weed control in potatoes - home | ahdb...

Weed control in potatoes

Prepared for the British Potato Council Crop Protection Treater Group* by Dr Ken Davies, SAC

and checked by Mark Ballingall, SAC, and the key product manufacturers

*The treater group is a voluntary committee that brings together expertise from R&D, Agronomy, Crop Protection, and Application Technology

March 2007


© British Potato Council 2007 2

• Weeds can affect yield and harvesting of potatoes and may encourage certain pests and diseases.

• Early weed control treatments need to maintain 6-8 weeks of control before the crop can compete through shading.

• Cultivation/re-ridging and thermal weed control are the main control options for organic crops.

• Where using herbicides consider: variety, end-use of the crop, whether early or main-cropping, use of covers, soil type/conditions, weed species and growth stage, and crop growth stage.

Contents Effects of weeds.............................................................................................................3

Timing of weed control..................................................................................................3

Late treatments...............................................................................................................3

Choice of herbicides ......................................................................................................4

Impact of crop type on herbicide choice........................................................................5

Seed crops ..................................................................................................................5

Early ware varieties....................................................................................................5

Growing crops under covers ......................................................................................5

Processing crops.........................................................................................................5

Other end markets ......................................................................................................5

Impact of soil conditions................................................................................................6

Precautions.....................................................................................................................6

Following crops .............................................................................................................6

Mixing herbicides ..........................................................................................................7

Herbicide selection checklist .....................................................................................7

Herbicide mixtures.....................................................................................................7

Table 1: Active ingredients and products used in potato crops ....................................8

Table 2: Weed control of active ingredients used in potatoes ....................................10

The authors do not accept liability for any error or omission in the content, or for any loss, damage or other accident arising from the use of techniques or products mentioned in this document.

USE PESTICIDES SAFELY, ALWAYS READ THE LABEL.



Effects of weeds Potato yields can be seriously affected by the presence of weeds; particularly where crop competition is reduced because of poor growing conditions or disease, or in varieties where haulm development is slow. Weeds can also have a serious affect on lifting the crop, entangling equipment and slowing operations. They can also encourage some pests and diseases such as slugs, free living nematodes, wireworm and rhizoctonia. The potato crop, once the haulm meets in the rows, is usually effective in reducing late weed emergence through shading. However, weeds emerging before row closure can grow above the crop canopy. Entangling weeds such as bindweeds and knotgrass can grow through the crop, and tall weeds, such as fat-hen, sowthistles, oilseed rape and grasses, can grow above the crop, shading and competing strongly for resources.

Timing of weed control Weed control needs to start whilst the weeds are accessible to treatment, that is, pre-emergence of the crop to before the crop shades emerged weeds. The treatment needs to be able to maintain weed control for up to 6-8 weeks after crop emergence to allow the crop canopy to close and shade further weed emergence. In less vigorous and less competitive varieties, canopy closure may take longer, or remain incomplete.

In conventional farming systems, treatments are based on herbicide treatment. This may be before weeds and crop emergence with a residual soil-acting herbicide which gives a number of weeks of control, depending on dose, soil type and weather conditions. Some residual herbicides have some foliar as well as root activity and can be used as the weeds and crop emerge. Mixtures with a contact herbicide are often used at this timing. This timing, slightly later than purely pre-emergence of the crop, allows the residual effect of soil-acting herbicides to persist for longer into the season. This is particularly useful for slow growing varieties or in slow growing conditions.

In organic farming systems, cultivations and thermal weed control systems are used. There is no residual effect from such treatment, so an alternative approach is needed. By planting under a low ridge which is built-up by successive shallow cultivations/ridging operations, good kill of seedling weeds is achieved, whilst reducing the danger of crop root pruning. However, care has to be taken as later cultivations may damage the crop. Just as the crop and weeds emerge, thermal weed control can be successful. This is best used whilst weeds are at seedling stages.

The skilled organic grower can achieve good weed control, but planting the crop in good conditions which allow rapid haulm development, good canopy development and thus good crop competition, is even more important than for the conventional grower.

Late treatments The importance of early weed control is emphasised by the lack of later back-up treatments once the crop is developing. For the conventional grower there are a few herbicides available for late grass weed growth, and for ware growers, two herbicides for later broad-leaved weed control. But they are limited in their activity. For the organic grower there is no such fall-back apart from hoeing or hand-weeding. This is



a particular problem where tall (e.g. fat-hen) and perennial (e.g. couch grass, thistles) weeds emerge late in the crop and are less affected by canopy development.

Choice of herbicides (see Tables 1 & 2) The range of traditional residual herbicide treatments upon which conventional potato growing relies have been greatly reduced in the past ten years. Modern growers are heavily reliant on two residual herbicides: linuron and metribuzin. There is some use of pendimethalin and clomazone, and a mixture of metribuzin with flufenacet. The future of linuron will depend on decisions made under the EU Council Directive 91/414/EEC ‘Plant Protection Products Directive’. This is intended to harmonise national arrangements for the authorisation of plant protection productions. This includes four phases of reviews of existing active ingredients. The third round of the review to accept products into Annex 3 includes linuron. Linuron is being supported by a manufacturer (Makhteshim-Agan) through the review, but this is not a guarantee of acceptance. Further the manufacturer’s support is for relatively low doses of linuron (up to 960g active ingredient/ha), which may be insufficient when used alone in some circumstances.

In 2007, however, a novel residual herbicide has become available, prosulfocarb, which further improves the options for growers. This has been widely available and used in Northern Europe for a number of years as an alternative to linuron. There are also other potential products in development.

Metribuzin also has some foliar/ root uptake activity which allows use on weeds as they emerge, and can be used post-emergence in the crop. The other residuals (clomazone, linuron, pendimethalin, prosulfocarb) are either strictly soil acting or have very limited post-emergence activity. If weeds have emerged then a foliar treatment needs to be added to the residual. This is traditionally paraquat or paraquat + diquat, although more recently carfentrazone – ethyl has become available. Glufosinate-ammonium can also be used in ware crops. Paraquat +/- diquat and carfentrazone-ethyl can be used as the crop is emerging :10% in seed crops and 40% in ware crops for paraquat +/- diquat; 5% in earlies and 10% in maincrop for carfentrazone-ethyl - although as the soil is cracking over the sprouting potatoes is the preferred timing for carfentrazone-ethyl.

Other foliar-acting treatments include the graminicides for control of wild-oat, volunteer cereals, rye-grass, black-grass and suppression of couch-grass : cycloxydim, propaquizafop and quizalofop-P-tefuryl. For broad-leaved weeds, bentazone can be used in some varieties of ware crops and rimsulfuron generally in ware crops. These have some useful activity but cannot be relied upon for broad-spectrum control and bentazone can scorch the crop.



Impact of crop type on herbicide choice

Seed crops Seed potato crops are more susceptible than ware crops to problems related to weed control practices. Any practice which delays tuber production will affect both seed size and yield of these early lifted crops. Herbicides which affect the crop foliage may render a crop liable to be rejected for seed use classification. The wrong herbicide can distort or discolour crop foliage, which may affect seed certification. Only herbicides recommended by the manufacturer for use on seed crops should be used. Table 1 indicates whether a herbicide can be used in seed crops.

• Do not use pendimethalin as it can cause viral-like curling of foliage.

• Preferably use metribuzin pre-emergence rather than post-emergence in seed crops as it can cause discolouration in stressed crops. Avoid use on light or stony soils as; if heavy rain follows treatment it can cause damage to emerging foliage.

• Do not use rimsulfuron or bentazone in seed crops as they can affect the foliage colour.

• Contact herbicides, paraquat +/- diquat, and carfentrazone-ethyl, are best used pre-emergence in seed crops. Post-emergence use may delay tuber initiation and bulking, and therefore harvest date. Do not use glufosinate-ammonium in seed crops.

Early ware varieties Early planted crops and early varieties are generally more sensitive to herbicide treatments. Metribuzin can only be used pre-emergence in named early and second-early varieties. Contact herbicides are best used before crop-emergence to reduce potential checks in growth.

Growing crops under covers The use of covers increases the potential vigour of weed growth and may reduce the tolerance of the crop. The use of a pre-emergence residual treatment is needed in most situations, but only metribuzin (eg Sencorex - check the product label for other products) and linuron (Afalon) have a specific pre-emergence recommendation for use in covered crops. For other products, check with the manufacturer.

Processing crops Where crops are gown on contract for processing, the processor should be consulted before a herbicide programme is initiated.

Other end markets The major supermarkets may have their own limitations on pesticide use in ware potatoes. These can vary widely. For example, one chain will not accept the use of linuron from 2007, and other supermarkets may limit its use. However, this is an ongoing situation and the packer should be consulted before a herbicide programme is initiated.



Impact of soil conditions • Residual herbicides require moist soils to work well. However, excessive rainfall

or heavy early irrigation on light or stony soils can move herbicides to the potato sets and damage the crop. Metribuzin should not be used on sands or very light soils in certain varieties and other residuals (clomazone, linuron and pendimethalin) can readily discolour or distort the foliage of the crop in such soils, and stony or gravelly soils, after heavy rainfall.

• Residual herbicides work poorly on cloddy tilths; a particular problem on heavier soils.

• Where growing on soils of >10% organic matter, the activity of residual herbicides can be seriously affected. Metribuzin is the only residual herbicide with a specific recommendation for organic soils – incorporation to 100-150mm just before or after planting. The contact/ foliar treatments can be used on emerging weeds after crop planting.

• Stone separation can affect herbicide performance where stones are left lying on the surface.

• Residual herbicides should not be applied until after final ridging-up or their effect will be greatly diminished. Do not disturb the soil in any way, for example by fertiliser injection, after residual herbicide use.

Precautions Where using herbicides, following the timing and conditions of use precisely as laid down by the manufacturer. All users must comply with the conditions of approval relating to use of agricultural pesticides.

All users of pesticides must comply with the new relevant statutory codes of practice for using pesticides: For England and Wales: ‘Pesticides: Code of Practice for using plant protection products’ by Defra, PSD, HSE and Welsh Assembly Government, published by Defra (www.defra.gov.uk). For Scotland: ‘Pesticides: Code of Practice for using plant protection products in Scotland’ by Scottish Executive Environment and Rural Affairs Department (SEERAD) and HSE, published by the Scottish Executive (www.scotland.gov.uk). Northern Ireland will produce their own updated version of these codes in due course. These codes replace and update three previous codes of practice, and include information on the legal requirements to keep spray records and advises on new laws regarding protection of ground water and waste management.

Following crops Residual herbicides may under some conditions persist to affect crops planted after the potato crop. Check the product label carefully for such precautions. This is particularly relevant in early growing areas. For example, in West Cornwall on soils with more than 5% organic matter where metribuzin has been used, the soil should be ploughed before planting summer brassicas.



Mixing herbicides • Mixtures can generally be considered between all the residual herbicides, and

between residual herbicides, and between residual herbicides and the contact herbicides. Some mixtures are less useful than others, depending on weed spectrum. The product label may indicate preferred mixtures.

• In mixing products note the most limiting varietal, timing, soil type and conditions recommendations, and adhere to those conditions of use.

• Consult with your merchant, packer or processor before using mixtures in case a part of the mix, or the mixture itself, is not acceptable.

Herbicide selection checklist • Variety • Soil type and conditions

• Varietal haulm vigour • Weather conditions

• End-use: seed/ware/processing/market • Weed species

• Early/second early/maincrop • Weed growth stage

• Whether grown under cover • Crop growth stage

Herbicide mixtures Examples of standard herbicide mixtures used early in potato crops include:

Pendimethalin + clomazone Pre-emergence

Linuron + paraquat ± diquat Pre-/early post-emergence

Pendimethalin + metribuzin Pre-emergence

Linuron + metribuzin Pre-emergence

Metribuzin + paraquat ± diquat Pre/early post-emergence

• Clomazone or prosulfocarb can be added to the mixtures, or metribuzin + flufenacet substituted for metribuzin, for cleaver control; use pre-emergence of the crop only.

• Carfentrazone-ethyl or glufosinate-ammonium can be used instead of paraquat ± diquat, preferably pre-emergence of the crop.

The advent of prosulfocarb in 2007 will add to available standards:

Prosulfocarb + linuron or metribuzin Pre-emergence

Prosulfocarb ± linuron or metribuzin + paraquat ± diquat

Pre/early post-emergence

These are just examples and Tables 1 and 2 can be used to assess options.



Table 1: Active ingredients and products used in potato crops Crop Application

Timing ACTIVITY/ Ingredient

Products

Marketing Company

Dose/ ha

Pre Plant

Pre-em

Post- em

Notes: SEED = can be used in seed crops.

RESIDUAL Clomazone Gamit 36CS

(360g/ℓ) Belchim 0.25ℓ • Before crop emergence only for broad-

leaved weeds. Not on Sands, VLS or >10% organic matter soils. Can cause transient bleaching if heavy rain after application, especially if seed near the surface. The crop grows through with no effect on yield. SEED

Linuron Various (450/500g/ℓ)

Various 1-6ℓ/ha •

○

Dose depends on soil type and planting date. Do not use on Sands, VLS or >10% organic matter soils. Use pre-emergence, up to 10% emergence in early crops or 20% emergence in maincrop, mixed with paraquat +/- diquat or glufosinate-ammonium. SEED Registration for re-approval has been applied for and is expected in early 2008, with a maximum 950g ai/ha (possibly a 2l/ha product dose). There will be a use-up period, so expect the current labels to be used until early 2009.

Metribuzin Various e.g. Sencorex (70% w/w)

Various e.g. Bayer

0.33 - 1.5 kg/ha (up to 2kg/ha in sequen-ces)

•

•

•

Only certain varieties may be treated (see label). Only use pre-emergence on first earlies. Pre-or post-emergence in second earlies. Maincrop: pre-emergence (except certain varieties on Sands/VLS) or post-emergence, on certain varieties only, before shoots reach 150mm. Use incorporated before or after planting and up to 100-150mm in fen and moss soils. Avoid stony soils. There are low dose sequential (x3) programmes available for long periods of weed emergence. SEED

Re-registration will begin in 2008, with possible approval in 2009. Due to operator exposure criteria, the submission is likely to be for a maximum dose of 0.525gai/ha (0.75kg product/ha) pre-emergence and 0.35kgai/ha (0.5kg product/ha) post-emergence in the first instance.

Metribuzin+ Flufenacet

Artist (24:17.5% w/w)

Bayer

•

Pre-emergence of crop and weeds. See label for list of tolerant varieties. Do not treat Maris Piper on VSL. Do not use on Sands. Avoid stony soils. SEED

Pendimethalin Various e.g. Stomp (400g/ℓ)

Various (e.g. BASF)

•

Apply up to 7 days before crop emergence. Effectiveness reduced on soils with >6% organic mater (OM); do not use on soils with >10%OM. Tank mixing with metribuzin recommended, but note that this effects varietal choice. There may be a novel pendimethalin - based mixture product being currently developed.



Crop Application Timing

ACTIVITY/ Ingredient

Products

Marketing Company

Dose/ ha

Pre Plant

Pre-em

Post- em

Notes: SEED = can be used in seed crops.

Prosulfocarb Defy (800g/ℓ)

Syngenta 4-5ℓ/ha •

Apply up to soil cracking over shoots recommended by manufacturer, but up to 10% crop emergence may be used in practice in mix with paraquat +/- diquat. Do not use on soils with > 10% OM. SEED

RESIDUAL + CONTACT Metribuzin as above

Various e.g. Sencorex

Various e.g. Bayer

•

•

•

See metribuzin above.

Mixtures of paraquat +/- diquat or carfentrazone ethyl or glufosinate-ammonium are possible with all the residual herbicides stated above.

•

•

CONTACT Carfentrazone-ethyl

Shark Belchim 0.33l • Best used before crop emergence on small weeds. It can be used to 10% emergence in maincrop and 5% emergence in earlies. Always use with residual partner. SEED

Glufosinate-ammonium

Harvest/ Kaspar (150g/ℓ)

Bayer/ Certis

3ℓ/ha • • • Preferable before crop-emergence, up to 10% emergence in earlier and seed crops, 40% (and 150mm tall) maincrop emergence.

Paraquat Various e.g. Gramoxone 100 (200g/ℓ

Various e.g. Syngenta

2-6ℓ/ha • • • Use before shoots are 150mm tall, before 10% crop emergence in earlier and 40% in maincrop. Preferably pre-emergence in seed crops. SEED

Paraquat + Diquat

Various e.g. PDQ (80:120g/ℓ

Various e.g. Syngenta

2-6ℓ/ha • • • Use before shoots are 150mm tall, before 10% crop emergence in earlies and 40% in maincrop. Preferable pre-emergence in seed crops. SEED

OTHER FOLIAR ACTING Bentazone Basagran

SG (87%w/w)

BASF • Use for some broad-leaved weeds on selected varieties of second early and maincrop. Do not use on seed crops or earlies. Use before crop is 150mm tall. Adjuvant assists fat-hen control in some varieties. Read label carefully regarding weather conditions. Consult packer/ processor before use.

Cycloxydim + adjuvant

Various e.g. haser (200g/ℓ)

Various e.g. BASF

• Dose depends on grass weed. 8 week harvest interval. Consult processor before use. SEED

Propaquizafop Various e.g. Falcon (100g/ℓ)

Various e.g Makhteshim Agan

• Dose depends on grass weed. 7 week harvest interval on early crops, 8 weeks on maincrop. Consult processor before use. SEED

Quizalofop-P Tefuryl

Panarex (40g/ℓ)

Certis • Dose depends on grass weed. 60 day harvest interval. Consult processor before use. SEED

Rimsulfuron+ adjuvant

Various e.g. Titus, Tarot (25% w/w)

Various e.g. Makhteshim Agan

• Use for some broad-leaved weeds. Use before any crop shoots reach 250mm height. Mix with an approved non-ionic wetter or adjuvant as supported by the approval holder. For example, Makhteshim Agan will support Torpedo or Buzz. The only supporters of a straight noni-ionic wetter are DeSangosse with Activator 90. Consult processor before use.

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ctiv

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gred

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s us

ed in

pot

atoe

s

Res

idua

l R

esid

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+ Fo

liar

Con

tact

Fo

liar

Clomazone

Linuron

Pendimethalin + metribuzin

Prosulfocarb

Flufenacet + metribuzin

Metribuzin pre-em

Metribuzin post-em

Carfentrazone-ethyl

Glufosinate-methyl

Paraquat +/- diquat

Bentazone + oil

Cyloxydim

Propaquizafop

Quizalofop-P-tefuryl

Rimsulfuron

Bla

ck-g

rass

-

- -

MS

S S

MS

- S

S R

S*

S*

S*

S*

B

lack

-bin

dwee

d M

S S

MS

MR

MS

MS

S S

S S

MS

- -

- M

S B

uglo

ss, v

iper

’s

- M

S S

- S

S S

- S

S -

- -

- -

Cha

rlock

-

S S

S S

S S

S S

S S

- -

- S

Chi

ckw

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com

mon

S

S S

S S

S S

- S

S S

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- S

Cle

aver

s S

MR

MS

S M

S -

- S

S S

S -

- -

S C

ouch

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mon

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R

R

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MR

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MR

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S M

S M

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M

S M

S S

S S

S S

S S

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M

S S

S M

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MS

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S -

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S S

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- -

Tim

ing

on w

eeds

Pre

Pre

Pre

Pre

Pre

Pre

Seedlings

Seedlings

Seedlings

Seedlings

Seedlings

Post-em

Post-em

Post-em

Seedlings

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Folia

r

Clomazone

Linuron

Pendimethalin + metribuzin

Prosulfocarb

Flufenacet + metribuzin

Metribuzin pre-em

Metribuzin post-em

Carfentrazone-ethyl

Glufosinate-methyl

Paraquat +/- diquat

Bentazone + oil

Cyloxydim

Propaquizafop

Quizalofop-P-tefuryl

Rimsulfuron

May

wee

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entle

ss

- M

S S

- S

S S

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S M

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MS

MS

S S

S S

S M

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R

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Net

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mal

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S S

S S

S S

S S

S -

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S N

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shad

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MS

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S S

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S S

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MR

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S S

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S

Tim

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Pre

Pre

Pre

Pre

Pre

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Seedlings

Seedlings

Seedlings

Seedlings

Post-em

Post-em

Post-em

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