoil movement is detrimental to propyzamide effectiveness (including ‘low disturbance’ legs).  

Dick Neale of Hutchinsons

“Shallow soil movement has been seen to deliver a significant reduction in larval infestation of the rape crop, although not always a reduction in adult feeding pressure, which is more driven by drilling date.”

“This reduction in soil movement also reduces the mineralisation of nitrogen within the seedbed and, when early drilling, this prevents rapid and excessively large growth, which is a good thing. Nitrogen, however, is a vital component in the successful establishment of WOSR, as the crop has a high requirement during the establishment phase. “

Nitrogen availability can be judged by:

•    WOSR establishment success
•    Previous crop yield i.e. 12+ tonnes/ha wheat yield will have ‘emptied’ the upper horizon N reserve
•    Straw removed?
•    Dry soil
•    Field history (background fertility)
•    Organic manure/digestate application

Mr Neale notes that Primary P placement fertiliser is extremely useful and it is becoming clear that soil Index is not the main driver for use, it is more about the ability of the plant to extract P and the soil’s ability to release P. Some N is also applied, but only a very small quantity.

“Where nitrogen is needed as a priority, Hutchinsons has found that one of the most effective ways of treatment during a dry August sowing period is to mix 10kg/ha of technical grade urea directly with the seed. This product is used in horticulture and the 25kg bags make it very easy to mix with seed in small batches and to transport from field to field.”

The product is technical grade and is coated - this prevents it from being rapidly hygroscopic and in a sealed tank will last at least 3 days without degradation. Mixing will obviously blow the urea directly down the pipe with the seed.

“The drill needs to be calibrated to deliver 14.25kg/ha of total seed/fertiliser mix and as the granule and seed sizes match well, there are no separation issues. “

Treatment of CSFB

With the release of neonicotinoid seed dressings no longer an option, all growers will have to be timely with their treatments for adult feeding, and the key timing as the crop begins to produce its first leaves says Mr Neale.

The need to switch to absolutely minimal soil movement, to be successful in crop establishment, cannot be emphasised enough.

“Although there is no available trials’ data, there is much anecdotal evidence to suggest that spraying the adults during the hours of darkness does help significantly, largely because the pest is most active and can be hit with the spray.”

“As with treatments for black grass, success comes from proper application. It is vital to create a degree of swirl and spray movement just above the soil surface to get spray deposition around and under plants and on to the soil.”

He says that operating at 20kph, with booms set at 75-100cm, just does not work! “The spray sheet breaks up and ‘swirls’ way before it is at soil level, leading to unwanted drift and limited control. “

“Efficacy critical applications should be applied in 100l/ha water volume through flat fan or angled flat fan nozzles, with booms at 40-50cms above the crop, with forward speeds from 8-12kph.”