Determination of Flora and Distribution of Weeds in Soybean Fields Using Geographic Information System (Case Study: Gorgan County)

Oilseeds are the second largest food sources in the world after cereals. Soybean (Glycine max L.) is the largest source of protein and oil in the world and is cultivated in some regions in Iran. Golestan province is one of the important agricultural regions in Iran with about 62.01% of the soybean cultivation area. Most of these areas are located in Gorgan County. There are a number of approaches that they use to management of weeds in agroecosystems. Management and protection of crops from weed damage depend on our knowledge about weed richness and their distribution status. For this purpose, generation of weed distribution maps provide very important information for weed management. Also, determination of weed flora help to managers and farmers to identify weeds and select the best method for weed control, especially invasive weeds in soybean fields. Therefore, the aim of study was to determination of flora and distribution of weeds in the current soybean fields in Gorgan County.

This research was carried out in 48 soybean fields of Gorgan County, during 2015-2016. In the field section, the W method used to collect weed samples in order to calculate the number, frequency, species density, etc. The spatial information of the fields was recorded by GPS and sampling was done by 0.25*0.25 square meters quadrate. Total weed samples moved to weed research laboratory of Gorgan University of Agricultural Sciences and Natural Resources (GUASNR) and identified in terms of scientific and family names based on taxonomical classification way. Then, all data entered to Microsoft Excel spread sheets var. 2010, and were averaged and arranged. In order to determine the weed flora status of these fields, some indices such as frequency, relative species frequency, species uniformity, relative species uniformity, density and relative species density were calculated. To better display of distribution maps, weeds were divided based on frequency index into three categories; 50-100%, 50-30% and less than 30% and weeds with a frequency of 50-100% were considered as important weeds of soybean fields in Gorgan. Finally, the data were processed in ArcGIS software var. 10.3, and then weed distribution maps of soybean fields were generated.

In this study, 21 types of weeds were identified from 14 plant families, which 14.28% were belonged to perennials group and the rest amounts were annual weeds. Also, 76.19% were belonged to dicotyledons and about 23.81% were monocots. The highest density was related to ground cherry (Physalis divaricate L.) with 16.68% compared to other weed species in the measured plots. Crab grass (Digitaria sangunalis L. Scop.) with a density of 0.94 and sorghum (Sorghum halepense L. Pers) with a density of 0.78 had the highest density than other weeds. The weeds of the studied fields included 23.80% of narrow-leaved weeds and 76.19% of broadleaf weeds. In terms of photosynthetic cycle, 28.57% of weeds classified to C4 cycle and 71.42% to C3 cycle. Also, the most important plant family was Poaceae with 19.04% of the observed weeds in soybean fields, compared to other plant families. The most dominate species of this family are sorghum, cockspur grass (Echinochloa crus-galli L. P. Beauv) and crab grass. The weeds with a frequency of 50-100% were included ground cherry, sorghum and wild melon (Cucumis melo var. agrestis L.) with a frequency of 91.66, 54.16 and 52.08%, respectively. Results of weed distribution maps showed that weeds with frequency of 50 to 100% were observed in the most of surveyed field and they were distributed across the county, from east to west.

In the study, the weed distribution maps showed the existence of broadleaf weeds such as ground cherry, sorghum and wild melon in the most of the studied fields. Also, we identified the large number of weed families in this study. Considering this high number of identified weed and the presence of some invasive weeds in the surveyed fields, these results can useful for weed management programs of soybean fields and other crops in Gorgan County. Therefore, it is necessary that farmers and agricultural managers of the Gorgan County care to distribution of invasive weeds. Finally, it concluded that a combination of local and longer-term weed management can possible to deliver reduced weed infestation levels. Also, integrated weed management is an essential practice for reducing the impact of pesticides in the agroecosystems.