Activity of mesotrione on resistant weeds inmaize†

Peter Sutton,1* Claire Richards,1 Larry Buren2 and Les Glasgow2

1Syngenta, Weed Science, Jealott’s Hill International Research Centre, Bracknell RG42 6EY, UK2Syngenta Crop Protection, Vero Beach Research Centre, Florida 32967, USA

Abstract: Mesotrione is a new callistemone herbicide that inhibits the HPPD enzyme (p-

hydroxyphenylpyruvate dioxygenase) and introduces a new naturally selective tool into weed-

management programmes for use in maize. Mesotrione provides control of the major broad-leaved

weeds, and it can be used in integrated weed-management programmes depending on the grower’s

preferred weed-control strategy. At post-emergence rates of 150gAI ha�1 or less, mesotrione provides

naturally selective control of key species that may show triazine resistance (TR), eg Chenopodium

album L, Amaranthus species, Solanum nigrum L, as well as species of weed that show resistance to

acetolactase synthase (ALS) inhibitors eg Xanthium strumarium L, Amaranthus spp and Sonchus

spp. The data presented show that resistant and susceptible biotypes of these species with resistance to

triazine herbicides, such as atrazine, simazine, terbutylazine and metribuzin, or ALS-inhibitor

herbicides, such as imazethepyr, remain susceptible to mesotrione. These results confirm that there is

no cross-resistance in biotypes with target site resistance to triazine or ALS-inhibiting herbicides. It is

important that herbicide choice and rotation becomes an integral part of planning weed management,

so as to minimise the risks of crop losses from weed competition, build-up of weed seed in the soil and

the further development of weed resistance across a range of herbicide modes of action.

# 2002 Society of Chemical Industry

Keywords: mesotrione; HPPD; weed management; resistance; triazine; ALS

1 INTRODUCTIONIntegrated weed management requires that the farmer

or field manager has a number of options and

alternative strategies available so that he can choose

an appropriate weed-management strategy for the site,

conditions and criteria.

In North American and European maize, weed

resistance has become a factor for two classes of

herbicides in particular, the triazines and the aceto-

lactase synthase (ALS) inhibitors1 (Table 1). Maize

crops in the USA and Europe are infested with a wide

range of summer annual grasses and broad-leaved

weeds. Table 2 shows the listing of major weeds

according to market research data.

Sustainable weed management strategies need to

consider:

. Crop rotation

. Crop competitiveness—varieties, spacing

. Cultivation—stale seed beds, no-till, inter-row

. Costs, timing and benefit (crop value)

. Weed population, biology and ecology

. Herbicide choice (spectrum, mode of action and

timing)

Continual use of exactly the same weed manage-

ment over 4 years has been shown to result in

increased weed numbers, weed shift and a consequent

reduction in maize yields.2 Introduction of meso-

trione3 in the USA and Europe for use in maize where

it is naturally selective provides a useful and flexible

addition to the products available as it enables good

control of some key aggressive weeds. It can also be

Table 1. Occurrence of triazine and ALS resistance

Species of Weed

Number of

countries

Number of

US states

TR ALS-R TR ALS-R

Amaranthus retroflexus 13 3 10 4

Amaranthus rudis 1 1 5 6

Chenopodium album 19 0 20 0

Solanum nigrum 10 0 0 0

Kochia scopularia 2 3 9 17

Sonchus oleracea 1 2 0 0

Xanthium strumarium 0 1 0 10

TR=Triazine resistant (See Reference 1).

ALS-R=ALS resistant.

(Received 2 March 2002; revised version received 13 May 2002; accepted 29 May 2002)

* Correspondence to: Peter Sutton, Syngenta, Weed Science, Jealott’s Hill International Research Centre, Bracknell RG42 6EY, UKE-mail: peter.sutton@syngenta.com† Based on a presentation at the Meeting ‘Resistance 2001’, organised by IARC, SCI and BCPC and held at IACR, Rothamsted, Harpenden,UK, on 24–26 September 2001

# 2002 Society of Chemical Industry. Pest Manag Sci 1526–498X/2002/$30.00 981

Pest Management Science Pest Manag Sci 58:981–984 (online: 2002)DOI: 10.1002/ps.554

used flexibly in terms of rate and timing (pre-

emergence as well as post-emergence), and it adds

diversity to the modes of action used in the field.

Consequently it will be used in a range of mixtures to

complete the weed spectrum.4

This paper illustrates these principles with examples

of mesotrione’s activity on triazine-resistant and ALS-

resistant weeds, when used alone or in mixtures.

2 METHODSThe results are taken from glasshouse tests either in

the UK or USA with susceptible and a resistant

biotype of species known to show target-site resistance

from previous testing. The glasshouse test design is a

randomized complete block with three or four

replicates, and the treatments were applied to weeds

at the four- to six-leaf stage using a single nozzle spray-

rig at a volume of 200 litre ha�1. Visual ratings, using a

0% (no injury) to 100% (dead plants) scale, were

recorded 20–28 days after application (DAA).

3 RESULTS3.1 Mesotrione efficacy on ALS-resistant weedsThe data presented in Table 3 clearly show that

mesotrione treatments are equally effective on the S

and ALS-R biotypes of Sonchus oleracea L and

Xanthum strumarium L, whereas imazethypyr has

failed to control the ALS-R biotypes.

3.2 Mesotrione efficacy on triazine-resistant weedsSimilarly, the data presented in Table 4 clearly shows

that mesotrione treatments are equally effective on the

S and TR biotypes of Amaranthus retroflexus L,

Chenopodium album L and Solanum nigrum L, whereas

atrazine has failed to control the TR biotypes.

3.3 Mesotrione mixtures on triazine-resistantweedsTesting mixtures on triazine-resistant weeds reveals

that mesotrione and photosystem II (PSII) inhibitors

are synergistic and give surprisingly good control of

both the S and the R biotypes. This is illustrated for

mesotrione/atrazine on Amaranthus (Fig 1) and for

mesotrione/terbuthylazine (Fig 2). Tests using

Chenopdium and Solanum also show the mixture of

HPPD inhibitor with PSII inhibitor is remarkably

effective on weeds that are resistant to PSII inhibitors.

4 DISCUSSIONThese results show briefly how herbicides with

differing modes of action can be combined in an

integrated programme to enhance the spectrum of

weed control, even where target-site resistance has

developed. Mesotrione offers potential for pro-

grammes with either pre-emergence, or post-emer-

Table 2. Major weeds in maize

USA France/Italy/Germany

Setaria spp Chenopodium spp

Xanthium strumarium Polygonum spp

Abutilon theophrasti Echinochloa crus-galli

Amaranthus retroflexus Digitaria sanguinalis

Chenopodium spp Setaria spp

Ambrosia artemissifolia Solanum nigrum

Helianthus annuus Amaranthus retroflexus

Sorghum halapense Stellaria media

Amaranthus rudis Matricharia spp

Digitaria sanguinalis Mercurialis annua

Table 3. Mesotrione efficiency on ALS-resistantweedsa

Treatment (gha�1)

Control (%)

Sonchus S Sonchus ALS-R Xanthium S Xanthium ALS-R

Mesotrioneb (75) 84 95 97 90

Mesotrioneb (100) 100 100 99 96

Imazethapyrc (100) 67 0 99 0

a S=susceptible, ALS-R=ALS-resistant.b Applied as 100g litre�1 SC (Callisto, Syngenta).c Applied with 2.5g litre�1 NIS.

Table 4. Mesotrione efficacy against triazine-resistant weedsa

Treatment (gha�1)

Control (%)

Amaranthus Chenopodium Solanum

S TR A TR A TR

Mesotrioneb (75) 76 96 100 100 100 100

Mesotrioneb (150) 100 100 100 100 100 100

Atrazinec (1000) 100 0 100 0 100 0

a S=susceptible, TR=triazine-resistant.b Applied as 100g litre�1 SC (Callisto, Syngenta).c Applied with 10g litre�1 COC.

982 Pest Manag Sci 58:981–984 (online: 2002)

P Sutton et al

gence timings to cope with the full range of weeds, soil

types and climatic conditions.

Overuse of specific herbicide modes of action

contributes to resistance build-up, and so herbicide

rotation has become a desirable aspect of weed

management. At this time ‘ALS’ resistance is spread-

ing in the US corn/soya rotation, whilst triazine

resistance has become widespread where continuous

maize is grown in both North America and Europe.

Factors that increase likelihood of resistance in maize

are:

. Continuous maize

. Continuous non-ploughing

. Reliance on herbicides only

. Reliance on herbicides with the samemode of action

. Reliance on herbicides where resistance is known

. High weed pressure

. Weeds with high seed production and low dor-

mancy.

Factors that decrease the likelihood of resistance in

maize are:

. Good crop rotation (including winter cropping)

. Annual ploughing

. Use of cultural control

. Use of non-selective herbicides for clean-up and

seedbed preparation

. Low weed pressure (=good husbandry)

The adverse factors are typical of many maize/corn

production systems and their flora. Consequently any

sustainable weed-management system in maize needs

to incorporate at least three, and preferably more,

modes of action for the control of key weeds.

Mesotrione and other p-hydroxyphenylpyruvatedeoxygenase (HPPD) inhibitors introduce a new

Figure 2. Mesotrione/terbuthylazine on Amaranthus (TR biotype).MST=mesotrione; TBA=terbuthylazine.

Table 5. Modes of action for use in maizea

Group

Mode of

action

Active on

ExampleGrasses

Broad-leaved

weeds

F2 HPPD ✓ ✓ Mesotrione

K3 Cell division ✓ ✓ S-Metolachlor

B ALS ✓ ✓ R Nicosulfuron

O Auxins ✓ Dicamba

C3 PSII ✓ Bromoxynil

C1 PSII ✓ ✓ R Atrazine

N Lipid synthesis ✓ ✓ EPTC

a R=resistance in the market.

Figure 1. Mesotrione/atrazine on Amaranthus retroflexus (S and TRbiotype). MST=mesotrione 2.7gha�1; ATR=atrazine 60gha�1.

Pest Manag Sci 58:981–984 (online: 2002) 983

Activity of mesotrione on resistant weeds in maize

mode of action to the crop—the other key modes of

action6 are given in Table 5.

Verdier et al. 4 suggested a technique for assessing

the risk of developing herbicide resistance in cereals,

Table 6 extrapolates that thinking into maize and may

be used for discussion of herbicide strategies.

5 CONCLUSIONSMesotrione has a distinct mode of action as an

inhibitor of HPPD and is highly active on triazine-

resistant and ALS-resistant weeds, with no suggestion

of target-site cross-resistance. This new callistemone

herbicide introduces a new naturally selective herbi-

cide tool into weed management in maize and it is very

effective in controlling problem broad-leaved weeds

that have become resistant to other modes of action.

It is important that herbicide choice and rotation

become an integral part of sustainable weed manage-

ment so as to minimise the risks of increased seed rain,

weed shift or weed resistance and the resultant losses

in crop yield and quality from weed competition.

REFERENCES1 Heap I, available at www.weedscience.com (2001).

2 Harvey RG, Weed species shifts following four years repeated

applications of common corn herbicides. Weed Science Society of

America Proc p 209 (2001).

3 Wichert R, Foxon GA, Townson JK and Bartlett DW, Technical

review of mesotrione, a new maize herbicide, in Proc Brighton

Crop Protect Conf—Weeds, BCPC, Farnham, Surrev, UK, pp

105–112 (1999).

4 Sutton PB, Foxon GA, Beraud J-M and Anderdon J, Integrated

Weed Management systems for maize using mesotrione, in Proc

Brighton Crop Protect Conf—Weeds, BCPC, Farnham, Surrey,

UK, pp 225–230 (1999).

5 Verdier JL, LeTerrier JL, Gasquez J and Roy C, Reasoning

process for prevention and management of herbicide resis-

tances, Association Francais de Protection des Plants—16th

Conference, COLUMA, pp 165–174 (1998).

6 Schmidt RR, HRAC Classification of herbicides according to

mode of action, in Proc Brighton Crop Protect Conf—Weeds,

BCPC, Farnham, Surrey, UK, pp 1133–1140 (1997).

Table 6. Maize herbicide resistance risks: Croppingassessed over 5 years and scored as 1, 3 or 5 forlow, medium and high risk

Factor affecting risk

Score

1 3 5

Number of maize crops 1–2 3 4–5

Spring (S) versus winter (W) crops W>S S>W S only

Plough (P) versus minimum tillage (MT) P>MT MT>P MT only

Inter-row cultivation, stale seedbed used >2 years 1–2 years 0

Number of MOA used per season >2 2 1

Successive use of MOA over years 1–2 3 4–5

Repeat use of MOA in 1 year 0 1 >1

Weed pressure Low Medium High

Quality of weed control Good Moderate Poor

Interpretation based on total score for the nine categories:

< 20= low risk of resistance developing for a given MOA/weed;

> 30=high risk of resistance developing (see Reference 4).

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