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:
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].
I
n arable fields, the generalized adoption of glyphosate-

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636
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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