Ndiob is a municipality in the department and region of Fatick, in west-central part of Senegal’s groundnut basin, located between latitudes 14˚26' and 14˚35'N and longitudes 16˚19' and 16˚24'W. It covers an area of 127 km² and comprises 18 villages (Figure 1).

Figure 1. Location of the municipality of Ndiob.

Mean monthly temperatures in the Fatick department from 1991 to 2020 show a maximum of 30.4˚C in May and June and a minimum of 25.5˚C in January.

A dry season runs from October to June and a rainy season from June to October. Over the period 1991-2020, mean annual rainfall totaled 604.6 mm. Rainfall is characterized, among other features, by (i) interannual variability in rainfall amounts was (336.6 mm in 2007, the driest year, and 922.5 mm in 2012, the wettest year), with more deficit years than surplus years; (ii) variability in the length of the cropping season (early and/or late onset and end dates); and (iii) frequent, often long (more than 7 days), dry spells. Weather data is provided by the National Civil Aviation and Meteorology Agency (ANACIM).

Vegetation consists of shrub savanna, with some tree cover. Two demonstration farm fields were established on slightly leached, sandy tropical ferruginous soil, one on slightly leached, sandy-clay tropical ferruginous soil, and one on non-leached, clay-sandy tropical hydromorphic ferruginous soil.

Pearl millet (Pennisetum glaucum (L.) R. Br.) is an annual cereal grass belonging to the family Poaceae and the order Poales ([5] Maiti & Bidinger, 1981; [6] Kadri et al., 2019). Genetic and archaeobotanical evidence places its domestication process in West Africa ([7] Oumar et al., 2008; [8] Burgarella et al., 2018). From germination onward, it develops a root system that provides extensive soil exploration capacity and considerable plasticity, conferring resistance under water deficit conditions ([5] Maiti & Bidinger, 1981; [9] Shrestha et al., 2023). Indeed, pearl millet is particularly well adapted to arid and semi-arid environments owing to morphological and physiological traits that allow it to maintain growth and production under water deficit ([10] Dancette, 1983; [9] Shrestha et al., 2023).

Its development cycle is characterized by three phases:

the vegetative phase, from coleoptile emergence to panicle initiation;the reproductive phase, ending with panicle flowering (female flowering followed by male flowering);the maturation phase, during which the grain develops and ripens ([5] Maiti & Bidinger, 1981).

Millet production remains strongly influenced by soil fertility. Improved nutrient availability and soil physical properties favor increased millet yields ([11] Mason et al., 2015; [6] Kadri et al., 2019). Millet is a thermophilic cereal ([12] Mohamed et al., 1988a), with optimal germination temperatures between 29 and 36.5˚C and maximum germination temperatures between 41 and 47˚C depending on genotype. It is an important cereal crop in the arid and semi-arid regions of sub-Saharan Africa ([11] Mason et al., 2015; [6] Kadri et al., 2019). In Senegal, millet accounted for 51% of the area sown to cereals in 2023 (DAPSA, 2023). It directly contributes to household food security and is consumed in various forms ([13] Vieira Junior et al., 2024; [14] O’Brien et al., 2022). Its processing is also an important economic activity, particularly for women involved in the processing and marketing of millet-based products ([14] O’Brien et al., 2022). Beyond its food and economic functions, millet holds a strong social and cultural significance, particularly for the Serer ethnic group.

Zaï is a technique for restoring sterile, hardened soils (zipellé) in the Sahelian environment, best known in West Africa (Niger, Burkina Faso, Mali, Senegal). It consists of digging pits 10 to 60 cm in diameter and 10 to 50 cm deep before the onset of the rainy season ([15] Bowers et al., 2024; [16] Danjuma & Mohammed, 2015; [17] Roose et al., 1993). An earthen ridge is formed downslope of the pit using the excavated soil. The pits are filled with organic matter. The main objectives are water conservation, improvement of soil structure, and stimulation of biological activity, among others.

2.4.1. Selection of Farmers and Demonstration Plots

The trial was conducted during the 2021 agricultural season and tested on four demonstration plots established across four soil types in the municipality of Ndiob. The experimental setup involved splitting each plot into two identical sub-plots (50 m × 25 m): one managed using the improved zaï method and the other using traditional farming practices. Four local volunteer farmers made four demonstration farm fields available for the zaï trial. To account for the variability between compound farm fields (near the village) and bush farm fields, one demonstration farm field was selected within the village and three in the bush. The compound farm fields are located in the immediate vicinity of the houses and are richer in organic matter due to regular contributions of household waste and manure from parked animals. The bush farm fields are in the remote outskirts of the village and are less fertile. Two demonstration farm fields were established on slightly leached, sandy tropical ferruginous soil, one on slightly leached, sandy-clay tropical ferruginous soil, and one on non-leached, clay-sandy tropical hydromorphic ferruginous soil.

2.4.2. Location and Digging of Zaï Pits

To mark the location of the zaï pits, perpendicular lines (a grid pattern) spaced 80 cm apart were laid out. The zaï pits were dug at the intersection points of these perpendicular lines.

2.4.3. Pre-Sowing Amendment

As organic matter was in short supply, each farmer spread the amount available on their field, prioritizing the areas they considered least fertile. Horse and donkey manure was spread on the control plots, although the quantity applied per unit area could not be determined. The zaï pits were amended with horse and donkey manure at a rate of 250 g per pit. Usually, the amount of organic matter applied ranges from 300 to 600 g per pits. However, farmers are very often faced with a shortage of organic matter. For this reason, the amount has been limited to 250 g per pits.

2.4.4. Sowing

Sowing was done by hand in the zaï pits and with a horse-drawn seeder on the control plots. Approximately, 10 to 20 millet grains are sown per pits, whether by hand (zaï) or using a horse-drawn seeder. In both cases, thinning out the seedlings helps to reduce the number of millet plants and thus limit competition for water and nutrients. Subsequent crop management was carried out identically on both plot types: first weeding, thinning, and second weeding.

2.4.5. Post-Sowing Amendment

After thinning and before the second weeding, 50 g of Organova—an organic amendment—was applied to each millet plant on both the zaï plots and the control plots.

2.4.6. Harvest and Yield Calculation

Grain yields were measured using the yield-square method ([18] Fermont & Benson, 2011). Two 5 m × 5 m yield squares were laid out and delimited with string in each of the 8 plots across the 4 demonstration farm fields (4 zaï plots and 4 control plots). They were positioned along the diagonal of each plot, avoiding border effects (a three-plant margin). Panicles within each yield square were harvested and weighed immediately, before being sun-dried on a tarpaulin for twelve (12) days. After drying, the panicles were weighed again before threshing. Grain weight was determined after threshing. Grain weight was extrapolated to a per-hectare yield for each plot using the formula below:

All weighing was carried out using a precision digital scale.

2.4.7. Perception Surveys

A perception survey was conducted using a standardized questionnaire administered to 113 farmers, who had trialled the improved zaï technology. The survey focused on heads of households, all of whom are men. The questionnaire covered several aspects, including information on the farmer, the demonstration plot, farming practices, the main constraints on farming, and the advantages and disadvantages of the zaï method compared with traditional farming practices, etc.

The questionnaire was designed using the ONA data collection, management, and visualization platform, compatible with Microsoft Excel, which allowed questions to be coded to facilitate subsequent processing. Data were cleaned and statistically analyzed using Stata. Graphs were produced using Microsoft Excel.

Grain yield in the zaï plots was higher than in the control plots in 3 of the 4 demonstration farm fields, with the difference ranging from 110 to 270 kg/ha (Figure 2). In contrast, for demonstration farm field 4 (a compound farm field), the yield of the control plot, cultivated using local traditional practices, was 1280 kg/ha versus 1190 kg/ha for the zaï plot, a difference of 90 kg/ha. The control plot’s superior performance in field 4 could be explained by the fact that compound farm fields are generally more fertile than bush farm fields ([19] Fatondji et al., 2006), and that the sowing density achieved with a seeder is higher than with the zaï method, which has a regular spacing of 80 cm between pits. The better performance of the zaï plots is likely attributable to the beneficial effects of water capture and use, localized organic matter input, and improved nutrient uptake by crops. Indeed, [20] Roose et al. (1999) demonstrated an increase in sorghum grain yield from 150 to 1700 kg/ha and in plant biomass from 500 to 5300 kg/ha on deep brown soil in Burkina Faso. Similar results were reported by [19] Fatondji et al. (2006) in Niger, with a 3- to 4-fold increase (up to 19-fold in one case) in grain yield under zaï compared with flat cultivation, both without amendment. With organic amendment, the same authors reported a 2- to 68-fold increase in grain yield compared with unamended conditions.

These results are also consistent with those of [21] Sawadogo et al. (2008), who found that organic matter significantly improves yield levels. [22] Mando (1997) attributes this phenomenon to the stimulation of soil biological activity: burying organic matter in the pits promotes nutrient mineralization and termite activity, which perforate the surface crust and thereby improve soil structure and infiltration capacity. [23] Somé et al. (2016) add that this could be explained by the combined effects of the improved soil moisture profile created by the zaï pits and the organic manure input.

Figure 2. Grain yield (kg/ha) across demonstration farm fields.

Increased agricultural yields were the main benefit of zaï reported by farmers during the perception survey.

Surveyed farmers attributed several benefits to the zaï practice (Figure 3). The most frequently cited were increased yields (95.4%), improved soil fertility (61.1%), and water retention (55.1%). Farmers also mentioned more efficient use of organic matter (33%), adaptation to climate change (12.8%), and faster crop growth (23.9%).

The ability to collect and retain rainwater in the pits was frequently cited as a benefit of zaï. This water retention capacity is linked to climate change adaptation, which was also considered a benefit by the farmers surveyed. In a changing climate marked by spatio-temporal rainfall variability, the ability of crops to cope with periods of water stress is vital. Water collection in the zaï pits helps maintain soil moisture for longer and supports adaptation to climatic hazards, particularly during deficit years. [24] Roose et al. (1994) confirm this finding, concluding that zaï can reduce the negative impact of climatic hazards and secure production. The ability of crops to withstand dry periods was observed in our study, in which crops experienced a 23-day dry spell during which plants in the zaï plots withstood the stress better than those in the control plots through a higher survival rate, certainly because of to the better water retention in the zaï pits. [20] Roose et al. (1999) affirms that zaï pits treatments increased infiltration of water and stores water for more periods compared to conventional tillage. Consistent with our findings, [25] GIZ (2012) states that, in the context of climate change, zaï is particularly valuable because it limits water losses.

More efficient use of organic matter, cited by 33% of respondents, is also considered an important benefit of zaï. Indeed, placing organic manure directly in the pit is more economical than broadcast spreading, which is the most commonly applied method in the area—particularly since the scarcity or insufficiency of organic matter was cited by 23% of respondents as a difficulty facing the agricultural sector. In this regard, [25] GIZ (2012) states that localized application of organic manure directly at the base of plants allows for its optimal use.

Ultimately, the benefits of the zaï practice observed in this study, corroborated by scientific findings on the subject, demonstrate the relevance of its introduction in the study area and its agronomic effectiveness.

Figure 3. Farmers’ perceptions of the benefits of improved zaï.

The benefits of improved zaï combined with organic manure convinced 77% of respondents of its effectiveness as a soil and water conservation response.

The main constraints of zaï (Figure 4) reported by surveyed farmers were: (i) the physical strain of the work (57.7%), its slowness (51.4%), lack of labour (45.1%), and insufficient organic matter (35.1%). Farmers also noted that the pits are easily spotted by grain-eating birds that dig up the seeds. Overall, 61% of respondents expressed a wish to adopt zaï technology, against 39% opposed. Implementing zaï technology requires considerable energy and time, particularly for digging the pits. Many studies confirm these findings, noting that zaï is very labour-intensive, particularly for the manual digging of pits ([26] Kimaru-Muchai et al., 2021); [16] Danjuma & Mohammed (2015); [27] Dugué (2014). To this must be added the insufficient availability of organic matter and labour in Senegalese agricultural communities. These constraints are consistent with the findings of [17] Roose et al. (1993), who state that zaï adoption is limited by the availability of organic matter and labour.

Figure 4. Constraints of improved zaï reported by respondents.