Wet weather this summer in many parts of the UK has led to large slug populations and means growers need to be extra vigilant in monitoring crops for damage from slugs to ensure crops are protected during early stages of growth.
The use of metaldehyde-based slug pellets last autumn led to exceedances of the Drinking Water Directive limit of 0.1 ppb in many areas and this has highlighted the need for close adherence to industry stewardship guidelines on metaldehyde use this coming autumn, says Dr David Ellerton, Hutchinsons Technical Development Director.
“Although the overall threat from slugs is potentially high this autumn, field monitoring and judging the risk of slug damage on a field by field basis remains critical to avoid unnecessary pellet applications, whilst still protecting crops from slug attack.”
Ideally, bait trapping for potential slug problems in oilseed rape should commence in the previous crop and also in stubbles for cereals, he advises.
“The thresholds for oilseed rape are four or more slugs per trap in the previous crop and one slug per trap in the previous stubble. The cereal threshold is four or more slugs per trap. However, trapping is only an effective means of monitoring slug activity when the soil surface is moist and slugs are active.”
“Crops are most vulnerable to slug damage in the first four weeks of growth - the cut off point for monitoring cereals is the start of tillering and for oilseed rape the four leaf stage.”
He points out that a risk assessment for slug damage, including the current and previous crops, field history, soil type, seedbed quality, weather conditions and planting date, can be used in conjunction with trapping to judge the need for chemical control.
“Slug pellets will continue to be the most important means of controlling slugs this autumn, ideally aiming for a minimum of about 40 pellets per square metre, but other measures including seedbed cultivations with adequate consolidation, seed dressings and depth of drilling can have a significant impact.”
There are now only two main active ingredients available for slug control - metaldehyde and ferric phosphate. For many years, metaldehyde has been the main active ingredient that farmers chose to use for slug control. Nevertheless it needs to be managed carefully to avoid problems with drinking water contamination.
An industry led initiative coordinated by the Metaldehyde Stewardship Group (MSG) has established clear guidelines for operators to follow – the aim being to avoid this product being restricted or possibly withdrawn from use completely.
Metaldehyde has four routes by which it can enter water:
• Direct – e.g. inadvertently spreading pellets into watercourses • Point source – e.g. spills on hard surfaces which eventually get into drains • Surface run off from fields following heavy periods of rain • Water moving through the soil that carries metaldehyde with it into the field drainage system.
Scientific studies have highlighted field drainage as the main route by which metaldehyde reaches water courses highlights Dr Ellerton. “Moisture moving down the soil profile will take metaldehyde into the drainage system and from there into ditches and streams.”
“In order to limit water contamination, the annual maximum metaldehyde dose for the calendar year has been set at 700g of active ingredient per hectare and a maximum total dose of 210g ai/ha between 1st August and 31st December, the period when there is the greatest risk of metaldehyde peaks occurring.”
Metaldehyde Stewardship Group (MSG) best practice application guidelines
• Use minimum active per hectare to avoid drainage and run-off losses • Maximum application rate 210g metaldehyde/ha* • Maximum total dose from 1st August to 31st December: 210g metaldehyde/ha* for additional protection of water, suppliers/ BASIS advisors may recommend rates reduced to 160g a.s./ha or less* • Maximum total dose rate: 700g metaldehyde/ha/calendar year* • No pellets to be applied within 6 metres of a watercourse • Do not apply when heavy rain is forecast • If drains are flowing do not apply metaldehyde based slug pellets *from any combination of metaldehyde products
A decision support tool to identify high risk situations for water contamination from a range of active ingredients including metaldehyde is the ‘Wate Aware’ App which has been developed by Adama and has now been amended and upgraded for autumn 2016 to include SlugAware.
It helps growers to assess the risk of water pollution from key products based on current and future weather forecasts, soil type and water deficit at specific locations and is available to download onto Apple or Android smart phones and tablets .This helps growers to identify the potential risk of water contamination from chemical applications and should be used to help avoid peaks appearing in water from metaldehyde and a range of oilseed rape herbicides.
Helping Water Companies
An additional way in which Hutchinsons agronomists are helping to reduce the movement of metaldehyde to watercourses is to provide information to water companies on molluscicide application timing, enabling them to predict high risk periods for metaldehyde reaching water.
“Those companies which abstract water from rivers into reservoirs are then able to only divert water when the risk of metaldehyde peaks are low, thereby reducing exceedances and the likelihood of restrictions on the use of metaldehyde in future, “ says Dr Ellerton.
“Subsequent to requests from water companies for such information, the Metaldehyde Stewardship Group (MSG) launched the ‘Get Pelletwise!’ Agronomic Update system last autumn, providing a number of water companies with weekly electronic reports from agronomists. “
“The reports detail regional agronomic information to help them anticipate metaldehyde usage and inform water abstraction decisions. This initiative will be extended this autumn to cover some 14 water companies, an increase of some 20% on last season and will look to include over 40 counties throughout the UK.”
“Currently the MSG and a number of water companies have set up metaldehyde pilot catchments in which high risk fields have been identified. Farmers with these fields are being requested to refrain from applying metaldehyde in order to protect water and levels in nearby water sources will be monitored to assess the impact of these measures. “
“There have been encouraging results to date, although the success or otherwise is largely dependent on the involvement of farmers in the catchment.” In the meantime, he points out that more needs to be done to ensure there is no repeat of metaldehyde peaks appearing in water this autumn.
“Clearly minimising the amount of active ingredient applied to fields will make a significant difference. Selection of high quality pellets to reduce breakdown and minimise dust during application may also help. Yet one of the key methods would be switching to other products with different modes of action, where there is a high risk to water.”
Alternative Product – Ferric Phosphate
The only other viable alternative for broad acre crops now is ferric phosphate which was launched in the arable market in 2009, the first new molluscicide for 30 years, he adds.
“Its key benefits are that it is as effective as metaldehyde, but is very specific to target only slugs and snails and so presents no threat to wildlife. It is also virtually insoluble in water and therefore may be used in situations at high risk of metaldehyde entering water; such as vulnerable water catchment areas, headland treatments adjacent to watercourses (where other pellets may not be used), and poorly drained heavy soils.”
“It is important to remember that, unlike metaldehyde, slugs which ingest ferric phosphate do not die on the surface of the soil where they can easily be found, but will crawl underground to die. However, they will rapidly stop feeding and so the crop will quickly recover following treatment.”
Preserving Active Ingredients
In summary if growers adopt sustainable slug control policies of only applying high quality slug pellets where risk of slug damage is high, abiding by the MSG guidelines and switching into an alternative mode of action in situations where there is a risk of metaldehyde entering water, it should be possible to preserve the remaining molluscicide active ingredients in the market place, contends Dr Ellerton.
“However, it is crucial that the agricultural industry joins together in adopting best practice strategies to minimise the risk of metaldehyde reaching water courses and so maintain this vital active ingredient in the battle against slugs.”
Growers must hold-off drilling winter wheat on land affected by black-grass for at least another month or risk fire-fighting this pernicious weed for the rest of the season.
That is the warning from Hutchinsons technical manager Dick Neale who says many growers still fail to recognise the importance of delayed drilling in controlling black-grass.
All too often the temptation is to push ahead with autumn cultivations and drilling whenever conditions allow for fear of the weather changing and getting “caught out” later. But high dormancy this autumn (see panel below) combined with dry soils in many southern and eastern areas means very little black-grass has yet emerged and drilling too early risks high levels of emergence within the crop, which will be harder to control and potentially exacerbate problems.
“Don’t be fooled. Although some growers who created stale seedbeds early, to the right specification and in moisture, have seen a good black-grass chit, the bulk of the population will not emerge until late September with a second major flush in the first week of October. If black-grass is a threat you can’t afford to drill before the 15th of October.
“Most soil will take two to three inches of rain with no problem and bring land back to the conditions required for a good seedbed creation and emergence. The best advice is to leave ground well alone and wait for black-grass to emerge. We’re only talking about delaying drilling on fields affected by black-grass, so you can still drill earlier elsewhere.” Mr Neale warns this autumn is very different to last year, when seed return from the previous year was lower and soil conditions favoured good pre-emergence herbicide efficacy. “But the weather this spring resulted in bad tiller control in a lot of places leading to high seed return, so the starting population for many growers has already been ramped up several times over.”
For those debating whether to apply a pre-emergence herbicide where seedbed conditions remain very dry, Mr Neale points to trials that show the efficacy of residual chemistry is reduced in dry years and he reiterates the need to wait for soil conditions to improve before drilling.
“If you’re drilling into dry seedbeds during September, when most black-grass emerges, then you’ve already gone wrong.”
He also advises avoiding winter cropping on the worst “red” black-grass fields, which should go into spring cropping to allow more time to reduce weed population pressure outside the crop and also introduce alternative forms of chemistry to help manage resistant populations.
Key tips: • Avoid drilling before mid-October where black-grass is present • Leave ground alone while black-grass germinates • Avoid deep cultivations (e.g. subsoiling) which can bring fresh seed to surface and disturb natural soil structuring • Spray-off seedbeds with glyphosate prior to drilling – follow the WRAG guidelines • Consider spraying-off flushes earlier if weed emergence justifies it (i.e. more than 200-300 plants/m2 emerge) • Minimise cultivation depth before and during drilling – black-grass germinates mainly from top 50mm • Use a pre-emergence herbicide e.g. products based on robust co-formulations of flufenacet with either picolinafen/ PDM/ DFF, tailored to suit the weed-spectrum • Follow-up with a robust post-em application based on a range of active ingredients • Put worst black-grass fields into spring cropping.
Black-grass dormancy this autumn is the highest since 2012 due to cooler than normal conditions during the key seed-set period in June, according to ADAS. Analysis of 30 samples shows an average germination of just 22%, which is well below the last three years (33-35%) and only slightly higher than 2012 (20%). However, ADAS warns that dormancy only applies to freshly shed seed and the effect may be “minimal” depending on the amount of seed already in the seedbank. “The key factor affecting germination this year will be moisture availability.”
Mr Neale also points out that dormancy tests can be influenced by the type of crop black-grass has grown in, so results should always be treated with caution and management decisions made on an individual field-by-field basis.
For example, black-grass that has grown under a large, early-sown oilseed rape canopy may have matured earlier under dull, damper conditions than black-grass in a spring-sown barley crop, which is likely to have emerged and set seed later, with heads above the canopy in full sun. In such cases, black-grass seed collected from the oilseed rape crop may exhibit higher dormancy than that from the barley.