Weed cover, frequency and diversity in field plots and edges in the soybean central region of Argentina

doi:10.4236/as.2012.35076

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Vol.3, No.5, 631-639 (2012) Agricultural Sciences

http://dx.doi.org/10.4236/as.2012.35076

W eed cover, frequency and diversity in field plots and

edges in the soybean central region of Argentina

Eduardo Puricelli1*, Delma Faccini2, Luisa Nisensohn2, Daniel Tuesca2

1Crop Production Department, Crop Protection, Rosario National University, Zavalla, Argentina;

*Corresponding Author: [email protected]

2Crop Production Department, Weeds, Agronomy, Rosario National University, Zavalla, Argentina

Received 17 April 2012; revised 23 May 2012; accepted 5 July 2012

ABSTRACT

A comparative survey of the weed species pre-

sent in field plots and edges was performed in

fields at Zavalla (Santa Fe) Argentina in the

soybean central region of the country in order

to determine changes in cover, frequency and

diversity of the weed communities. Five to

twelve soybean fields were surveyed in 2006,

2007, and 2009. Weed surveys were carried out

in the soybean fallow in winter and after soy-

bean planting in spring. In edges, species rich-

ness was higher than in field plots in spring-

summer but diversity showed an erratic response.

The weed community cover showed a shift in

weed vegetation composition relativ e to the field

plot. Our results indicate that the community in

crop edges relative to field plots differs in

structure and abundance and that many weed

species are only present either in crop edges or

in field plot s.

Keywords: Weed Community; Richness;

Glyphosate

1. INTRODUCTION

Agricultural practices have caused major changes in

the composition and species richness of weed communi-

ties in the field [1,2]. Arable weed species play an im-

portant role in supporting biological diversity in agro-

ecosystems [3,4].

Weed species that thrive in the field edges and may

colonize cropped plots include Avena sterilis and Galium

aparine [5], Conyza canadensis [6] and Senecio vulgaris

[7]. In other studies, plant populations in field edges have

not resulted in weed infestations in the adjacent crop in

many studies [8].

There is evidence that herbicide efficacy, increased

crop competition and changes in cropping patterns have

resulted in a gradual decline in weed abundance and di-

versity over recent decades [9-11]. Herbicide use is a

widespread practice detrimental to weeds [12] and con-

tinued use of a single herbicide often results in weed

composition shifts from highly susceptible species to

those having greater tolerance to the herbicide [13]. The

most used herbicide in arable crops in Argentina is gly-

phosate which provides application flexibility, lacks of

rotational restrictions and controls a broad spectrum of

weeds [14]. However, as a result of repeated use, species

difficult to control with glyphosate have become more

common in many countries [15-17] and in Argentina as

well [18].

Weed diversity may be concentrated in the crop edges,

especially in the weed communities of conventional ce-

real fields [19,20]. In Argentina, crop edges are narrow

areas that are taken out from agriculture. In crop edges,

insecticides, fungicides are not used and occasionaly,

herbicides are applied. In the field plot, the most used

herbicide is glyphosate-alone or in combination with

residual herbicides [21]. The objective of this study was

to analyze weed abundance and diversity as well as the

frequency of weeds tolerant or resistant to glyphosate in

field plots and edges in the soybean central region of

Argentina.

2. MATERIALS AND METHODS

A weed survey set up in Zavalla (Santa Fe, Argentina)

(Lat. 33˚01'S) was designed to measure the weed com-

munity in field plots and edges. The survey was carried

out across 5 to 12 fields chosen to represent the diversity

of cultural practices and environmental conditions pre-

sent in arable fields in the region. The survey was done

each year approximately 30 days after soybean planting

in late spring (December 2006 and 2007) and in winter in

fallows between summer crops (June 2007 and 2009).

These two sampling dates made it possible to account for

seasonal variations in weed populations (i.e. weeds asso-

ciated with both autumn-winter and spring-summer

E. Puricelli et al. / Agricultural Sciences 3 (2012) 631-6 39

632

cropping patterns). Surveys were generally made after

herbicide treatments.

In each arable field plot, in the field plot, an area of

approximately 20 ha subjected to normal field manage-

ment practices was surveyed, positioned at least 20 m

from field edge to avoid field border effects. The border

of each arable field consisted of narrow areas taken out

from agriculture as no herbicides, insecticides, fungi-

cides or fertilizers were applied and no crop was planted.

The same fields were sampled in the successive sam-

plings. In each field, both, field plot and edges surveys

were performed by two or more trained persons walking

across the survey area sampling randomly all species in

50 records each 20 m in a semicircle 1 m in diameter

(3.14 m2). The method takes into account the cover in

percentage of each species in each semicircle. A few

plant records determined only at the genus level were

discarded from the analysis.

For each species, % frequency of occurrence was cal-

culated using Eq.1.

% frequency of occurrence = (number of fields species

was detected/total fields sampled) × 100.

Also, each species average cover was calculated using

Eq.2.

Average cover = Σ (cover from each field where spe-

cies was present)/number of fields species was detected.

Multi response permutation procedure (MRPP) was

used to analyze differences between weed cover in the

edge and field plot communities. MRPP was conducted

using PCORD [22] software. Euclidean distance was

used as the distance measure. MRPP is a nonparametric

procedure that does not depend on assumptions such as

normally distributed data or homogeneous variances, but

rather depends on the internal variability of the data [23].

MRPP evaluates the uniqueness of a priori defined

groups relative to all other possible permutations among

groups of objects within the sample that have the same

size of the proposed classification [24].

Multivariate analysis of data was carried out to parti-

tion the respective importance of field plots and edges on

weed species composition. Data were classified with the

minimal variance method [25], using a resemblance ma-

trix of standarized Euclidean distances [26] and were

ordered with PCA (Principal Component Analysis) [27]

using a species covariance matrix. Analyses were done

using PC-ORD programs [22]. Biplots of samples and

species dispersion diagram were made on the plane of

the first two axes. In field plots and edges in each date,

richness, Shannon’s and Simpson’s diversity indexes

were analyzed using a t-test.

3. RESULTS

3.1. Richness and Diversity

The total number of species recorded was 76 in spring-

summer and 66 in winter. Richness and diversity indexes

in spring-summer were generally higher in edges relative

to field plots and in winter the opposite trend was ob-

served (Table 1).

In field plots the percentage of annuals was higher

than perennials. Perennials showed higher percentage in

edges compared with the field plots (Table 2). Of the

plant species recorded in the survey, only 8 were grasses

in spring summer and 5 in winter.

3.2. Cover

Species composition in crop edges is often different

relative to the cropped plot [2,19]. In our study, MRPP

showed significant differences between weed cover in

field plots and edges. In spring-summer: 2006 (p <

0.00003), 2007 (p < 0.00007), in winter 2007 (p < 0.01),

2009 (p < 0.007).

In both, spring-summer and winter surveys when the

overall analysis of community compositional differences

using PCA was conducted, weed cover in communities

under field plots was distinctly separated from edges

along the first canonical variable. In spring-summer in

2006 the first axis explained 54.1% of the variation and

corresponded to differences between field plots and crop

edges (Figure 1).

Bromus catharticus, Sorghum halepense and Cynodon

Figure 1. Overall analysis of community weed cover composi-

tional differences using PCA of weed cover in communities

nder field plots and edges in spring-summer (2006-2007). u

E. Puricelli et al. / Agricultural Sciences 3 (2012) 631-6 39

633

Table 1. Richness (S), Shannon’s (H), and Simpson’s diversity indexes (D).

S H D

Plot 9.8 (6.02) * 1.9 (0.56) NS 0.80 (0.11) NS

December 2006

Edge 15.8 (5.26) 1.9 (0.43) 0.75 (0.10)

Plot 10.4 (2.84) * 2.1 (0.27) NS 0.86 (0.04) NS

December 2007

Edge 21.4 (4.45) 2.4 (0.32) 0.84 (0.06)

Plot 15.0 (2.1) * 2.1 (0.41) * 0.78 (0.14) *

June 2007

Edge 10.0 (2.4) 1.5 (0.54) 0.62 (0.20

Plot 10.4 (5.5) NS 1.9 (0.79) NS 0.80 (0.18) NS

June 2009

Edge 11.0 (6.6) 1.8 (0.39) 0.77 (0.11)

For each column and date * indicates significant differences using a t tet (p = 0.05).

Table 2. Percentage of annual and perennial weed species in

field plots and edges.

Summer 2006 2007

Field plot Edge Field plot Edge

(%)

Annuals 63.6 57.9 72.2 61.8

Perennials 36.4 42.1 27.8 38.2

Winter 2007 2009

Field plot Edge Field plot Edge

(%)

Annuals 68.1 64.5 57.6 46.4

Perennials 31.9 35.5 42.4 53.6

dactylon were only present in edges. Iresine diffusa

showed much higher cover in edges and the species most

associated with crop field was Eleusine indica. The sec-

ond axis explained 22.1% of total variation. In 2007 the

first axis explained 60.3% of the variation and corre-

sponded to differences between field plots and crop

edges. Results were similar to 2006 but some other spe-

cies showed relative high weed cover. Sphaeralcea bon-

ariensis was only present in edges and Parietaria d ebilis

in both treatments but more associated to edges. The

second axis explained 12.9 % of total variation.

Figure 2. Overall analysis of community weed cover composi-

tional differences using PCA of weed cover in communities

under field plots and edges in winter (2007-2009).

3.3. Frequency

In spring-summer, the most frequently encountered

species in both treatments and both years were Anoda

cristata and Portulaca oleracea were observed in greater

than 50% of the fields. C. dactylon, I. diffusa, P. debilis,

S. halepense, Carduus acanthoides, Verbena litoralis

occurred only in edges. Euphorbia hirta was the only

species present with frequency higher than 50% only in

the field plot (Tab le 3). Within the annual grassy weeds,

Digitaria sanguinalis, Echinochloa colona and Eleusine

indica showed the highest frequency.

In winter in 2007 the first axis explained 59.8% (Fig-

ure 2). The same the species were found both in the field

plots and edges. Bowlesia incana was more associated to

crop edges while Poa annua showed more cover in field

plots. The second axis explained 17.3% of total variation.

In 2009 the first axis esplained 62.7 and the second axis

36.2% of total variation. B. catharticus was only present

in edges and P. debilis was more associated to edges but

also present in the field plot. In winter, P. debilis was th only species present with e

OPEN A CCESS

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634

Table 3. Mean maximum frequencies of the most common field plot and edge species in spring-summer in 2006 and 2007.

2006 2007

F. Plot Edge F. Plot Edge

Althernanthera philoxeroides (Mart.) Griseb. - 15 - 9

Amaranthus quitensis Kunth 31 38 45 6

Ammi majus L. - 23 - 9

Anoda cristata (L.) Schltdl 54 54 64 64

Artemisia annua L. - 15 - 9

Baccharis salicifolia (Ruiz & Pav.) Pers. - - - 18

Bidens subalternans DC. - 23 - 18

Brassica rapa L. - 23 - -

Bromus catharticus Vahl. - 53 - 100

Capsella bursa-pastoris (L.) Medik. 31 0.01 36 18

Carduus acanthoides L. 16 54 0.01 82

Centaurium pulchellum (Sw.) Druce - 7 - 9

Chenopodium album L. 30 8 0.01 18

Chenopodium pumilio R. Br. - 15 - 9

Cirsium vulgare (Savi) Ten. 0.01 31 9 9

Commelina erecta L. 10 77 0.01 10

Convolvulus arvensis L. 7 31 - 0.01

Conyza bonariensis (L.) Cronquist 15 85 81 27

Coronopus didymus (L.) Sm. 23 8 9 9

Cynodon dactylon (L.) Pers 15 62 0.01 100

Cyperus esculentus L. 47 46 0.01 47

Cyperus rotundus L. - - - 36

Cyclospermum leptophyllum (pers.) Sprague 8 24 0.01 19

Datura ferox L. 15 8 - -

Dichondra microcalyx (Hallier f.) Fabris 15 8 27 18

Digitaria sanguinalis (L.) Scop. 77 35 54 100

Echinochloa colona (L.) Link 77 38 63 55

Eleusine indica (L.) Gaertn. 69 54 35 55

Eleusine tristachya Lam. 8 - 54 -

Eryngium eburneum Decae. - 15.4 - 9

Euphorbia hirta L. 61 31 72 36

Euphorbia serpens Kunth 0.01 31 72 27

Gamochaeta subfalcata (Cabrera) 0.00 8 90 -

Iresine difusa L. 15 69 0.01 100

Ipomoea nil (L.) Roth. 0.01 8 9 9

E. Puricelli et al. / Agricultural Sciences 3 (2012) 631-6 39 635

Continued

Ipomoea purpurea (L.) Roth. - - - 27

Ipomoea rubriflora O Donell - 23 - 18

Jaborosa runcinata Lam. 15 8 - -

Lamium amplexicaule L. 15 - 18 -

Lolium multiflorum L. - 23 - 36.

Modiolastrum gillesii (Steud.) Krapov. - - - 36

Nicotiana longiflora Cav. - 23 - -

Oenothera indecora Cambess. 0.01 8 27 -

Parietaria debilis G. Forst. 0.01 51 27 91

Physalis viscosa L. 23 46 45 10

Portulaca oleracea L. 77 54 72 51

Polygonum aviculare L. 8 - - -

Rapistrum rugosum (L.) All. - 15 - 45

Rumex crispus L. 0.01 8 9 36

Senecio grisebachii Baker - 15 - 27

Solanum diflorum Vell. - 15 - 36

Sonchus oleraceus L.E. 61 31 54 64

Sorghum halepense (L.) Pers. - 61 - 63

Sphaeralcea bonariensis (Cav.) Griseb. - 30 - 45

Trifolium repens L. 15 15 36 36

Urochloa platyphylla (Nash) R.D. Webster 15 8 18 -

Verbena litoralis Kunth 15 51 0.01 100

Verónica persica Poiret. 15 - - -

Especies with frequency lower than 10% in both years Amaranthus viridis L.; Ambrosia tenuifolia Spreng; Ammi visnaga (L.) Lam.; Chloris canterae Arechav.;

Festuca arundinacea Schreb.; Geranium dissectum L.; Ipomoea grandifolia (Dammer) O Donell; Linaria canadensis (L.) Dum.Cours; Nothoscordum gracile

(Dryand. Ex Aiton) Stearn; Oxalis micrantha Bert. ex Colla; Plantago lanceolata L.; Setaria viridis (L.) P. Beauv.; Sida spinosa L.; Solanum sisymbriifolium

Lam.; Taraxacum officinale Weber; Urtica urens L.; Verbena bonariensis L.; Wedelia glauca (Ortega) Hoffman.

frequency higher than 50% only in edges. B. incana, C.

bursa-pastoris, C. didymus and S. oleraceus were ob-

served in greater than 50% of the fields (Table 4). Rap i s-

trum rugosum ocurred only in edges. The only annual

grassy weeds were B. catharticus and Poa annua.

4. DISCUSSION

Overall, MRPP indicated variations in species compo-

sition between edges and field crops. Two weed commu-

nities were identified according to season: Spring-sum-

mer (soybean crop) and winter (fallow). Variations in

weed species composition between seasons were also

observed in another study [28].

The higher richness and diversity in spring-summer

observed for edges relative to field plots concurs with

other studies [8,19,20,29,30] which may be due to the

absence of chemical control in edges as ocurred in an-

other study [11]. Crop fields are characterised by consid-

erable herbicide applications which may partially explain

why their weedy vegetation is different from edges not

subjected to these inputs [31]. However, in our study in

winter, the opposite trend was observed as higher or

similar diversity values were determined for field plots

relative to edges. The effect of herbicides on weed diver-

sity has also been erratic in other studies where herbicides

applied over more than one year either reduced [32] or

maintained [33] diversity. In both spring-summer and

winter surveys and in both years between 52% and 67%

of the observed species were annuals which concurs with

another study in arable fields [34].

n arable fields, the generalized adoption of glyphosate-

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Table 4. Mean maximum frequencies of the most common field plot and edge species in winter in 2007 and 2009.

2007 2007

F. Plot Edge F. Plot Edge

Althernanthera philoxeroides (Mart.) Griseb. 54 9 - -

Ammi majus L. 0.01 9 20 -

Amaranthus quitensis Kunth - - 60 20

Artemisia annua L. 0.01 9 20 -

Baccharis salicifolia (Ruiz & Pav.) Pers. - 18 - 20

Bowlesia incana Ruiz & Pav. 82 91 55 61

Brassica rapa L. 18 36 - -

Bromus catharticus Vahl. 9 40 - 60

Capsella bursa-pastoris (L.) Medik. 82 45 60 -

Carduus acanthoides L. 18 36 60 60

Cirsium vulgare (Savi) Ten. 82 45 0.01 20

Conyza bonariensis (L.) Croquist 100 63 20 40

Coronopus didymus (L.) Sm. 91 55 80 20

Convolvulus arvensis L. - - 20 20

Cotula australis (Sieber ex Spreng.) HooK. F. 64 36 - -

Cynodon dactylon (L.) Pers. - - 20 60

Cyperus esculentus L. 36 27 80 -

Cyclospermum leptophyllum (Pers.) Sprague 9 36 - -

Dichondra microcalyx Meisn. 27 9 20 -

Eryngium eburneum Decne. 9 36 - -

Eryngium horridum Malme - - 20 40

Fumaria capreolata L. 9 9 0.01 40

Gamochaeta subfalcata (Cabrera) Cabrera 91 91 40 20

Geranium dissectum L. - 27 - -

Geranium molle L. - - - 20

Hypochoeris brasiliensis (Less.) Benth. et Hook. - - - 20

Hybanthus parviflorus (Mutis ex L.f.) Baill. - - - 20

Jaborosa runcinata Lam. 28 27 - -

Lamium amplexicaule L. 100 27 20 -

Linaria canadensis (L.) Dum. Cours. - - 20 -

Lolium multiflorum L. - 9 - -

Medicago lupulina L. - - 40 -

Modiolastrum gilliessi (Steud.) - - 20 -

Modiola caroliniana (L.) G. Don 9 27 - -

Nicotiana longiflora Cav. - 18 - -

E. Puricelli et al. / Agricultural Sciences 3 (2012) 631-6 39 637

Continued

Oenothera indecora Cambess. 9 27 20 -

Oxalis articulata Savigny - - - 20

Oxalis cordobensis Knuth. 45 - - -

Parietaria debilis G. Forst. 36 55 10 100

Physalis viscosa L. 0.01 9 20 -

Plantago lanceolata L. 9 18 - -

Poa annua L. 73 27 40 -

Rapistrum rugosum (L.) All. - 18 - 40

Rorippa bonariensis Poir. Macloskie 27 - - -

Rumex crispus L. 9 - - 40

Senecio grisebachii Baker 64 - 40 60

Senecio vulgaris L. 55 - - -

Side rhombifolia L. - - - 40

Solanum diflorum Vell. - - - 20

Sonchus oleraceus L. 100 - 80 80

Sorghum halepense (L.) Pers. - - 40 -

Stellaria media (L.) Villars 82 - -

Taraxacum officinale Weber 18 - 0.01 20

Trifolium repens L. 18 - 20 -

Urtica urens L. 36 - 20 40

Verbena litoralis Kunth 27 - 0.01 20

Veronica persica Poiret 73 - - -

Veronica peregrina L. 9 - 20 -

Species with frequency lower than 10%: Anagallis arvensis L.; Anthemis cotula L.; Centaurium pulchelum (Sw.) Druce; Datura ferox L.; Gnaphalium gaudi-

chaudianum DC.; Mollugo verticilada L.; Polygonum aviculare L.; R a p hanus s a ti vus L.

resistant soybean resulted in a less dense and variable

weed community in many other studies [14,21,35-38].

Glyphosate shows very effective control of a wide range

of species including non-target species and changes in

weed populations in response to the adoption of gly-

phosate-resistant soybean has been reported elsewhere

[39,40]. Although there is no evidence to suggest that

herbicides such as glyphosate lead to the elimination of

species at the field scale [41], in the present study, weed

cover in average in the field plot was always low. The

absence of glyphosate application in edges may favour

potentially rare arable, broad-leaved weeds.

The crop planted at high density and the use of herbi-

cides and fertilizers favour crop production, and increase

the growth of the crop relative to the weed species [42]

which can account for the low weed cover in the field.

In our study the species composition differed between

years. Some of the most common species in spring-

summer were A. cristata and P. oleracea which are also

quoted as important weed species in other studies [43,44].

In our study, the most comon grassy perennial weeds

were S. halepense, C. dactylon and B. catharticu s. Those

grassy perennials species usually form a dense canpy

which exerts high competitive pressure for many other

weeds. S. halepense and C. dactylon aerial biomass is

killed by frosts but the dead canopy remains during win-

ter. B. catharticus is a short-lived perennial densely-

tufted and robust. Its aerial biomass is not killed by frosts

and consequently its green canopy is present during the

whole year. Some species common in crop edges are

adapted to grow under the canopy of perennial species

[28]. Among these especies, P. debilis is a broad-leaved

annual species which showed higher cover in edges rela-

tive to the field crop. The canopy of the grassy preenial

E. Puricelli et al. / Agricultural Sciences 3 (2012) 631-6 39

638

weeds can accout for this different behaviour as edges

protect P. debilis from freezing, allowing plants to

achieve more biomass accumulation than in the field

plots [45]. Another annual broad-leaved species common

in winter was B. incana, present in both the crop field

and edges. This species is also mentioned as an important

winter weed in the region in another study [37].

The most frequent annual grassy annuals were D. san-

guinalis, E. colo na and E. indica. In the last years, grassy

annuals are more abundant due to the adoption of no-

tillage systems [21]. Increases in summer grassy annual

density when tillage is eliminated has been shown in

previous research [46].

In our study, the only species found with high fre-

quency that is tolerant to glyphosate was A. cristata. No

resistant weeds were detected. In edges, several species

tolerant to glyphosate in high frequency were observed:

C. erecta, C. dactylon and P. debilis.

Our results indicate that the community in crop edges

relative to field plots differs in structure and abundance

and many weed species are only present either in crop

edges or in field plots.

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