Study area and selection of the surveyed sites
This study was conducted in two Sahelian countries, namely Senegal (14°40′N17°25′W) and Mali (12°39′N8°9′W), which are located in West Africa. In each country, two important sesame-growing regions were selected for the survey.
Mali
The Segou region (13°22′5″N5°16′24″W) covers 64,821 km2 and is located in south-central part of Mali. It is a strategic and cosmopolitan area for the country as it constitutes the transit area between the northern part and the southern part, bordering Mauritania and Burkina Faso, respectively. The region is characterized by a semi-arid climate with two seasons: a rainy season and a dry season. The average yearly rainfall is about 513 mm. Agriculture is the most important activity in this area as Segou produces most of Mali’s national food, including sedentary cattle farming [14, 15]. The major crops grown are cereals (sorghum, corn, fonio, rice), legumes and fruit (watermelon).
The Koulikoro region (13°56′41″N 7°37′28″W) covers 90,210 km2 and is located in the Western part of Mali. It is part of the tropical area with a rainy season and a dry season. The vegetation pattern shows a decreasing northward humidity gradient as a result of the joint effects of the different climatic zones and the soils: Sahelian zone in the north (150–550 mm rainfall), the Sudanian zone in the centre (550–1150 mm rainfall) and the pre-Guinean zone in the southern part (more than 1150 mm rainfall). The population in these regions consists of diverse ethnic groups and agriculture is the most important economic activity [16]. Different crops are cultivated including cereals (sorghum, millet, maize, and rice), legumes (peanut, bean etc.) as well as cash crops (cotton, sesame etc.).
Five villages in Segou (Boua-were, Tongo, Minankofa, N’gakoro and Kondogola) and in Koulikoro (Kalifabougou, Gouana, Tiendo, Kosa and Gountou) were selected for this study (Fig. 1).
Senegal
The Sine-saloum region or the peanut basin (14°11′N16°15′W) is located in the East of Senegal encompassing 24,000 km2 and includes the localities of Kaffrine, Kaolack and Fatick. In this region, sesame is widely cultivated mainly in the southern part. The climate is of a Sudano-Sahelian type with an annual rainfall lower than 700 mm. The “Wolof” ethnic group is the most predominant. Agriculture is one of the major economic activities, and millet is the prominent cereal grown in this area followed by maize.
Located in the southern part of Senegal near the Gambia, Casamance (13°0′54″N15°19′4″W) is a big region which covers 28,350 km2 (approximately 1/7 of total Senegal’s land) and includes Sedhiou, Kolda and Ziguinchor localities. The climate is a sub-Guinean type with an ample rainfall between 800 and 1300 mm, and the area is covered by dense vegetation. The major ethnic group in Sedhiou locality is the “Mandingue” and the main crops produced are rice, maize, fonio, groundnut, cotton and sesame, whereas in Kolda locality, the ethnic group “Peulh” is the most represented and is devoted to agro-pastoralism. The main crops grown in this area are maize, sorghum, rice, cotton and sesame [17].
Twenty-three villages in Casamance (Thies-pakane, Tenghori, Tambacouda, Soutouyel, Saré-koutayel, Saré-hamidou, Saré-amadou-diallo, Saoundé-popodié, Safane, Medina-souané, Médina-sadio, Kounaya-mankagne, Kégnéto, Kandiadiou, Kamboua, Kabaar, Faoune, Darou-salam, Boydo-malick-counda, Boutoupa-kamara, Boutolate, Bani and Bambadala) and 14 villages in Sine-saloum (Tewrou-mbeyenne, Niambour1, Ngettou-farba, Mbelbouck, Sahm, Sagna, Immidine, Dianké-souf, Mbaye-mbaye, Pirame, Patoulou-peulh, Diam-diam, Palangué-manding and Kalbirom) were selected for this study (Fig. 1).
Data collection
A preliminary survey was carried out in each region with local extension service agents during April–May 2016 to identify villages to be investigated. The villages where sesame is widely produced were selected, and the more distant ones were retained in order to cover a wide geographic area in each region.
The sampling of farmers per village was not totally random: we included 30% of women in the participants as well as young people and adults. The participants included were individuals who have a good experience in sesame cultivation and have been fully identified, thanks to village chiefs’ help. The questionnaire was pre-tested in two villages (not included in this study) from each region and adjusted prior to data collection.
Data collection was conducted in May–August 2016 and in total, 146 farmers (76 in Koulikoro and 70 in Segou) were surveyed in Mali whereas in Senegal, 110 farmers (69 in Casamance and 41 in Sine-saloum) were surveyed.
After obtaining their prior verbal informed consent, each participant was interviewed with a semi-structured questionnaire.
The questionnaire, comprising 71 items, was designed to obtain information about (1) the socio-demographic characteristics of the participants (name, age, sex, ethnic group, origin, educational attainment, contact, household size, principal and secondary activities, land owned etc.), (2) the agronomic practices (practices refer to the full range of field level management strategies, technologies and methods employed by the farmers) for sesame production (the varieties grown and their origins, agricultural practices, the type of ploughing, the use of pesticides and fertilizers, the period of harvest, the time and place of drying, conservation etc.), (3) the processing, the valorization and the market system, (4) the constraints and the farmers’ preference for the most important traits in sesame to be improved. Some sesame cultivars samples were collected from the different participants for future work.
Data analysis
Statistical analysis
The survey data were doubled-checked and inputted using Excel 2007. Thereafter, we used a range of descriptive statistics to characterize and summarize the farmers’ responses in each region. Categorical variables (items) were expressed as frequencies and percentages. To test whether some items varied significantly across the regions, we performed the one-way analysis of variance (ANOVA) with the Minitab® 16 software. The partitioning of the means was made with the Tukey test at a 5% probability level. To identify the most important traits in sesame to be improved and the main constraints the farmers faced during sesame production, we defined the main variables (items) based on the preliminary survey and asked the farmers to rank them. Data were analysed by adopting Garrett’s ranking technique [18]. The advantage of this technique compared to simple frequency distribution is that items are arranged based on their importance from the point of view of the farmers.
Garrett’s formula for converting ranks into percentage is:
$${\text{Percentile}}\,{\text{position }} = \frac{{100 \times \left( {Rij - 0.5} \right)}}{Nj }$$
where R
ij
= rank given for ith item by jth individual; N
j
= number of item ranked by jth individual.
The percentile position of each rank was converted into scores referring to the table given by Garrett [18]. For each item, the scores of each respondent were added up and divided by the total number of the respondents for whom the scores were added. These mean scores for all the items were arranged in descending order; the items were ranked accordingly and the most important ones identified.
Sesame seed sample preparation and biochemical analysis
The widely cultivated sesame varieties from each of the four regions were selected for seed biochemical analyses. Dust and stones were manually removed from the seeds. All mature and well-rounded seeds were kept for the analysis. Approximately 5 g of seed samples were manually ground to fine powder with liquid nitrogen. Seed oil was estimated by the conventional Soxhlet method using petroleum ether as the extraction solvent [19]. The total protein content was determined using the Kjeldahl method according to AOAC [20]. Each sample was tested in triplicate.