population dynamics of rumex obtusifolius under contrasting lucerne cropping systems

Population dynamics of Rumex obtusifolius under contrasting

lucerne cropping systems

J. PINO, F. X. SANS AND R. M. MASALLESDepartament de Biologia Vegetal, Universitat de

Barcelona, Diagonal 645, E-08028 Barcelona,Spain

Received 8 November 1996Revised version accepted 24 September 1997

Summary

The population dynamics of Rumex obtusifolius

L. was analysed in a lucerne:winter cereal croprotation by means of a matrix population modelthat takes into consideration two crop rotation

periods: the lucerne (Medicago sativa L.) crop-ping period and the cereal cropping period.Several transition matrices based on life-cycle

stages were calculated for each cropping periodusing experimental data and were used in theconstruction of a model that analyses the pop-ulation dynamics of R. obtusifolius under di�er-

ent harvest dates and lengths of lucerne croppingperiods. Model projections showed that popu-lations of R. obtusifolius increased during the

lucerne cropping period regardless of harvestdate and decreased during the cereal croppingperiod. Under a late harvest date, populations

decreased at each crop rotation when lucernewas grown for 3 years, remained close to theequilibrium when lucerne was left to grow for

5 years, and increased for longer lucerne crop-ping periods. In contrast, populations of R. ob-tusifolius decreased even with a lucerne croppingperiod of 9 years under an early harvest date.

The signi®cance of these results in relation to thebiology and the non-chemical control of thespecies is discussed.

Introduction

Rumex obtusifolius L. (broad-leaved dock) isa perennial herb indigenous in central andnorthern Europe that has been introduced into

many temperate areas of the world (Cavers &Harper, 1964). The species commonly grows inwaste and disturbed ground, ®eld borders and

hedgerows, and is considered to be one of themost troublesome weeds in intensively managedgrasslands (Haggar, 1980; Hongo, 1986; Jean-gros & NoÈ sberger, 1990), where it can reduce

grass herbage (Oswald & Haggar, 1983), and itslong-term control has proved variable in e�ec-tiveness (Speight & Whittaker, 1987). In recent

decades, the species has also colonized the irri-gated lucerne crops (Medicago sativa L.) in cen-tral Catalonia (north-east of Iberian Peninsula),

where it has become a noxious weed that reducesforage quality.

The development of e�ective control pro-

grammes for weed species depends largely on theknowledge of their population biology comple-mented by mathematical modelling (GonzaÂ lez-AnduÂ jar & FernaÂ ndez-Quintanilla, 1991). An

early model (Doyle et al., 1984) provides usefulinsights into control by herbicide of R. obtusi-folius in grasslands. However, it is not adaptable

to the lucerne cropping conditions in Spain as itdoes not take into account two important agro-nomic factors, the harvest date and the length of

the cropping period, that exhibit a great vari-ability in Spanish lucerne crops. In our study, weattempted to analyse the role of these factors onthe population dynamics of R. obtusifolius by

means of a mathematical simulation based onexperimental data. The ecological implicationsof results are discussed, and strategies for im-

proving the non-chemical control of the speciesare also proposed.

Weed Research, 1998, Volume 38, 25±33

Ó 1998 European Weed Research Society

Materials and methods

Study species

Rumex obtusifolius (Polygonaceae) is a medium-sized (0.5±1.5 m height) iteroparous herb witha life-span of more than 5 years (Cavers &

Harper, 1964). Mature plants have an under-ground structure with a stout taproot and abranched rhizome above the root collar (Pino et

al., 1995). Above-ground shoots arise in the apexof the rhizome and develop a panicle. Matureplants growing in lucerne crops produce seeds in

several fruiting waves from May to October,despite the application of periodical harvesting(Pino et al., 1993). Grassland populations an-

nually produce from fewer than 100 to morethan 60 000 seeds per plant, depending on indi-vidual size (Foster, 1989). Seeds do not haveinnate dormancy mechanisms and germinate

soon after release at surface conditions (Roberts& Totterdell, 1981), whereas they remain viablefor many years when deeply buried (Roberts &

Neilson, 1980). Seedling emergence in denseswards depends largely on vegetation gaps(Cavers & Harper, 1967; Panetta & Wardle,

1992), and new individuals usually remain at therosette stage for more than 1 year before an-thesis (Cavers & Harper, 1964). Mature plantsalso propagate vegetatively by fragmentation of

the underground structure after necrosis of theoldest rhizome branches (Pino et al., 1995). In-deed, fragments of underground structure se-

vered accidentally and containing rhizomebranches are able to regrow after damage(Roberts & Hughes, 1939; Pino et al., 1995).

Study site

The population dynamics of R. obtusifolius wasstudied in the irrigated area of Urgell (centralCatalonia, north-east of Iberian Peninsula), witha Mediterranean continental climate. In this

area, lucerne is commonly grown in a crop ro-tation with winter cereal, which has been takenas a reference for the study. Two periods can be

distinguished during this crop rotation: the lu-cerne cropping period and the cereal croppingperiod. During the lucerne cropping period, the

crop is left to grow for 5±6 years, during whichtime it is harvested at a height of 4±5 cm every30±40 days from April to October. The harvestedcrop is left to dry in the ®eld for a few days

before baling and removal. During the cereal

cropping period, the old lucerne crop is plougheddown, a winter cereal crop is established and,after harvesting and ploughing down the cereal,

a new lucerne crop is sown. Irrigation is appliedfortnightly or monthly, depending on season,from April to October during the lucerne crop-ping period and from March to June during the

cereal cropping period.

Experimental design and sampling

The lucerne cropping period. The demography ofR. obtusifolius during the lucerne cropping peri-

od was studied by means of an unreplicated ex-periment carried out in a 2-year-old crop fromMarch 1992 to March 1994. Two adjacent plots,20 m ´ 20 m each, were marked out in an ho-

mogeneous area in which lucerne cover wasgreater than 90% and density of adult plants ofR. obtusifolius was 40.12 m)2 (SD � 10.92). In

these plots, weed cover and composition weresimilarly related to their homogeneity in soilparameters and previous agricultural practices.

Plots followed contrasting treatments. In oneplot (`Control' plot), R. obtusifolius shootsfruited normally until the harvest, after which

they were left to dry with the crop. In the otherplot (`No seed' plot), shoots were cut and re-moved before each harvest, thus preventing seeddrop. No di�erences between plots were found in

initial adult density of R. obtusifolius (F � 5.41,P � 0.06).

The demographic performance of the species

under a late and an early harvest date was ana-lysed in two consecutive years. In 1992, the ®rstlucerne harvest took place on 25 April, whereas

in 1993 lucerne began to be harvested on 1 April.Harvests were taken every 35 days until Octoberduring the 2 years of study. No di�erences inlucerne and weed cover were observed from one

year to the other. The temperature range wasalso similar in the two years of study. Irrigationwas applied with a similar pattern in 1992 and

1993, beginning in early April and taking placeevery 15±21 days until early October (Pino,1995).

The demography of several life-cycle stages ofthe species was studied both in the `Control' andthe `No seed' plots. In the `Control' plot, shallow

soil seedbank censuses were annually performedin March, before the onset of R. obtusifoliusfruiting, by collecting 60 soil samples of 4 cm

26 J. Pino et al.

Ó 1998 European Weed Research Society, Weed Research 38, 25±33

depth and 5 cm diameter. Seeds were separated

by washing through a 0.5-mm-mesh sieve, andcounted. Their viability was tested by exposurein a growth chamber to optimal conditions for

germination, which are a daily 14-h photoperiodwith temperatures of 25 °C in light and 10 °C indarkness (Roberts & Totterdell, 1981). The adultpopulation was monitored in eight subplots

(1.5 m ´ 1 m) arranged in two rows. Just beforeeach harvest, plant fecundity was estimated byrecording stem basal diameter of all their fruiting

shoots as this parameter shows a close allometricrelationship with the number of seeds producedper shoot (Pino, 1995). The adult size, based on

the maximal diameter of the undergroundstructure, and mortality were annually recordedin March. Seed shed was estimated using 20 seedtraps (14 cm diameter and 5 cm height) buried at

ground level, whose contents were collected aftereach crop removal after harvesting and baling.The seeds collected were counted and their via-

bility was tested in a growth chamber with thesame procedure as for the seedbank estimation.Seedling demography was also monitored in two

50 cm ´ 25 cm permanent quadrats within each1.5 m ´ 1 m subplot. Samplings were taken afterharvests, every 4±6 weeks, as recording was only

feasible when the crop had recently been cut.The `No seed' plot was basically used to

analyse the demography of the shallow soilseedbank without new seed inputs. Thus, data

were obtained from the middle of the20 m ´ 20 m plot in order to exclude seed inputsfrom adjacent areas. Only seedbank, seed shed

and seedling emergence were studied. The seed-bank was periodically estimated at the samedates and using the same procedure as for the

`Control' plot. The seedling emergence wasmonitored in 16 permanent quadrats(25 cm ´ 25 cm), every 4±6 weeks after cropharvests, by counting and removing new indi-

viduals. In order to know the seed shed in theplot, 20 seed traps were monitored as in the`Control' plot. As the values recorded were, in

most samplings, fewer than 10 seeds m)2, newseed inputs in the `No seed' plot were considerednegligible.

The cereal cropping period. The population dy-namics during the cereal cropping period wasanalysed from 1993 to 1995 in an adjacent ®eldwith an old lucerne crop. In June 1993, the lu-

cerne was ploughed, and afterwards wheat

(Triticum aestivum L.) was sown in October 1993

and harvested in June 1994. A new lucerne cropwas sown in April 1995, after ploughing inSeptember 1994.

Because of the application of ploughings thatmade the use of permanent plots impossible,studies were limited to estimating the shallowsoil seedbank, and seedling and adult popula-

tions in March 1993 (before ploughing the oldlucerne) and in April 1995 (after sowing the newlucerne). The seedbank was assessed by collect-

ing 60 soil samples and by using the same pro-cedure as for the lucerne cropping period.Seedling and adult densities were estimated by

counting the number of plants present in 75 plotsof 1 m ´ 1 m size, regularly placed in rows of 15.Plots were placed in the same position in bothsamplings of March 1993 and April 1995.

Matrix construction

For each crop-rotation period, time-invarianttransition matrices based on life-cycle stageswere constructed using experimental data. Stage-

classi®ed matrix models have been shown to bethe most suitable for analysing population dy-namics of perennial species (Caswell, 1989). Six

life-cycle stages were considered for R. obtusifo-lius: the shallow soil seedbank, the seedling stageand four adult size stages de®ned according tothe maximum diameter of the underground

structure. Thus, adults of stage 1 (with un-branched rhizome) had rhizomes of less than20 mm in diameter, whereas the rhizomes of

plants of stages 2, 3 and 4 (with branched rhi-zome) were from 21 to 50 mm, from 51 to80 mm and more than 80 mm in diameter res-

pectively.

The lucerne cropping period. The annual proba-bilities of remaining in the same life-cycle stageand of transition to other stages were calculated

using data from the `Control' plot, except inthe case of the probabilities of remaining in theseedbank and of seedling establishment from

the seedbank, which were obtained using `Noseed' data. The number of seeds incorporatedinto the seedbank per individual in each adult

stage was given by:

Bi � Si � P�p; b� �1�where Si is the number of seeds shed per indi-

vidual in each adult stage (i), calculated as the

Population dynamics of Rumex obtusifolius 27


product of the mean seed production per indi-

vidual and the probability of seed shedding (ra-tio between the number of seeds m)2 shed andproduced from time t to t + 1). P(p,b) is the

probability that a shed seed becomes incorpo-rated into the seedbank, calculated as:

P�p;b� �SBt�1 ÿ P�b; b� � SBt

Sht; t�1�2�

where SBt and SBt�1 are, respectively, the `Con-trol' seedbank at time t and 1 year later, andSht;t�1 is the seed shed during the year. P(b,b), theprobability of remaining in the seedbank during

the year, corresponds to the quotient between the`No seed' seedbank at time t + 1 and at time t.

The number of seedlings established per plant

in each adult stage (i) was given by:

Sli � Si � G �3�

where Si is the number of seeds shed per indi-vidual in each adult stage (i), as previously de-scribed in eqn 1. G is the proportion of seedlings

germinated from the seed shed and alive at theend of the year and was calculated as:

G � Slt; t�1 ÿ Slt; t�1=SBt

Sht; t�1�4�

where Sht;t�1 is the seed shed during the year.Slt;t�1 and Slt;t�1=SBt are the number of seedlings

emerging during the year and alive at the end ofthis period that come from the seed shed and theseedbank respectively. Slt;t�1=SBt was given by:

Slt; t�1=SBt � P�b; sl�SBt �5�

where SBt is the `Control' seedbank at time t andP�b;sl� is the seedling establishment probability

from the seedbank, calculated as the quotientbetween the number of seedlings emerging andalive at the end of the year, and the seedbank atthe beginning of the year in the `No seed' plot.

Two transition matrices with a 1-year pro-jection interval were constructed for the lucernecropping period. One, with data obtained in

1992, analyses the population dynamics ofR. obtusifolius under a late harvest date and theother, with data obtained in 1993, explores this

population dynamics under an early harvestdate. For each transition matrix, the intrinsic

rate of population increase (k), the stable-stage

distribution and the reproductive value of eachlife-cycle stage were calculated using MATLAB.The elasticity of k with respect to changes in the

matrix elements, which measures how importantare the di�erent demographic processes onpopulation growth (Caswell, 1989), was alsoobtained.

The cereal cropping period. The rate of increasein each life-cycle stage was calculated as thequotient between the number of individuals at

the end and at the beginning of this period(March 1993 and April 1995 respectively).A diagonal matrix with a 2-year projection in-

terval was constructed with the probabilitiesobtained. Two main assumptions were made inthe construction of this matrix.

1. The rate of increase of each life-cycle stageintegrated the gains and losses in each stageduring the cereal cropping period.

2. As adult size could not be measured in themajority of cases because most individualshad been partially buried by ploughing; only

individuals with unbranched and branchedrhizome could be distinguished. For thisreason, the rate of increase of the second,third, and fourth adult stages, corresponding

to individuals with branched rhizome, had tobe calculated as a whole.

Model construction

The matrices obtained in each cropping periodwere used in the construction of a matrix popu-lation model that explains the population dy-

namics of the species in a complete crop rotation.The matricial expression of the model was

An X �6�

where A is the transition matrix for the lucerne

cropping period, n corresponds to time in yearsof lucerne cropping and X is the transition ma-trix for the cereal cropping period. The modelwas calculated for each transition matrix of the

lucerne cropping period and for n � 3, 5, 7 and9 years of lucerne cropping. In each case, theintrinsic rate of population increase was calcu-

lated and an arbitrary population vector (seed-bank � 20 seeds m)2; immatures and ®rst adultstage � 0 individuals m)2; second, third and

28 J. Pino et al.


fourth adult stages � 0.33 individuals m)2

each) was projected using MATLAB.

Results

The lucerne cropping period

The transition matrices obtained for the lucernecropping period are shown as life-cycle graphs in

Fig. 1. In the majority of cases, the transitionprobabilities obtained for both harvest dateswere quite similar. The lowest transition proba-

bilities were those related to prereproductivephases, such as the probability that a seed re-mains in the seedbank (8.8%) or gives a seedling

(2.3±2.5%), and the probability that an adultplant is recruited from the seedbank (0.5±2.2%)or from the seedling stage (3.3±3.8%). In con-trast, transition probabilities related to adult

stages, such as remaining in the same stage or thetransition to immediately larger diameter stages,generally showed higher values. The number of

seeds incorporated into the seedbank per indi-vidual was also similar in both matrices for eachadult stage, and increased in relation to plant

size. However, the number of immature plantsestablished per individual was, for each adult

stage, more than ten times higher under a late

than under an early harvest date.Both matrices calculated for the lucerne

cropping period were primitive and irreducible

and, consequently, their ®rst eigenvalue corre-sponded to the intrinsic rate of populationincrease (k) and the associated left and righteigenvectors were, respectively, the reproductive

value and the stable-stage distribution vectors(Caswell, 1989). The ®rst eigenvalue was >1 forboth matrices (Table 1), indicating that the

population size increased under both early andlate harvest dates during the lucerne croppingperiod. The increase was, however, more im-

portant under a late than under an early harvestdate (k � 1.288 and k � 1.078 respectively).For each matrix, the di�culties in establishmentand recruitment gave rise to a stable-stage dis-

tribution (W) greatly skewed to the prerepro-ductive stages, in which more than 95% of thepopulation was concentrated, and an increase in

the reproductive value throughout the develop-ment, but mainly from the seedbank to the ®rstadult stage. The elasticity of k in relation to the

matrix components was greatest for the remain-ing probabilities of the last adult stages (Fig. 2),although the values corresponding to the

Fig. 1. Life-cycle diagram of R. obtusifolius during the lucerne cropping period. The nodes represent stages and the arrows indicatetransitions (probabilities or fecundity coe�cients) between stages. For each transition, values obtained under a late (above) and anearly (below) harvest date are shown.



remaining probabilities of the other adult stages,the transition probabilities between consecutiveadult stages and the immature recruitment werealso important.

The cereal cropping period

There was a general decrease in population sizeduring the cereal cropping period (Fig. 3). Theseedbank decreased by 90% and the complete

depletion of the seedling population and an

extreme reduction of the ®rst adult stage were

recorded, probably as a result of ploughings. Incontrast, the population of adults with branchedrhizome maintained 44% of its initial size. The

adult population at the end of the cereal crop-ping period was made up by survivors, individ-uals with branched rhizome incorporated fromthe ®rst adult stage, and plants deriving from

adult fragmentation.

Model projections

The projection of an arbitrary mature popula-tion under di�erent harvest dates and lucerne

cropping lengths is shown in Fig. 4. For eachcropping length, the population increased duringthe lucerne cropping period and decreasedsharply during the cereal cropping period, giving

a saw-toothed graph. Under an early harvestdate, the population decreased over the croprotations for each lucerne cropping length, al-

though the longer the cropping period the lessimportant was the population decrease. The in-trinsic rate of population increase (k) calculatedfor all the crop rotations was always <1.In contrast, under a late harvest date, the

Table 1. Intrinsic rate of population increase (k), stable-stagedistribution (W) and reproductive value (V) for each life-cyclestage of Rumex obtusifolius during the lucerne cropping periodunder a late and an early harvest date. SB, seedbank; Sl,seedlings; Ad1, Ad2, Ad3 and Ad4: adult size-stages orderedfrom the smallest to the biggest

Late harvest date(k = 1.2884)

Early harvest date(k = 1.0773)

W V W V

SB 0.7651 0.0344 0.9379 0.0004Sl 0.2046 0.5169 0.0414 0.0127Ad1 0.0124 12.9147 0.0028 0.2943Ad2 0.0078 20.9094 0.0027 0.3628Ad3 0.0075 33.3226 0.0027 0.4054Ad4 0.0032 33.1197 0.0125 0.4446

Fig. 2. Life-cycle diagram of R. obtusifolius during the lucerne cropping period showing the elasticity of k in relation to the matrixcomponents. Elasticities have been scaled so as to sum 1 and to give the proportion of k contributed by the corresponding transition.For each transition the elasticity of k under a late (above) and an early (below) harvest date are shown. The highest values areindicated in bold and their corresponding transitions by bold lines.

30 J. Pino et al.


population decreased when lucerne was grownfor 3 years (k of crop rotation � 0.46), re-mained almost stable when the crop was left to

grow for 5 years (k � 0.97) and increased whenlucerne was grown for 7 years or more(k � 1.67 and k � 2.78 for 7 and 9 years re-

spectively).

Discussion

While bearing in mind that the results obtained

must be interpreted with caution because of thebrevity of the observation period, some trends inthe population dynamics of R. obtusifolius in

lucerne crops may still be observed. One of the

Fig. 4. Projection of a Rumex obtusifolius population in a lucerne-winter cereal crop rotation under an early and a late harvest datefor di�erent lengths of the lucerne cropping period.

Fig. 3. Diagonal transition matrix of R. obtusifolius describing population changes from time t (columns) to t + 1 (rows) during thecereal cropping period. Only non-zero values are shown. SB, seedbank; Sl, seedlings; Ad1, Ad2, Ad3 and Ad4: adult size stagesordered from the smallest to the biggest.



most striking features was the signi®cant varia-

tion in population behaviour between crop ro-tation periods. During the lucerne croppingperiod, populations of R. obtusifolius increased

for both harvest dates analysed, despite the highmortality rates of prereproductive stages (80%for seeds in soil and 60% for seedlings) and thedi�culties of recruitment (only 0.1% of seeds

from the seedbank and 3.5% of seedlings annu-ally reaching the ®rst adult stage). As can bededuced from the results of elasticity of k in re-

lation to the matrix elements, the stability ofadult plants, which showed annual mortalityrates of less than 5%, was largely responsible for

population increase. The high longevity of adultplants, combined with their iteroparous strategyand their increase in fecundity with size, ensureda regular, important seed input each year that

counteracted the low e�ectiveness of recruit-ment. A similar life strategy is exhibited by otherperennial herbs, such as Viola sororia Willd.

(Solbrig et al., 1980), and several Ranunculusspp. (SarukhaÂ n & Harper, 1973), which com-monly grow in relatively stable environments

where intense competition is responsible fora high, density-dependent mortality in prere-productive stages (Bell, 1976).

Species growing in competitive habitats usu-ally have populations near the carrying capacityof the environment, as has been observed in Vi-ola ®mbriatula Sm. and Astrocaryum mexicanum

Liebmann ex Martius, which show intrinsic ratesof population increase near to 1 (Pinero et al.,1984; Solbrig et al., 1988). In grasslands, several

authors have observed that the proportion ofR. obtusifolius seedlings expected to survive tomaturity also depends on the distance to adult

plants (Makuchi & Kanda, 1980) and on theseverity of competition from other species in thesward (Cavers & Harper, 1967). Incorporatingthese results in a density-dependent population

model, Doyle et al. (1984) simulated the infes-tation of a grassland by R. obtusifolius. Theirresults suggested that no increase in population

is expected when the percentage of ground coverof the species reaches 22%.

The behaviour of R. obtusifolius populations

in lucerne crops was, however, clearly di�erentas the intrinsic rates of population increase are>1, even when initial adult densities (40 in-

dividuals m)2) corresponded to a ground coverpercentage higher than 30%. Periodical har-vesting may contribute to a reduction in

competition and to the maintenance of an in-

trinsic rate of population increase >1 by en-hancing seedling establishment, as has beenobserved in frequently cut grasslands (Hongo,

1989). Intrinsic rates of population increase >1have also been recorded in other species growingin periodically disturbed environments, such asAndropogon brevifolius Schwarz and A. semi-

berbis (Nees) Kunth from periodically burntsavannas (Silva et al., 1991; Canales et al., 1994).

During the cereal cropping period, a signi®-

cant decrease in population size, mainly due toploughings, was recorded. The decrease was es-pecially important in the immature and ®rst

adult stages, leading to their virtual disappear-ance. In contrast, almost half the adults of theother stages survived this period by means oftheir branched rhizome, which has a great ability

to produce roots and aerial shoots and this en-hances the regeneration of individuals rooted outand fragmented by ploughing (Pino et al., 1995).

Therefore, the substitution of one lucerne cropwith another after a year of cereal cultivationdoes not necessarily involve the complete elimi-

nation of the weed, whereas the infestation of ayoung lucerne crop is usually initiated by a re-sidual adult population inherited from previous

crops.The results obtained on R. obtusifolius popu-

lation dynamics can be used to suggest non-chemical control procedures based on agronomic

practices. As a regular seed input is needed tomaintain population increase, there are two maintargets to control population growth: (a) the

annual seed shedding and (b) the adult popula-tion. A large reduction in the annual seed input isachieved by crop harvesting and baling, which

remove 75% of seeds produced each year fromthe ®eld. However, this reduction may be en-hanced further by advancing the harvest date asthis prevents the maturing of seeds developed in

the ®rst fruiting wave, which represents 80% ofthe annual seed production (Pino et al., 1993).On the other hand, the adult population may be

successfully controlled by limiting the lucernecropping period. As rhizome branching does notbegin until the third year of life (Pino et al.,

1995), the restriction of the lucerne croppingperiod means that a great number of individualscannot reach adult stages with the capacity to

survive ploughing. No information is currentlyavailable concerning the intercalation of two ormore years of cereal between two lucerne crops.

32 J. Pino et al.


However, as only 64% of adults with branched

rhizome survive each ploughing (Pino, 1995), thelonger the cereal cropping period the greater willbe the decrease in population. Further studies are

also needed to elucidate the economic implica-tions of these measures on the control of R. ob-tusifolius in a lucerne:winter cereal crop rotation.

Acknowledgements

The authors wish to thank Dr J. RaventoÂ s for hisadvice in constructing the matrix model. The ®-nancial support of the Catalan Government is

also gratefully acknowledged.

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population dynamics of rumex obtusifolius under contrasting lucerne cropping systems

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