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Prolonged dry spell – what can be done?

4 July 2018

We’ve had calls from across the country for what can be done in the face of the continued dry weather.  We asked a few experts for their advice:

David Lawson, Grassland Specialist at SRUC

The current heatwave that we have been experiencing won’t in itself restrict the growth of the principal grass ie ryegrasses. In fact their optimum growth temperature is 25 C. The main issue is lack of soil water. This is exacerbated in continually grazed swards where the grass rooting depth is severely restricted by continual loss of leaf material. Thus the grass is more prone to drought. One major advantage of rotational grazing is that it allows the grass to put down roots to a greater depth.

Silage and hay crops are not so badly affected by drought. However, re-growth after first silage cut may well be slow.

Lack of water also means lack of nutrient uptake. In dry conditions the grass turns yellow through lack of nitrogen , not water. So when the rains do arrive there will be flush of growth from the un-used nitrogen reserve in the soil. Once this flush has declined it is then advisable to return to the usual fertiliser programme with the aim of applying the same amount of NPK as in ‘normal’ years

It is generally the case that the total amount of grass growth in a year is not significantly affected by the weather conditions , the main difference is on when it grows.


Robert Logan, Animal Health and Welfare topic lead

Our team has produced a series of articles over the last couple of months to look at this issue:

Animal Welfare Updates July 2018 

Animal Welfare Updates June 2018 


Fiona Burnett, Crop Health topic lead

Winter crops are holding up not too badly in the heat and dry weather but crops on lighter land or gravelly parts are dying back rapidly. Winter barley is past the stage for any inputs and most winter wheat crops have had ear sprays. The dry weather reduces the risk of ear disease but T3 treatment is still worthwhile at flowering to protect the foliage from any later infections on leaves and ears when rain forecast for later in July arrives. Don’t forget about doing a mycotoxin risk assessment on wheat as this is compulsory before passing wheat into the food chain.

Spring crops such as spring barley and oats are very stressed by the dry conditions and the need for fungicide inputs is reduced by the dry weather and low disease levels. Be aware that in hot conditions even usually benign sprays can cause scorches so avoid any applications in the middle / heat of the day. Ramularia has appeared in winter barley (probably linked to earlier wetness and current crop stress). It is unclear this year what the risk of ramularia in spring barley is but the symptoms are made worse by strong sunlight and by crop stress so on that basis it is worth protecting the crop. Given resistance to SDHI and azole fungicides the inclusion of the only remaining active ingredient with efficacy (chlorothalonil) is essential.

There have been no confirmed reports of blight in Scotland to date. Check the AHDB Fight against Blight outbreak maps regularly for the most up to date information.

Potential virus-carrying aphid species have now been caught in Scottish aphid suction traps and yellow water traps, and aphid numbers are helped by warm dry weather. Peach-potato aphids are relatively scarce so far, although some have been caught in the suction trap at Dundee, and in yellow water traps near Elgin, Crieff and Duns.

Aphicide treatments with a weekly pyrethroid insecticide (esfenvalerate or lambda-cyhalothrin) should now be underway, with the inclusion of thiamethoxam, acetamiprid, pymetrozine, thiacloprid, spirotetramat or flonicamid every 14 days where peach-potato aphids have been seen in local water traps or suction traps.

Note that thiamethoxam, acetamiprid, pymetrozine, thiacloprid, spirotetramat or flonicamid should not be used in consecutive sprays: i.e. thiamethoxam should not be followed by thiamethoxam; acetamiprid should not be followed by acetamiprid and so on.

Monitor regrowth after burning down and consider an aphicide to prevent virus transmission

Potatoes – slugs
Dry weather has reduced the number of slugs visibly grazing top growth but goodly numbers are still present lower down the soil profile. Test and bait to estimate numbers. Effective slug management in potatoes relies on applying molluscicide pellets at the best timing. Trials over several years have demonstrated that the key timing for pellet application is just before the crop canopy meets across the rows. Missing this timing will not provide significant reductions in slug damage even if pellets are applied several times after this timing. Trials over two seasons funded by AHDB Potatoes have shown that 3x pellet applications: just before crop canopy meets; 3-4 weeks later if there has been rainfall; and at burning down of the crop, has given good reductions in slug damage. Both ferric phosphate and metaldehyde are effective molluscicides, and can be used in a programme (e.g. ferric phosphate, metaldehyde, ferric phosphate). If planning to use a metaldehyde product note the updated guidelines issued by the Metaldehyde Stewardship Group and in particular the requirement for a 10 metre buffer zone to the edge of the field where no metaldehyde pellets should be applied.

Forage and vegetable brassicas
Warm dry weather has lead to higher numbers of Diamondback moths, Silver Y moths and cutworm moths which have been found at our monitored crops. Growers should check crops for eggs and young caterpillars and apply control measures where necessary. Note that diamondback moth are capable of laying eggs through some types of mesh or if mesh is damaged or stretched (and so can cabbage root fly). Note that diamondback moth caterpillars may well be resistant to pyrethroid insecticides, so growers may need to use spinosad (Tracer, Exocet), indoxacarb (Explicit, Steward) diflubenzuron (Dimilin Flo) or Bacillus thuringiensis (Lepinox Plus) as an alternative. Check product labels for specific crop approvals and also check for crop specific EAMU’s for these and other insecticides such as chlorantraniliprole (Coragen).

Soft fruit
The warmer weather will be encouraging aphids, mites and blossom weevils to move into strawberries. Crops should be checked for these pests and any treatments applied before flowering. Slugs will be active too, especially if rain or irrigation is occurring so consider applying pellets but avoid pellets getting caught in the flowers.

Growth splits in raspberry canes are prime egg-laying sites for cane midge. Check these splits for any sign of eggs, and apply an insecticide to target the adult midge before further egg-laying can occur. Note that chlorpyrifos products are now no longer available, and options are limited. The EAMU’s for indoxacarb (Explicit 2013/1369 and Steward 2013/0988) cover insect pests on raspberry so may have an effect, but bear in mind use would be at growers’ risk. Deltamethrin used for raspberry beetle control may have an effect on cane midge.

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