Simulation of herbicide use in a crop rotation with transgenic herbicide-tolerant oilseed rape

Simulation of herbicide use in a crop rotationwith transgenic herbicide-tolerant oilseed rape

K H MADSEN, W M BLACKLOW*, J E JENSEN & J C STREIBIGDepartment of Agricultural Sciences (Weed Science), The Royal Veterinary and Agricultural University,

Frederiksberg C, Denmark, and *Faculty of Agriculture (Plant Sciences), Nedlands WA 6907, Australia

Received 6 November 1997

Revised version accepted 16 November 1998

Summary

The potential impact of herbicide-tolerant winter oilseed rape (Brassica napus L.) on future

herbicide use was investigated with a simulation model. The model uses a sigmoid function to

simulate the growth of crops and weeds that compete for a maximum yield potential. Thresholds

for weed control are based upon critical levels of weed biomass. The dynamics of the weed

population are determined by the e�cacy of representative herbicides on individual weed species

and by seedbank parameters. Herbicide e�cacy is determined by a log-logistic dose±response

curve for each species. Simulation of a rotation with winter oilseed rape/wheat/wheat/barley

showed contradictory predictions of herbicide use, because herbicide use in a rotation with either

glyphosate- or glufosinate-tolerant oilseed rape was not reduced in the amount of kg a.i. ha±1

compared with a traditional treatment, whereas the treatment frequency (number of standard

recommended doses per unit area) decreased.

Keywords: models, simulation, population dynamics, weeds, dose±response, seed bank, herbicide

resistance, transgenics.

Introduction

Oilseed rape (Brassica napus L.) is grown on »5% of the arable land in Denmark (Anonymous,

1997b). Few herbicides are currently available for dicotyledonous (dicot) weed control in oilseed

rape. These include propyzamide (registered for winter oilseed rape cultivars only), which

controls a small number of dicot weed species; napropamide, which is used pre-emergence for

control of broad-leaved and non-perennial monocotyledonous weeds; and clopyralid, which is

selective mainly against Asteraceae. It is therefore di�cult to control common species such as

Sinapis arvensis L., Raphanus raphanistrum L. and Capsella bursa-pastoris (L.) Medicus

(Kristensen, 1997). Introduction of oilseed rape varieties tolerant to broad-spectrum herbicides

may therefore be of bene®t to Danish farmers, at least in the short term.

Correspondence: K H Madsen, Department of Agricultural Sciences (Weed Science), The Royal Veterinary and

Agricultural University, 40 Thorvaldsensvej, DK-1871 Frederiksberg C, Denmark. Tel: (+45) 35 28 34 45; Fax (+45) 35

28 34 68; E-mail: [email protected]

Ó Blackwell Science Ltd Weed Research 1999 39, 95±106 95

Since 1987, a Danish regulatory policy has been implemented to reduce pesticide use by 50%

over a period of 10 years and to encourage use of more environmentally and toxicologically

benign pesticides (Bennekou, 1997). This policy explains in part why only a few herbicides are

now available for use on oilseed rape. To assess the e�ectiveness of the policy, two measures of

annual herbicide usage are used. The ®rst measure is the total annual sales of active ingredients

of pesticides, and the second one is a so-called treatment frequency, which is de®ned as the

number of standard recommended rates with which the total agricultural area is treated annually

(Haas, 1989). The target of 50% reduction in herbicide use has almost been reached in terms of

herbicides measured in kg a.i. ha±1; however, treatment frequency has only decreased slightly

over the 10-year period (Anonymous, 1998). Consequently, Danish agriculture is under pressure

to reduce pesticide usage further, in particular that of herbicides, which make up about 67% of

the total sale of pesticides (Anonymous, 1998). In view of the constraints Danish farmers are

facing as regards reducing herbicide usage with fewer available compounds, the potential value

of growing transgenic herbicide-tolerant oilseed has to be taken into account.

In recent years, transgenic oilseed rape with tolerances to the otherwise non-selective

herbicides glyphosate and glufosinate has been developed and introduced (Moll, 1997; Rasche &

Gadsby, 1997). The future growing of these herbicide-tolerant crops has caused public debate

and focused research on the putative uncertainties about the long-term consequences of growing

herbicide-tolerant oilseed rape.

Growing transgenic oilseed rape creates a potential for spreading resistant plants because seed

shed during harvest can create a volunteer problem in succeeding crops (Lutman, 1993).

Furthermore, herbicide-tolerant oilseed rape can hybridize with conventional oilseed rape

cultivars and with related weed species such as Brassica campestris L. (Jùrgensen & Andersen,

1994; Mikkelsen et al., 1996). In a recent botanical survey of the weed ¯ora on arable land,

B. campestris was present in 6% of ®elds with non-cruciferous crops (Andreasen, 1990),

indicating that this species was not a major weed. Since then, however, the area under winter

oilseed rape has increased at the expense of spring cultivars, and the consequences of this shift on

the distribution of B. campestris have not yet been evaluated.

In this paper, a simulation model was used to address the following two questions. Will

herbicide use change with the introduction of transgenic herbicide-tolerant oilseed rape

compared with the same rotation with non-transgenic crops? And will populations of volunteer

oilseed rape and hybrids with wild relatives become a severe agronomic or environmental

problem in the future? These questions can best be addressed experimentally by multiyear

experiments, but these experiments are costly and require approval of ®eld releases.

As an alternative, we can simulate the consequences of di�erent herbicide strategies in a

common North European crop rotation. We here present a simple empirical model for assessing

the cumulative usage of herbicides and the selection of herbicide-resistant weeds in transgenic

herbicide-tolerant winter oilseed rape in comparison with traditionally grown oilseed rape.

Materials and methods

Prerequisites

The model (Fig. 1) is empirical and based on alleged relationships, comparative studies between

transgenic and non-transgenic crops, and practical experience from current crop rotations. The

model requires few growth-related parameters and does not take climatic and soil conditions into

96 K. H. Madsen et al.

Ó Blackwell Science Ltd Weed Research 1999 39, 95±106

account, in contrast to the ecophysiological model developed by Spitters & Krop� (Krop�,

1993).

The model was based on the following actual data: the crop rotation was selected from a

range of Danish crop rotations based on the Danish agricultural statistics (Anonymous, 1994;

Madsen et al., 1997). Maximum obtainable yields for crops were based on representative yield

levels. Selection of weed species (Table 1) was based on a recent Danish weed survey (Andreasen,

1990). Initial levels of di�erent volunteer crops were estimated as a percentage of the crop seed

loss (Table 1). The death rate of seeds in the soil was assumed to be exponential (Roberts &

Fig. 1 Overview of the model that simulates crop/weed/herbicide interactions in a rotation with winter cultivars of

oilseed rape (OSR)/wheat/wheat/barley. Y, growing season, STEME, Stellaria media, BRACA, Brassica

campestris, ELYRE, Elymus repens, SU-resist, sulfonylurea-resistant. rs, heterozygous for the resistance gene, rr,

homozygous for the resistance gene and ss, susceptible. Arrows from biomass pool to seedbanks indicate

Mendelian segregation of the produced seed.

Simulation of herbicide use in transgenic oilseed rape 97


Tab

le1

Selectedinputvalues

andparametersusedto

de®neseedbankdynamics,growth

(growth

rate

gandmaxim

um

yield

Ymax)andherbicidee�

cacy

(ED

50isthe

herbicidedose

thatcausesa50%

reductionin

biomass)oncropsandweedsin

thesimulationmodel

ofarotationwithwintercultivars

ofoilseed

rape/wheat/wheat/

barley

Initia

l

Cro

p

seed

s

(Surv

ive

to

d

(Seed

e

g

(Gro

wth

Harv

est

Benazo

lin+

clo

pyra

lidG

lyphosate

Glu

fosin

ate

seed

loss

seedbank)

decay)

(Germ

ination)

rate

)Y

max

index

ED

50

ED

50

ED

50

Specie

sno.

m±2

%%

%year±

1%

day

±1

tha

±1

%g

a.i.

ha

±1

ga.i.

ha

±1

ga.i.

ha

±1

Win

ter

oils

eed

rape

±10

25

90

20

0.0

714

25

±207

372

Win

ter

wheat

±10

15

99.9

99

33

0.0

418

40

±73*

171*

Win

ter

barley

±10

15

99.9

99

33

0.0

514

40

±73*

171*

S.

media

10800

±50

33

50.0

75

50

84

73

65

C.

alb

um

1000

±50

33

50.1

5

850

317

106

161

C.

burs

a-p

ast.

3600

±85

33

50.0

75

25

600*

49

64

M.

arv

ensis

3400

±60

33

30.0

85

25

150*

82

57

B.

cam

pestr

is200

±80

33

50.0

68

25

±207*

372*

Hybrid

F1à

±±

25

33

20

0.0

714

5±

±±

E.

repens

5kg§

±2.5

±±

0.0

38

±±

150*

372*

*Estim

atedparameter

value.

Summer

annual.

àHybridbetweenoilseed

rapeandB.campestris.

§Bankofpropagulesin

kgha±1.



Feast, 1973). Crossing frequencies between oilseed rape and B. campestris were based on

experimental data (Jùrgensen & Andersen, 1994), from which inverse linear curves were

extrapolated with the presence of one species relative to the other as the predictor. Data for a

naturally occurring sulfonylurea-resistant Stellaria media (L.) Vill. in Denmark were taken from

Kudsk et al. (1995), and the initial frequency was set at a high level (10±4) corresponding to a

worst-case scenario, which in the model caused resistance problems within 20 years in a

hypothetical crop rotation with the use of tribenuron-methyl in all crops. The time period of

20 years for all simulations was chosen arbitrarily.

Herbicide-resistant biotypes have similar ®tness to the non-resistant biotypes of both crops

and weeds (Jensen, 1993; Madsen, 1994; Fredshavn et al., 1995); however, biomass and seed

production appear to be reduced in second-generation hybrids (F2 or BC1) between oilseed rape

and B. campestris (T. Hauser, pers. comm.). To account for the observed yield depression in later

generations, which are not modelled separately, the harvest index from the hybrid was reduced in

the model (Table 1). Seeds from the second-generation hybrids were directly transferred to the

B. campestris seed pool. The resistance trait segregated according to Mendelian principles. The

recommended dose used for calculation of treatment frequency was 406 g a.i. ha±1 for the

mixture of benazolin and clopyralid, and 375 g a.i. ha±1 for ¯uazifop-P-butyl (Anonymous,

1995). As no recommended dose is yet available for the transgenic oilseed rape, we chose to use

the Canadian recommendations for spring cultivars of glyphosate-tolerant `Roundup Ready

Canola' and the glufosinate-tolerant `Liberty Link Canola' for which the recommended dose was

up to 445 g a.i. ha±1 for glyphosate and up to 600 g a.i. ha±1 for glufosinate (Anonymous,

1997a).

Model development

The model (Fig. 1), programmed in STELLA II (Peterson & Richmond, 1994), simulated the

growth of crops and weeds in a rotation with oilseed rape. It included six `model' weed species

(Table 1).

Seedbank

The weed species and volunteers regenerate in succeeding crops from a seedbank (or for Elymus

repens (L.) Gould from vegetative propagules). The number of seeds entering the seedbank yearly

was quanti®ed as a linear function of the biomass present at harvest time:

d�Ninput�dt

� 1000

TCWHI � s � Yt�h �1�

where Ninput is the number of seeds m±2 that enter the seedbank; TCW is the 1000 grain weight

according to Korsmo et al. (1981); HI is the harvest index (Table 1); s is the proportion of seeds

that survive from mature seed to seedbank (Table 1); and Yt� h is the dry-matter production

g m±2 [100 ´ (t ha±1)] at harvest time.

Seed loss from the seedbank was determined by the exponential rate of seed decay and the

proportion of germination:

d�Noutput�dt

� NTotal ��1ÿ exp

� log�1ÿ d�365

�� et�s

��2�



where Noutput is the number of seeds m±2 day±1 that leave the seedbank; NTotal is the total number

of seeds m±2 in the seedbank of the species; d is the annual seed decay (Table 1); et� s is the

proportion of seeds that germinate when the crop is seeded (Table 1)

Growth and competition

Sigmoid growth curves described the growth of the di�erent species that competed for a yield

potential determined by a maximum potential biomass of the crop:

d�Yt�1�dt

� g � Yt � �YMax ÿ YTotal�YMax

�3�

where dYt+1 is the dry-matter production (t ha±1) from day t to day t + 1; g is the relative

growth rate day±1 (Table 1); Yt is the dry-matter production (t ha±1) up to time t; YMax is the

maximum attainable biomass production (t ha±1) for the species (Table 1); and YTotal is the

accumulated biomass produced (t ha±1) of all species in the model.

The simple competition model (Eqn 3) allowed the individual weed species to compete with

each other as well as with the crop in order to obtain a share of the maximum available yield

potential (Table 1.). The growth stops when YTotal equals YMax of the largest biomass producer,

the crop.

Weed control

At a ®xed threshold of weed biomass, each crop was sprayed with herbicides. All weeds were

assumed to emerge simultaneously and the e�cacies varied with weed species, herbicide and

dose. Timing, according to practice, and numbers of sprayings were based on the following weed

biomass thresholds: propyzamide, 0.5 t ha±1 of biomass of weeds and volunteers during winter;

benazolin + clopyralid, 1 t ha±1 of dicot weeds followed by ¯uazifop-P-butyl at 0.5 t ha±1 of

monocotyledonous weeds; glyphosate and glufosinate, 1.4 t ha±1 weeds and volunteer crops ha±1.

E�cacy of glyphosate, glufosinate or the mixture of benazolin + clopyralid in the oilseed rape

crop was determined by dose±response curves (greenhouse experiments described in Madsen &

Streibig, 1999) for S. media, C. album L., C. bursa-pastoris, Myosotis arvensis (L.) Hill and

oilseed rape (data are not shown); details on comparison of herbicide e�cacy with the log-logistic

dose±response model are found elsewhere (Streibig et al., 1993; Madsen & Jensen, 1995). The

dose±response curves for B. campestris were assumed to be identical to the curve for oilseed rape.

Dicot weeds in the barley and wheat crops were sprayed with a sulfonylurea herbicide,

tribenuron-methyl (threshold, 0.1 t biomass ha±1), with e�cacy levels based on dose±response

curves (P. Rydahl, pers. comm.). If the fraction of sulfonylurea-resistant:sensitive weeds

exceeded 0.25 in the cereal crops, then ¯uroxypyr (144 g a.i. ha±1) was added to the treatment.

Gressel & Segel (1990) assumed in their model that problems with resistant weeds would become

apparent when resistance exceeded a level of 10±30% of the population. Where no

dose±response relationships were available, the e�cacy according to agronomic practice was

used (propyzamide 500 g a.i. ha±1 and ¯uazifop-P-butyl 188 g a.i. ha±1). Elymus repens growing

in cereal crops was controlled with a pre-harvest application of glyphosate (800 g a.i. ha±1) when

the development of this grass weed exceeded 0.3 t biomass ha±1 at the end of the growing season.

The model simulated four scenarios for weed management in a rotation with winter cultivars

of oilseed rape/wheat (Triticum aestivum L.)/wheat/barley (Hordeum vulgare L.):

1. A non-transgenic oilseed rape sprayed only with propyzamide and ¯uazifop-P-butyl against

E. repens and cereal volunteers.



2. A non-transgenic oilseed rape sprayed with a mixture of benazolin + clopyralid against dicot

weeds and ¯uazifop-P-butyl against E. repens and cereal volunteers.

3. A glyphosate-tolerant oilseed rape sprayed only with glyphosate.

4. A glufosinate-resistant oilseed rape sprayed only with glufosinate.

Results

The simulated growth of crops and weeds (Fig. 2) was used to evaluate scenarios with di�erent

herbicides in oilseed rape and a common herbicide for the cereal crops. The adjusted threshold

enabled the timing of applications and the number of sprayings to simulate common practice

with the traditional herbicides over the 20-year simulation period. When the threshold triggered

a spraying, it reduced the biomass of the individual weed species on the basis of the dose±

response curve for this weed and herbicide combination. The remaining crop and weeds took up

the biomass of the controlled weeds. However, the biomass of weeds, S(Yt) of the weed species,

relative to Yt of the crop had diminished, which favoured growth of the crop (Eqn 3).

By including the dose±response curves (Fig. 3) in the model, it was possible to evaluate the

e�ect of dose on accumulated herbicide use over 20 years for the scenarios with benazolin +

clopyralid, glyphosate or glufosinate. This made it possible to compare herbicide scenarios. At

the assumed recommended dose (based on Canadian recommendations), the amount of herbicide

used over 20 years was similar for the scenarios with traditional and glyphosate-tolerant oilseed

rape, whereas it was higher for the scenario with glufosinate-tolerant oilseed rape (Figs 4 & 5).

Pre-harvest spraying of glyphosate to control E. repens in wheat crops caused the accumulated

curves to rise steeply, e.g. for the scenarios with benazolin, glyphosate and glufosinate in year 3

and the glufosinate scenario in year 14 (Fig. 4). The preset thresholds caused the curves for

accumulated herbicide use vs. the fraction of recommended dose (Fig. 5) to be uneven as minor

changes in e�cacy in¯uenced the population dynamics and in turn initiated or avoided a spray

operation.

If herbicide use was measured as treatment frequency, the rotation with glyphosate- or

glufosinate-tolerant oilseed rape had consistently lower treatment frequencies at the di�erent

fractions of the recommended dose (Fig. 6).

Fig. 2 Simulated growth curves for

winter oilseed rape, dicotyledonous

weeds and monocotyledonous weeds

in scenario 1, growing season 1 (year

1±2).



The response to decreased herbicide dose (Figs 5 & 6) could leave the impression that total

herbicide use can be decreased by lowering the dose inde®nitely, as there is a positive correlation

between the fraction of recommended dose and the accumulated herbicide use. However, when

rates were reduced below 0.3±0.5 times recommended dose, the number of sprayings increased

radically because of the preset threshold levels for weed biomass, with a consequent

impracticable increase in application costs (these are, however, not included in the model).

Resistant volunteers and hybrids between oilseed rape and B. campestris were present in the

model but did not cause a problem in the ®eld [i.e. exceeded 25% of the dicot weed biomass

(Gressel & Segel, 1990)] in any of the four scenarios.

In the scenario in which oilseed rape was sprayed with benazolin + clopyralid, a high number

of tribenuron-methyl applications in the cereal crops imposed a high selection pressure on the

weeds that favoured growth of sulfonylurea-resistant S. media. At high rates (fraction of

recommended dose > 0.75) the model added ¯uroxypyr to the sulfonylurea herbicide once or

twice in order to control the resistant weeds. No such problems occurred in the scenarios with

glufosinate or glyphosate-tolerant oilseed rape over the simulated period of 20 years.

Fig. 3 Dose±response curves for

non-transgenic oilseed rape, Stellaria

media and Chenopodium album

sprayed with glufosinate. Green-

house experiments. Points are

averages of four replications and are

shown on a relative scale for fresh

weights, based on the upper limits

for the log-logistic regression

analysis.

Fig. 4 Simulated herbicide use over

20 years for a crop rotation with

winter cultivars of oilseed rape/

wheat/wheat/barley. The four

di�erent scenarios are oilseed rape

sprayed with: a mixture of benazolin

+ clopyralid, propyzamide,

glyphosate and glufosinate.

Accumulated herbicide use is based

on e�cacy data from greenhouse

experiments.



Discussion

Compared with glyphosate and glufosinate, the traditional herbicides, benazolin + clopyralid

and propyzamide had low e�cacy on several of the modelled weed species, resulting in a greater

treatment frequency in the succeeding cereal crops with tribenuron-methyl. Use of sulfonylureas

adds little to the amount of herbicide used but is equal to the other herbicides in terms of

treatment frequency. This explains the apparently con¯icting results concerning herbicide use.

In practical agriculture, the average treatment frequency in 1996 was 0.95 in winter oilseed

rape and 1.33 in winter cereals, which, `all other things being equal', means that the accumulated

treatment frequency over 20 years is 25 standard recommended rates for this particular crop

rotation (Anonymous, 1997b). The model predicted an accumulated treatment frequency of 25

for the traditional scenario with propyzamide and 27 for the scenario with benazolin +

clopyralid. This indicates that the model simulated the crop rotation in accordance with current

agricultural practices.

Resistant volunteers and hybrids between oilseed rape and B. campestris did not become a

problem in this crop rotation, because tribenuron-methyl, which was used to control dicot weeds

in the cereal crops, controls Brassica species e�ciently. This emphasizes the importance of

Fig. 5 Accumulated herbicide use at

di�erent fractions of recommended

dose in oilseed rape sprayed with

either benazolin + clopyralid,

glyphosate or glufosinate in a crop

rotation with winter cultivars of

oilseed rape/wheat/wheat/barley.

Accumulated herbicide use over

20 years is based on e�cacy data

from greenhouse experiments.

Fig. 6 Accumulated treatment

frequencies at di�erent fractions of

recommended dose for herbicide

strategies in oilseed rape sprayed

with either benazolin + clopyralid,

glyphosate or glufosinate in a crop

rotation with winter cultivars of

oilseed rape/wheat/wheat/barley.

Accumulated treatment frequency

over 20 years is based on e�cacy

data from greenhouse experiments.



studying consequences for the complete rotation system when a transgenic herbicide-tolerant

crop is introduced. However, this model neither includes interactions with other oilseed rape

®elds, nor considers secondary e�ects on weed control in other herbicide-tolerant crops [e.g.

sugar beets (Beta vulgaris L.)] with the same mechanism of resistance.

Sulfonylurea-resistant S. media became a problem in the scenario where oilseed rape was

sprayed with high doses of benazolin + clopyralid. It should be noted that the initial frequency

of sulfonylurea-resistant S. media, found in the literature, is normally assumed to be 105±10±9

(Saari et al., 1994), but in the model it was adjusted to the high level (10±4) in order to simulate a

worst-case scenario with development of resistance within 20 years if a sulfonylurea herbicide

was used in all crops.

Sensitivity analysis of single parameters in the model showed that overall model behaviour

was sensitive to changes in growth parameters, because growth is de®ned only by two

parameters: relative growth rate g and total maximum potential biomass Ymax (Eqn 3).

Sensitivity of the dependent variable (number of sprayings) varied with the independent variable

(dose) according to location on the dose±response curve for the individual herbicide/weed species

(Fig. 2). At high rates, close to the lower limits of the curves, the changes in number of sprayings

were negligible, but if the dose varied along the steep part, around the ED50, then number of

sprayings changed dramatically, because of the preset thresholds for weed biomass.

Conclusions

The model described here is our ®rst attempt to integrate knowledge about herbicide-tolerant

crops with known agricultural practices. It should be emphasized that the model needs validation

before any reliable predictions can be made about long-term consequences of growing herbicide-

tolerant oilseed rape. A crude ®rst validation of the model is that the treatment frequency in the

traditional scenarios of the model was similar to the average treatment frequency of 1996. To

more thoroughly validate and adjust the model, short-term simulation results should later be

compared with ®eld data from crop rotations with transgenic herbicide-tolerant crops after their

release on the market. Bearing these precautions in mind, we believe that the model outcome

represents a tool that can supplement expert opinions on long-term e�ects of growing herbicide-

tolerant crops and thus prevent problems with unwise herbicide use and resistance in weeds and

volunteers.

Acknowledgements

The authors wish to thank P Rydahl, the Danish Institute of Agricultural Sciences, for providing

the dose±response curves for tribenuron-methyl and the Danish Environmental Protection

Agency, the Danish Agricultural and Veterinary Research Council and the Grain Research

Committee of Western Australia for ®nancial support.

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Simulation of herbicide use in a crop rotation with transgenic herbicide-tolerant oilseed rape

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