Checklist for the crop weeds of Paraguay

Abstract Paraguay, a country whose economy is based mainly on agriculture and livestock for export, has experienced a major expansion in mechanized crops during the last few decades. Despite being heavily dependent on agriculture, Paraguay has very limited research on crop weeds, in spite of these having a high economic impact on production. This work aims to update and enhance the knowledgebase on the most common weeds affecting productive fields throughout the different ecoregions of Paraguay. We present here the first checklist of crop weeds for the country, which includes a total of 256 taxa (189 species, 10 subspecies, 54 varieties and 3 forms), with the most species-rich families being Poaceae and Asteraceae followed by Malvaceae, Amaranthaceae, Fabaceae and Solanaceae. The list includes three new records for the country. Synonyms, distribution details within Paraguay, habit and a voucher specimen are provided for each taxon.


Introduction
Weeds can be defi ned as plants (not necessarily alien) that grow on sites where they are not wanted and which have a detectable economic and/or environmental impact (Richardson et al. 2000, Pyšek et al. 2004. Th ey overlap partly with invasive plants, which are introduced species that cause problems, either in agriculture or in natural areas. According to Pyšek et al. (2004), there have been terminological misunderstandings in studies dealing with plant invasions, partly caused by diff erent perceptions of plant invasions by particular biological disciplines and viewpoints. As shown by these authors, plants encroaching on habitats in which they were not present before can be assessed from the ecological point of view (and termed colonizers), from the biogeographical point of view (invaders, or alien plants in a more general sense), or from the anthropocentric point of view (termed weeds, harmful species, problem plants, noxious plants, pests, etc.).
Paraguay has a total area of 406,752 km 2 and is divided into two regions by the Río Paraguay: the Oriental Region and the Occidental Region. Th e economy is based mainly on agriculture and livestock for export; between the years 2013-2014, approximately 52,381 km 2 were dedicated to temporary and permanent crops (DGEEC 2015), particularly soybean (35,000 km 2 ), corn (8,000 km 2 ) and wheat (5,600 km 2 ). During the second half of the twentieth century, global soy production grew tenfold from 27 million tons to 269 million tons (WWF 2016). Th is increased production was the result of a major expansion in mechanized crops, including other crops that form part of this complex such as wheat, corn, sorghum, canola, among others. It is expected that production will double by 2050 (Bruinsma 2009, WWF 2014. Indeed, Paraguay plays a strong role in this soybean industry; during the 2012-2013 harvest, Paraguay ranked sixth as a global producer and fourth in exports. Itapúa and Alto Paraná have been the highest producing departments, while expansion currently occurs in the departments of San Pedro, Canindeyú and Alto Paraguay (WWF 2016).
Agriculture in Paraguay is carried out on a variety of very distinct soil types in both natural regions. Th e soils in the Oriental Region are for the most part reddish in colour, resulting from geological events during the Precambrian era and Jurassic and Cretaceous periods (Bertoni and Gorham 1973). Th ese soils include the leptosols and arenosols, which are generally poor in nutrients, and the loamy sand and sandy loam regosols, which are rich in nutrients and where most of the mechanized crops in the region have been carried out (González Erico 2007). In the Occidental Region, most of the soil types are brackish, with a variable content of clay, including planosols, solonetz, gleysols and regosols, where mechanized crops are carried out (Proyecto Sistema Ambiental del Chaco 1992-1997.
Despite being mainly an agricultural country, Paraguay does not have much in terms of research on crop weeds, even in cases where these have a high economic impact on production. Only two sources can actually be considered as references on the subject: Lurvey (1983), the only weed guide available for the country, and the Flora del Paraguay collection (Spichiger and Bocquet 1983-1985, Spichiger 1987-1989, Spichiger and Ramella 1990-2002, Ramella and Perret 2008 that superfi cially covers the behavior of some species, in terms of their weediness, within the families included in the collection. Even herbarium data has proved to be of little use to determine the level of weediness of a species, due to the well-known biases in plant collection towards natural areas. Weeds have a particular set of skills for survival through mechanisms such as high competitive ability, high seed production, seed production for as long as growing conditions permit, rapid growth throughout vegetative phase to fl owering, germination requirements fulfi lled in many environments, adaptations for short and long dispersal and great longevity of seeds, self-compatible but not completely autogamous or apomictic, and when cross-pollinated, either pollinated by unspecialized visitors or by wind (Baker 1965, Baker 1974, Lorenzi 2000. Th eir development in productive areas tends to reduce the quality and quantity of the crop yield, making it diffi cult to harvest, and in some extreme cases even making it unviable (Guglieri-Caporal et al. 2011). According to Pott et al. (2006), success in controlling invasive plants begins with the fl oristic inventory of the infesting species and knowledge of the biology of those species that are predominant.
Because of their numerous interactions with humans, their biology, ecology, evolution and community dynamics, weeds are an important subject of study (Kuester et al. 2014). In fact, the study of plant species considered to be weeds in agro-ecosystems is a subject of high interest in the area of invasion ecology, not only because of their natural ability to become established in natural and semi-natural environments (Figueroa et al. 2004, Pyšek et al. 2004) with the consequent great ecological harm they can cause to ecosystems, but also because of the special attributes that make these plant species more invasive (Rejmánek and Richardson 1996), and the way they respond to human disturbance (Hill et al. 2002). Other studies have also focused on revealing taxonomic patterns among natural area invaders and agricultural weeds (Daehler 1998, Kuester et al. 2014, while Conservation Agriculture builds upon weed ecology to generate integrated and sustainable weed management strategies (Bajwa 2014).
Regarding the increasing importance of weed science in a country such as Paraguay, which depends heavily on agricultural production, this work aims to update and enhance the knowledgebase of the most common weeds aff ecting productive fi elds throughout the diff erent ecoregions of the country, as well as the generation of relevant information to facilitate their identifi cation and management.

Methods
Paraguay is a land-locked country located between 19° and 28° south latitude and 54° and 63° west longitude at the heart of the South American continent, and lies entirely within the Río de la Plata drainage system, second only in size to the Amazon basin. Th e country is divided into two regions by the Río Paraguay: the Oriental Region, or eastern region, also known as the Paraná region, and the Occidental Region, or western region, also known as the Chaco (part of the Gran Chaco Americano shared by Argentina, Bolivia and Paraguay). Paraguay is divided into 17 departments, 14 of which are located to the east of the Río Paraguay (De Egea et al. 2012).
While building the checklist, we included: 1) a survey of all plant species occurring in key sites, mainly in areas of mechanized soybean, sorghum, sugarcane, corn and rice, where weed management and control was being periodically implemented ( Figure 1, Table 1). Each site was surveyed once, at a time when crop fi elds presented a high level of weed infestation and prior to any weed control activities. Sampling was carried out between 1-3 days, depending on the size of the available crop area per site, by surveying a series of random points (up to 8 per day) to collect the weeds occurring within a 50 meter radius. Th e data collection included the random collection of plant specimens, georeferencing of the records, the identifi cation of specimens and the resolution of taxonomic problems. Where possible, specimens were collected in quadruplicate; the set of originals were stored in FCQ and duplicates were sent to BM, G and CTES; and 2) the systematization of data available in the literature. Th ree sources were considered as reference: Lurvey (1983), the Flora del Paraguay collection, and the database of Th e International Survey of Herbicide Resistant Weeds (Heap 2016), containing a list of the more resistant weed species in the region. Online databases and herbarium resources such as TROPICOS (2016) and the Catálogo de las plantas vasculares del Cono Sur (Zuloaga and Belgrano 2015) were also consulted.  Data was compiled in a MS Access database. Flowering plant family circumscription follows APG III (Stevens 2001 onwards) and pteridophyte classifi cation follows Smith et al. (2006). In the checklist, families are sorted alphabetically and genera are sorted alphabetically within families. For each taxon we present the accepted name, place and date of publication, synonyms, habit, general distribution in Paraguay, residence status (all species are native except where indicated), and one voucher specimen. Relevant bibliographic citacions are included for taxa recorded in literature, but not found during fi eld surveys nor in the consulted herbaria (BM, CTES, FCQ, and PY). Synonymy follows Zuloaga and Belgrano (2015), except for those taxa covered in more recent literature. Herbarium acronyms used in the text follow Index Herbariorum (Th iers, continuously updated). New reports of taxa for Paraguay are marked with an arrowhead (►) and those taxa not cited in Zuloaga and Belgrano (2015) for Paraguay but recorded in other publications or databases are marked with an asterisk (*).

Data resources
All occurrence data underpinning the checklist have been uploaded to the Natural History Museum Data Portal (http://dx.doi.org/10.5519/0060042) and are provided as a data supplement to this paper.

Results and discussion
Th e list includes a total of 256 taxa (189 species, 10 subspecies, 54 varieties and 3 formas), making up 38 families and 141 genera, of which all except one are angiosperms. Th ere is only one pteridophyte listed, Pteridium arachnoideum (Kaulf.) Maxon, a very close relative of Pteridium aquilinum (L.) Kuhn, a cosmopolitan species with a vigorous vegetative reproductive system (Baker 1974), which is a very common weed worldwide.
Th e most species-rich families are Poaceae, Asteraceae and Malvaceae, followed by Amaranthaceae, Fabaceae, Solanaceae, Polygonaceae and Cyperaceae (see Table 2). According to Baker (1974), Malvaceae, Amaranthaceae, Cyperaceae and Poaceae are the families which have a higher number of weed species in warm regions, in line with our results.
Nine species were found to be the most frequent and abundant within all crops surveyed: Amaranthus hybridus L., Bidens pilosa L., Conyza bonariensis (L.) Cronquist, Commelina erecta L., Eleusine indica (L.) Gaertn., Euphorbia heterophylla L., Digitaria insularis (L.) Fedde, Richardia brasiliensis Gomes, and Solanum americanum Mill. Th ese species were found in both natural regions of Paraguay and on diff erent soil types, showing a greater plasticity/resilience than the other species found. Overall, they are species with high adaptability, fast growth, resistance to drought and high biomass and seed production. It is also likely that the seeds of these weed species accompany the seeds of the harvested grain crops. All these have been listed and highlighted in regional literature on weeds as being problematic species (Ferrari 2012, Guglieri-Caporal 2011, Lorenzi 2000, and with the exception of Commelina erecta and Richardia brasiliensis, are considered among the weeds with greatest herbicide resistance according to Th e International Survey of Herbicide Resistant Weeds (Heap 2016).