Floristic study of weed species in common bean (Phaseolus vulgaris) fields of Azna

Introduction There are many different factors that affect yield and quality of agricultural crops, including weed management. The abundance, distribution, density and composition of weed species in a cropped field varies due to the nature of the crop, cultural practices and cropping pattern/system, soil type, moisture availability, location and season, therefore identification of weed flora is important to identify the proper weed control options and enables farmers to use the best management strategies. Common Bean (Phaseolus vulgaris L.) is an increasingly important cash crop for growers in western provinces of Iran. Due to the lack of basic information about the weed diversity in common bean fields in Lorestan province which is the mean producer of bean in Iran), the current study was conducted for exact identification of flora condition of weeds.   Materials & Methods The present investigation was carried out to find out weed floristic composition in common bean fields of Azna (33°45΄ N latitude, 49°45΄ E longitude, with a mean altitude of 1871 m above sea level), in the North of Lorestan province during 2014. In this study, 34 fields of bean using method of stratified random-sampling, and by assigning the suitable number of the samples for each level, were selected. The longitude and latitude and sea level of different fields were recorded exactly by global positioning system (GPS). In order to determine the density and composition of weeds, sampling in each field done using the pattern of W. According to the pattern of the W, 9 points in each field were selected and the distance between two sampling points was 20 meters and in each point by using a 0.5 m × 0.5m quadrate the density and composition of weed species was determined. For data analysis of weed flora in the fields, frequency, abundance index, distribution uniformity and mean density were calculated using appropriate equations. Weed frequency indicates the proportion of fields where the species was found. The diversity of weed species was described by species richness and weed density was determined by counting the number of plants in each sampling quadrate. Cluster analysis was done by JMP software using the data related to species richness index, species diversity and similarity.   Results & Discussion In the current experiment, 23 weed species from 14 families were collected and identified at the different bean fields in the studied region. The highest number of weed species belongs to Poaceae family (five species) and Asteraceae family (three species). Based on results, 78% and 22% of weed species were dicotyledonous and monocotyledon respectively. Among the identified weed about 65% were annual and the remaining species were biennial or perennial. It seems that intensive and high input cultivation of common bean in the studied region affected the composition and diversity and density of weeds. Previous studies also showed that perennial weed communities under less cultivation and reduced tillage systems are often more frequent whearase in conventional systems the annual weed are dominant. Kenaf (Hibiscus cannabinus L.), Field Bindweed (Convolvulus arvensis) and Redroot Pigweed (Amaranthus retroflexus L.) had the highest and Black Nightshade (Solanum nigrum L.) and Goat's Beard (Tragopogon pratensis L.) had the lowest density, uniformity and abundance in the common bean fields. The density of the weed species varied considerably in the different fields and the maximum density of weed (5.5 plants. m2) in the fields was recorded for Hibiscus trionum. The most important noxious weed of studied regions were Field Bindweed (Convolvulus arvensis), (Sorghum halepense (L.) Pers.)، (Cynodon dactylon (L.) Pers.) and (Echinochloa crus-galli (L.) Beauv.). The highest value of species richness index (27.5) and Simpsons diversity index (0.94) were recorded for Southwest, whereas the lowest value of species richness index (17.14) and Simpsons diversity index (0.86) calculated for Southeast regions. Results of another study showed that species richness is related to geographical condition and increase with the height. Decrease of species richness in plain areas suggests that agronomic management has a strong effect on species diversity and weed abundance during the time.   Conclusion The occurrence of numerous weed species in bean fields means that different methods of weed control must be selected because some of these weeds such as redroot pigweed (Amaranthus retroflexus L.) can produce a lot of small seed which may increase the problem of weeds in further years. The higher number of dicotyledonous weeds means that special cautions for the use of chemical herbicides is necessary because many of recommended herbicides for broadleaf weeds could result in serious damages to the common bean fields. It seems that in these conditions considering the integrated weed management (IWM) methods is justified.