Crop Watch: Slug warning for slow-emerging cereals

The increased threat from slugs is a key concern for our agronomists this week, as it is not too wet or cold for their activity in crops that farmers have managed to get in.

Crops that are slow to emerge are at higher risk, with those that were drilled towards the end of October in the West only just emerging from the cold, wet soils.

In oilseed rape, signs of phoma are being seen, but one agronomist urges close inspection of crops. What appears to be phoma could be downy mildew lesions, which can look very similar.

North: Mary Munro

AICC/Strutt and Parker (Perthshire)

Mary Munro

Mary Munro

Another four weeks of wet weather with perhaps one spraying day in the last month….but Scotland is still better off than many parts of the UK.

See also: Norfolk grower trials closed-transfer sprayer system

The more northern arable regions are further on than central and southern Scotland, and some have finished all the planned autumn work. In Perthshire most potatoes have been lifted and wheat drilled behind them, but in Fife and points south, things are still well behind.

Sowing winter wheat in December is not new, and many Scottish growers will keep going even into January if possible. It is an amazingly resilient crop and will cope with conditions that are far from ideal. Seed rates should be increased with later sowing to compensate for slugs, wetness and frost.

The frustrations are not new – soils drying out to the point of being able to travel over and then it rains and we are back to square one.

As each week goes by it seems less likely that all the outstanding “autumn” jobs will be done before Christmas, but there are many alternatives – later herbicide applications, adjusting rates and using different products if necessary.

Spring barley

The main concern is the amount of spring barley that will be sown in the UK for 2020 and the impact that will have on the market.

 It is inevitable that spring barley will be the main replacement for winter wheat. Spring wheat and oats can be a good fit although later to harvest, and the break crop options are worth considering too. It is important to check that the alternatives will also have a market to go to.

Slugs love the wet conditions, of course, and it is important to continue to monitor them, especially as the crops are now growing slowly and many have not nearly reached the two-leaf stage.

I have abandoned metaldehyde and am recommending ferric phosphate pellets to my growers. The pellets last longer in the wet and do at least as good a job.

Pesticide stores are somewhat full, therefore, with supplies of slug pellets and chlorothalonil, and the various wheat and oilseed rape herbicides that should have been on by now.

West: Antony Wade

Hillhampton Technical Services (Herefordshire/Shropshire)

Antony Wade

Antony Wade

Whenever I sit down to write my monthly article, I think I’m not going to go on about the weather! But we work in an industry where the weather can actually prevent us producing the product that produces income.

Weather affects many other industries such as the construction industry, but the effects usually only result in delays to project timelines, rather than preventing the end result.

November has followed the pattern of October with the odd few dry days at the start of weeks followed by a breakdown to rain, which undoes any drying that occurred in the previous days.

So far in November, 103mm has meant very little progress with any drilling and increasing unlikeliness that any more will be done this year. However, with spring seed availability and pricing reducing the incentive to switch to spring cropping, we are still hoping we might get some wheat planted before February.

Cereals that were drilled towards the end of October are only just emerging from the cold wet soils, but in areas in fields that are waterlogged or that have had flood water across them there are losses to seed rot, resulting in poor germination.

Slug threat

Slugs are requiring repeated ferric phosphate applications, especially in wheat after oilseed rape. So it is still too early to assess how successful establishment will be in these crops and whether they will need redrilling or patching up in the spring.

Ground conditions have not allowed any spraying to be done, but fortunately wet cold soils have seen a reduced germination of weeds so far.

Oilseed rape crops are generally improving slowly after the early flea beetle damage and continued slug damage, but plants in wet patches are turning purple. Closer examination reveals some larvae in leaf petioles, while looking on one farm club root galls were regularly found.

These crops are by no way guaranteed to have survived in the spring, which makes deciding how much to spend on them more difficult.

Plans were made for propyzamide, now soil temperatures are dropping, and fungicide, as phoma is at threshold in many crops, but even thinking about spraying seemed to induce another 16mm of rainfall.

All we can hope for is that 2020 starts drier than 2019 is ending.

East: Marion Self

Prime Agriculture (Suffolk)

Marion Self

Marion Self

Despite growers’ frustrations they have made progress with drilling, albeit slow and in difficult conditions without finesse.

My estimate would be that most growers in Suffolk have drilled somewhere between 50% and 85% of the planned winter cereal area with much of the “drilling” occurring during three days at the end of October.

Barley drilling dates were stretched until the end of October (as long as we dared), but plenty of seed is left in the shed. 

With reduced day length and falling soil temps, these October-drilled crops have been slow to emerge, although a dabble in the mud usually reveals strong shoots that will reveal themselves soon and, no doubt, boost our mood.

I urge you to monitor slug populations carefully – it is not too wet or too cold for their activity. Hollowed seed can be found and although most emerged crops are establishing well, there are significant areas that have been thinned due to slugs, especially after rapeseed or where the drilling technique has left a loose, rough environment. 

Well-manufactured ferric phosphate-based pellets are most appropriate. They provide more resilience in wet and cold conditions, therefore offering better value and less concern regarding water quality issues. 

If conditions are favourable, most growers will continue to drill planned winter wheat crops knowing that the latest safe drilling date for most varieties is the end of January or mid-February.

Spring crop options

However, as time moves on it will become important to compare likely gross margins of a lower-yielding winter wheat crop with spring drilling options. The best cropping option will vary according to the individual field and/or business needs. 

On planted crops persevere with pre-emergence applications and where required, any outstanding barley yellow dwarf virus and manganese applications. For rapeseed, soils are now cool enough (10C or below at 30cm depth) to maximise blackgrass control with propyzamide.

Astrokerb is proven to give good control of some broad-leaved weeds, although control can be disappointing where these weeds have become too big pre-application. In difficult weed situations control can be widened by tank mixing with Belkar (halauxifen + picloram).

Beyond this our thoughts move to planning spring logistics and agronomic strategies, including early tiller management of winter wheats with rolling, well timed and appropriate doses of nitrogen and plant growth regulators adapted to the season and crop growth.

In summary, conditions in East Anglia are challenging. While we haven’t been able to achieve all we want, in the conditions we would like, our thoughts are with those in other areas who have been less fortunate than us to the point that many are still under water.

South: Richard Harding

Procam (Sussex)

Richard Harding

Richard Harding

Some very useful rain has fallen over the Downs this weekend, which will greatly aid residual herbicide efficacy applied to what are, without doubt, excellent seed-beds this autumn. Soil temperatures are slowly beginning to cool but are still around 12-13C at 30cm.

With much glyphosate being used now in “stale seed-beds” and stubbles, it is now very important to be vigilant for the first signs of glyphosate resistance. Some reduced sensitivity has already been identified in brome populations. As such, it is important to maintain the correct dose rate.

For small flushes of blackgrass, the aim should be for a minimum of 540g/ha glyphosate. While for a typical stubble/stale seed-bed application the dose should not be less than 720g/ha.

The continued high air temperatures mean bird cherry oat aphids have shown a big increase in numbers and are above the 10-year mean for us in the south of England. This will have implications for early-sown and emerging cereals. Numbers of Myzus persicae are also on the rise.

Attention to detail will be vital for barley yellow dwarf virus control this autumn and the AHDB’s T-Sum calculations will help to determine the optimum spray dates when secondary aphid spread is about to occur.

Other key considerations to minimise disease transmission will be in leaving a four- to six-week gap between ploughing grass or weedy stubbles and drilling, where green bridge transmission is likely, or to switch to using a rapid-acting desiccant.

Oilseed rape

Oilseed rape crops, on the whole, look excellent and continue to grow apace, with later-drilled crops now shrugging off any remaining flea beetle and slug pressure.

Reasonable levels of phoma are now starting to appear. Some crops are already at the threshold of 10% infected plants for a variety with a low resistance rating, while 20% infection can be tolerated on varieties with higher scores.

 Where thresholds have been met, this is prompting an application of either straight difenoconazole on less developed crops, or prothioconazole and tebuconazole where some canopy manipulation is required.

The option then remains to come back later in the autumn to top up phoma control and cover light leaf spot overwinter at the conventional propyzamide timings, which are likely to be towards the middle/end of November.

However, crops require close inspection, as what appears to be phoma can actually be downy mildew lesions, which look very similar, but without the tell-tale black pycnidia.

Downy mildew on OSR © Blackthorn Arable

The downy mildew lesions will also typically have some fluffy, greyish mycelium on the underside of the leaf. This can often cause over-estimation of phoma levels in the early part of the season.

Leaf miner damage is also now easy to find in oilseed rape crops. Despite the unsightly mines, it is unlikely that these pests ever justify insecticide treatment.


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