Survey of Weeds in Sesame in the Major Growing Area of Ethiopia

: The survey of weeds was conducted in high sesame growing areas of Ethiopia namely, Metekel, Gonder, and Humera Zones during 2018/19 cropping season to assess the abundance and distribution. It was done at the crop seedling stage using a systematic sampling technique and in quadrant counts (0.5 m 2 ). Weed species composition and their quantitative measures were calculated using descriptive formulas. The result showed 23 weed families that comprised 135 weed species. The most dominant families were Commelinaceae (5.92%), Asteraceae (11%), Cyperaceae (7.40%), Fabaceae (12.59%)


Introduction
Weeds are the most underestimated pest in tropical and sub-tropical agriculture, but they influence human activities more than other crop pests and contribute towards lowering the harvestable yields [1].Weeds compete with cultivated food crops for limited resources such as water, nutrients, and light [2].Weed infestation is one of the major factors limiting the yield of sesame, once the cotyledons of sesame emerge, they are small compared to other crops and do not grow fast.Agricultural productivity is geared to a high yield and quality level, but weeds, insect pests and diseases can confront these objectives.Therefore, the protection of crops from these agents is very important.Sesame (Sesamum indicum L; Family Pedaliaceae) is one of the oldest crops known to humans.There are archaeological remnants of sesame dating to 5,500 BC in the Harappa Valley in the Indian subcontinents [3].It is one of the world's oldest oil seed crop grown mainly for its high oil content that consists approximately 52 -57% oil and 25% protein [4].The plant is an erect herbaceous annual with either single stemmed or branched growth habits and has two growth characteristics of indeterminate and determinate, reaching up to 2 m height.It has a large tap root of up to 90 cm [5].According to Food and Agricultural Organization of the United Nations, 4.8 million metric tons of sesame was produced worldwide in the year 2016 [6].India, Japan, and China are the largest exporters.In Africa, Nigeria is the second largest producer after Sudan and ranking seventh in the world [5].Sesame is mainly cultivated between the 25° N and 25° S latitudes, but it grows in tropical to temperate zones from 40° S to 40° N latitude.Areas with annual rainfall of 625-1100 mm and temperature of >27°C is most conducive for sesame production [3].The crop is tolerant to drought, but not to water logging and excessive rainfall.It is also productive under high temperature and can grow on residual moisture during the end of a rainy season.This slow development is compounded by the nature of the drought resistance of sesame in that it will decrease large portion of photosynthetic resources to create more root mass to penetrate the soil as quickly as possible to reach moisture.In the first 30 days, sesame plants reach about 28 cm in height and yet will double to 60 cm in 41 days, triple to 90 cm in 49 days, and quadruple to 120 cm in 58 days after emergence (DAE).At this point the sesame plant begins developing canopy.Depending on row spacing and phenotype, mechanization of sesame requires good weed control for 50 to 60 days after planting (DAP) [7].The survey was conducted with the following objectives: 1) To collect, identify and provide updated information on weed species composition in major sesame production areas of Ethiopia.2) To determine abundance and distribution of commonly occurring and newly occurring weeds in sesame production of Ethiopia.3) To prioritize weed problems in sesame growing areas of Ethiopia.

Materials and Methods
Weed survey was conducted in sesame growing areas of Ethiopia such as in Pawe, Guba and Dangur districts of Metakel Zone in the Benishangul Gumuz region; Qafta Humera, Welkayit and Tsegade districts of Humera Zone in the Tigray region, and Sanja, Soroqa and Jawi districts in Gonder Zone and Awi Zone in Amhara region.Metekel Zone is located at latitude/longitude 11°19'N and 36° 24'E and altitude of 1120 masl.The average minimum and maximum temperature were 16.3°C and 32.6°C, respectively.The maximum rainfall is 1,587 mm while the length of growing period is 5-7 months in a year.The major soil types are nitosol, veritisol and alluvial soils.Humera Zone is located in western Tigray at longitude and latitude 14°18′N 36°37′E and altitude range of 560-1849 masl.The mean minimum temperature ranges between 17.5°C and 22.2°C, while the mean maximum temperature varies between 33°C and 41.7°C with average annual rain fall that ranges from 400 mm to 650 mm.Gonder Zone is located at altitude that ranges from 12°36N 37°28E with an elevation of 2133 mas.The average temperature ranges from 12°C to 25°C.Mean annual rain fall is 1295 mm.In general, the study area is predominantly categorized as hot and warm sub-moist plain agro ecological zone.The soil texture is mainly salty clay loam slightly alkaline, with a pH range from 7.8 to 8.

Method of Data Collection
Weed assessment was conducted on both small and largescale sesame farms at 10 km intervals along the main road.A quadrat of 0.5 m x 0.5 m size was randomly thrown along transects on ten spots per each field, and the weed species type and plant population count were recorded, following the first sample in a quadrat was taken following the procedure of [8].Where the surveyor walks 50 paces along the edge of the field, then turned right angle and walk 50 paces into the field and sampling started.During the survey all the weeds present in each quadrat were removed, collected, and kept separately species wise in polythene bags for counting.The weeds were separated species wise using the existing experience and available weed identification guidelines, identified and counted [9,10].Any plants found in the field that could not be identified on the spot were tagged, pressed, and sent to the national Herbarium of Adds Ababa University.When the plant specimens are completely dry, they were mounted on herbarium sheets and labeled.The species wise classified, weed count data were used to calculate frequency, relative frequency, density, relative density, abundance, dominance, relative uniformity, relative main field density, important value index and similarity index values of the differently located fields.
Mapping of weed species abundance and distribution was made to transfer field observations data records into spatial explicit points on a geographical map.Geographical map indicate where a given weed species' present or absent.In contrast, distribution maps from surveys on field indicate whether a species occurs as a weed in a specific region.
Weed management requires accurate information on weed infestation and distribution within agricultural fields.A geographical positioning system (GPS) latitude and longitude coordinates or the map (Figure 1) were used for survey location sampling points and on field experimental site at Tana Belles in Pawe district, Metekel Zone in Benishangul Gumuz region.

Method of Data Analysis
The data on weed survey was organized and summarized as frequency (F), abundance (A), dominance (D), important value index (IVI), and similarity index (SI) [10].
1) Frequency (%): is the percentage of sampling plots on which a particular weed species is found in a field.It shows how often a weed species occur in the survey area.

Frequency = × 100
Weed frequency indicates the ratio of the number of fields having specific weed species to the total number of the fields surveyed and is expressed in terms of percentage.It is concerned with the presence or absence of a species in a randomly through quadrat in sesame field.It is the percentage of sampling plots of visited field in which a particular weed species is found.F = X / N *100 where, F = frequency of particular weed specs.X = number of samples in which a particular weed species found.
N = total numbers of samples in a field.
2) Relative frequency (%): also expressed as a percentage, is the degree of dispersion of individuals (target) species in the sampling unit in relation to the number of all the species occurred.Where: D = dominance of a particular species.A= abundance of the same species.∑A =total abundance of all weed species 5) Similarity Index: Similarity of weed communities between any two different locations, soil types, surveys, and crop stages in terms of weed composition.

SI
./0 )** 1./02.2./ 3 Where, SI= similarity index, Epg = number of species found in both locations, Epa = number of species found only in location a, Epb = number of species found only in location b. 6) Important Value Index (IVI) Where, IVI = Important Value Index RD= Relative Density RF = Relative Frequency and RA= Relative Abundance Quantitative weed measurements i.e., abundance, dominance, frequency, relative density, relative uniformity, and relative main field density were calculated using MS-Excel from the collected data of the survey.

Results and Discussion
The weed survey was conducted on 60 fields in 46 kebeles of 10 districts, four zones and three regions, and collected and identified 135 weed species.Specimens of any plant encountered in sampling field but couldn't identified on the spot were collected, tagged, and pressed using plant press.
When the plant specimens optimally dry, they were labelled and mounted on herbarium sheets.Most of the weed species except 44 of them were identified on field condition by the team involved on the survey.Out of the 44 weed species, 34 of them were identified by experts at Ambo Agricultural Research Centre and 10 of them were identified at the National Herbarium of Addis Ababa University.These weed species were belong to 23 plant families.The colour of the flower, fruit, fragrance or any special features of the plants collected were documented on the field notebook, The list of weed species under the plant families and their characteristic classes have been presented in Table 1.Based on their morphological classification 88 weed species (65.18%) were found broad leaved.followed by 33 grasses (24.44%) and 14 (10.37%)sedges weed types.This result is also in agreement with that of Welday et al. [11] that reported 99 weed species.In their study, (67%) were broad leaved followed by 40 species of grasses (27%) and 9 weed species (6%) of sedges in their weed flora survey.The survey result indicated that dicotyledonous species were more dominant than monocotyledonous ones in the survey area.
This might be due to lack of proper cultivation of the fields.Plantations of sesame in the study area were made after one time cultivation followed by furrowing.The result also indicated that the occurrence of weed flora in medium altitude had more weed species diversity as compared to the low altitude areas.However, the weed species diversity in the crop fields within medium altitudes was found similar in most cases.Out of the total 23 plant families recorded in table 2, three dominant families were identified based on the number of tax contained.The plant families that consisted of more than 10 weed species were included: Poaceae (25), Fobaceae (17) and Asteraceae (15).These families constituted of a total 57 weed species of the total 135 weed flora.Therefore, in sesame field the weed species in Poaceae family was found as the most dominant followed by Fobaceae and Asteraceae 15 (Table 1 and Table 2).This might be due to the higher seed bank of these weed species families in the sesame fields.
Weed flora composition in sesame field.
The survey results showed that 51 weed species were recorded in Metekel Zone in Benishangul Gumuz region; 48 weed species in Humera zone in Tigray region, and 36 weed species in Gonder zone in Amhara region.
The occurrence of annuals was higher than perennial weeds which might be due to late weeding at which most of the annual weeds were abundantly grown.In addition, these families are very rich in species diversity, so it is usual that they contain many plant species.Asteraceae, Poaceae, and Fabaceae families were also reported to be economically important and common in different parts of the country [3].Frequency, Abundance and Dominance of weed species in Sesame.

Importance Value Index (IVI)
Among the frequent weed families Asteraceae, Poaceae, Cyperaceae and Fabaceae were found the most important in sesame field in Benishangul Gumuz, Tigray and Amhara regions.They were very rich in species combination than other families.[12] reported that weeds belong to the families Asteraceae, Poaceae, and Fabaceae were the most important in arable fields of northern and eastern Ethiopia.
The quantity of weed species from a family does not essentially represent the significance of that family in conditions of aggressiveness.For instance, the top five most important weed species in the sesame field that have scored more than 40 important value indices were Cyanotis sp (87.17),Medicago denticulata (76.94),Digitaria sanguinalis (46.33),Cyperus rotundus (45.77) and Oplismenus burmannii (41.04) (Table 6).
For these grounds, even if Tilaceae family was the most dominant family, its complete species except Cucurbitaceous Turneraceae, and Typhaceae others were not important weeds in the same way.Compositae was the next significant family in the field but there are no such important weed species of this family in their aggressiveness other than Commelinaceae.
This diversity of weed species in fields might show poor weed management, high accumulation of soil weed seed banks, high and frequent precipitation with warm weather conditions, and high fertilization.
In this study, 7 families were most diversified and contributed to about 104 species which is 72.22% of the total flora.The families Asteraceae, Cyperaceae, Commelinacee, Fabaceae, Poaceae and Malvaceae were the richest taxa contributed more than half of the whole flora in the sesame field (Table 2).
In this study Medicago denticulata, Cynotis spp, Cyperus spp and Vicia sativa were found the most diverse weed species within the counters.These species were found as the most dominant species in almost all the surveyed sesame fields in Ethiopia [13].

Weed Similarity Index
Weed species similarity index between districts of Benishangul Gumuz, Amhara, and Tigray regional states were described following the method used by Taye and Yohannes (1998).Weed communities having similarity indexes > 60% is assumed to be similar in terms of their species composition.Thus, when the weed similarity index is greater than 60%, it is considered that the same control method can be applied.However, if the similarity index is < 60%, on the other hand, it is assumed that the two locations have different weed communities, thus, different control measures should be applied.
The results from this study showed that the highest similarity index was obtained between Pawe, Soroqa, Sanja, Qafta Humera, Humera, Wolkite, and Tsegade districts from medium altitude, while the lowest similarity was observed between Dangure and Guba, and Dangure and Jawi districts.Therefore, a strategic plan of similar management options can be designed for locations having similarities in species composition.The top-ranking weed species were the most aggressive and difficult weeds to control in different surveyed areas.The high frequency of these weeds showed that they are a serious problem in all agricultural fields.The results showed (table 7), that greater than 60% similarity index were recorded among most surveyed districts from medium altitudes, while the lowest similarity index (<60%) was obtained between Dangur and Guba followed by Dangur and Jawi districts, and with different altitude ranges.
The SI result showed that weed species recorded in Dangur District were considerably different species from the higher altitude areas like Guba and Jawi districts.This result implies that districts with such <60% SI located at different altitudes require different weed management techniques while trying to implement control measures in the crop field.Weed succession and distribution patterns in sesame fields are dynamic in nature and weed flora composition may be region-dependent [13,14].Those structures of weed communities and their diversity are determined by environmental and management factors and by interspecific competition between weeds and crop plants and intraspecific competition between plants of the same species.[15][16][17] stated that there was a broader spectrum of variation and diversity between weeds of sesame from direct seeded areas as compared to transplanted areas.Other studies also depicted that weed flora composition in agricultural systems results from seasonal changes, crop rotation, and long-term environmental changes such as soil erosion and climate change.

Conclusion
The weed species composition recorded in sesame in the study areas is found diverse and extensive which demonstrates that weeds are still a significant problem in sesame field.A total of one hundred thirty-five (135) weed species were collected, identified, and characterized from the surveyed 60 sesame fields in the selected 46 kebeles of 10 districts of Metekel, Gonder and Humera Zones.These weed species were distributed in 23 families.The most aggressive and difficult to control weeds have been identified along with their distribution and measures of abundance, dominance, and similarity indices in different locations.Information on weed species composition and their quantitative measures in sesame along with climate, soil, and altitude in each district would be beneficial in designing weed management strategy that would best fit farmers' interest and efficiently control weed problems in sesame.
This survey revealed that Sorghum halepense, Convolvulus arvensis, Amaranthus spp and Cyperus rotundus were becoming a problem of sesame production areas.Weeds which have started increasing over the past few years are now known and a base for future weed surveys has been established.Since the weeds are becoming common and constant constraints of sesame production in the country it is recommended to carry out successive surveys of weeds in all sesame growing areas across the country to have a complete picture of the importance of the weeds across sesame agroecologies.
: Population density of a weed species expressed as the number of individuals of weed plants per unit area.A= (∑w)/n where A = Abundance.∑w = sum of individual of a particular weed species across all samples.N = total number of samples in a field.4) Dominance (%): abundance of an individual weed species in relation to total weed abundance.

Figure 2 .
Figure 2. Pictures depicting data collection on weed species composition and quantitative measures at Qafta Humera in sesame field during the survey in 2018.

Table 1 .
List of weed species under the plant families and their characteristic classes.

Table 2 .
Weed families, richness, and relative diversity of weeds in sesame field during 2019.

Table 3 .
The top ten of frequency, abundance, dominance, and IVI of the weed species in three regions of Ethiopia.

Table 4 .
Composition, and importance of weed species in sesame field of Metekel Zone, Benishangul Gumuz Region, and Ethiopia.

Table 5 .
Composition, and importance of weed species in sesame field of Humera zone, Tigray region, Ethiopia.

Table 6 .
Composition and importance of weed species in sesame fields of Gonder Zone, Amhara Region, Ethiopia.

Table 7 .
Weed similarity index among ten districts of Amhara, Benishangul and Tigray regions in Ethiopia during 2018 crop season.