Contribution of sorghum to productivity of small-holder irrigation schemes: On-farm research in the Senegal River Valley, Mauritania

Agricultural Systems 115 (2013) 72–82

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Contribution of sorghum to productivity of small-holder irrigation schemes: On-farm research in the Senegal River Valley, Mauritania E. García-Ponce a,b, H. Gómez-Macpherson a,⇑, O. Diallo b, M. Djibril b, C. Baba b, O. Porcel a,b, B. Mathieu a,b, J. Comas c, L. Mateos a, D.J. Connor d a

Instituto de Agricultura Sostenible, CSIC, P.O. Box 4084, 14080 Córdoba, Spain Ministère du Développement Rural, Nouakchott, Mauritania Universitat Politècnica de Catalunya, Campus del Baix Llobregat, Avinguda del Canal Olímpic s/n, 08860 Castelldefels, Spain d School of Agriculture and Food Systems, University of Melbourne, Victoria 3010, Australia b c

a r t i c l e

i n f o

Article history: Received 2 March 2012 Received in revised form 15 July 2012 Accepted 11 September 2012 Available online 23 October 2012 Keywords: Water use efficiency West African Sahel Participatory research Gross margin Irrigated sorghum

a b s t r a c t In Mauritania, most irrigated land was designed for, and remains devoted to, rice (Oryza sativa L.) cultivation. Decades after introduction, however, yield remains below expectations, irrigated land is gradually being abandoned, and now crop diversification is promoted to improve sustainability of irrigated agriculture. This paper presents evidence of the potential, and limitations, of sorghum (Sorghum bicolor L. Moench) cropping in small-holder irrigation schemes along the Mauritanian side of the Senegal River Valley. Results are based on 3-years of on-farm participatory research (2007–2009) carried out at five irrigation schemes in collaboration with farmers and national research and extension services. Grain yield, water productivity and gross margin of sorghum and rice were compared at plot level. Global average grain yield over years and schemes was 2.5 t ha1 for sorghum, ranging from 1.7 t ha1 to 3.2 t ha1, compared to 5.6 t ha1 for rice, ranging from 4.0 t ha1 to 7.3 t ha1, even though both crops had similar total above-ground biomass at maturity. Sorghum required less irrigation water than rice (435 vs. 601 mm) but the smaller yield resulted in similar irrigation water productivity (0.87 vs. 0.96 kg m3) and fuel (pumping) productivity (1.71 vs. 1.93 kg MJ1). Despite smaller yields, however, sorghum profitability was significantly greater than rice (1172 vs. 788 € ha1), due to higher market price and, in the case of one scheme, lower irrigation costs. Main constraints identified in sorghum cropping were (i) poor crop establishment because of late sowing and water logging; (ii) neglected weed management; and (iii) mismatch between irrigation delivery schedules and water requirements. The causes of these constraints are particularities of rice production systems (design and heavy soil) and farmers’ habits acquired with traditional rainfed sorghum cropping during the wet season. The analyses presented here reveal that sorghum cropping is a profitable option to rice for small-holder farmers, particularly on light-textured soils within the irrigation schemes. Furthermore, large variability of results among sorghum farmers and the high above-ground biomass at maturity suggests scope for improving grain yield and water productivity. Challenges remain, however, for adoption of sorghum in irrigated agriculture in Mauritania. National agricultural policies must ensure access to credit and agricultural inputs (seeds and fertilizers) and consider specific requirements for crop diversification (type of soil, irrigation distribution) in both rehabilitation of existing schemes and in design and construction of new ones. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction After severe famines during the 1970s and 1980s in the western Sahel, irrigation schemes were developed for rice (Oryza sativa L.) production with the general objective of enhancing food security ⇑ Corresponding author. Tel.: +34 9557 49 92 76; fax: +34 957 49 92 52. E-mail addresses: [email protected] (H. Gómez-Macpherson), jordi.comas-angelet @upc.edu (J. Comas), [email protected] (L. Mateos), [email protected] (D.J. Connor). 0308-521X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.agsy.2012.09.009

in the region. One third (40,261 ha) of the estimated potentially irrigable area (136,500 ha) on the Mauritanian side of the Senegal River valley was developed to irrigation. Of this area, less than half remains under irrigation (FAO, 2005). Notwithstanding, Government assistance focuses almost exclusively on rice production providing seasonal credit to farmers to purchase inputs (notably fuel, fertilizer and seeds) and hire machinery for soil preparation. Despite this intervention, rice yield rarely exceeds 4 t ha1 (Haefele et al., 2001; Van Asten et al., 2003; García-Bolaños et al., 2011; Comas et al., 2012) well below attainable yield of 8–10 t ha1

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(Haefele et al., 2001; FAO, 2011). The low productivity is among the reasons for degradation and abandonment of these schemes in Mauritania (García-Bolaños et al., 2011). In response, the Mauritanian Government has revisited its development objectives and implemented a program of rehabilitation of irrigation schemes that includes intensification and diversification of crop production (UNCTAD, 2010; IFM, 2011). Rice remains the focal crop, however, because during the last decade (1998–2007) imported rice still provides half of domestic consumption (FAO, 2011). Most rice is imported from Thailand and sells at higher price than local rice because it is more suitable for preparing local dishes with fish. A major reason for uneconomic rice production is its high irrigation water requirement, especially on relatively common lighttextured soils (Fox and Rockström, 2000). Considering that about one third of rice production costs correspond to pumping irrigation water (Comas et al., 2012), introduction of less water-demanding crops adapted to medium and light textured soils should increase productivity and sustainability of irrigated agriculture in Mauritania. To this end, Comas and Gómez-Macpherson (2007) suggested that adapted sorghum cultivars (Sorghum bicolor L. Moench) could be an appropriate option to rice. This crop is highly appreciated by the local population as a traditional staple food, and traditional cultivars are currently grown under rainfed and flood recession conditions along the Senegal River Valley. Sorghum is also recognized by various characteristics that confer drought tolerance (Blum, 2005) and good response to supplemental irrigation (Farah et al., 1997) so its water requirements should be less than for rice. The few studies on irrigated sorghum crops in the Senegal River Valley (ISRA, 1981; Ndiaye, 1992) were carried out on research stations, but there are no references on water productivity or on-farm adaptation to real conditions of small-holder irrigation schemes. The objective of the present study was to evaluate the potential role of irrigated sorghum on productivity of community-managed, small-holder, irrigation schemes (less than 100 ha) in Mauritania. Grain yield, water productivity and profitability of irrigated sorghum and rice crops were analyzed comparatively at plot scale using a combination of conventional and participatory research, in collaboration with farmers, farmers’ cooperatives, and local researchers and extension agents.

2. Materials and methods 2.1. Study area: climatic and soil conditions, seasonal rainfall The study was carried out during 3 years (2007–2009) on five small, community-managed, irrigation schemes in mid Senegal River Valley: Bélinabé and Rindiaw–Silla in Gorgol Region and Wabounde, Dagveg and Bakaho in Brakna Region. The schemes (with sizes between 20 and 73 ha) were managed as cooperatives, each governed by a Cooperative Board and General Assembly. Each scheme had between one and four motor pumps that supplied water directly from the Senegal River. The distribution systems were open, unlined canals, except in Rindiaw–Silla, where the main conduit was a low-pressure pipe. Other characteristics of the schemes are described in García-Bolaños et al. (2011). Soil formation in these irrigation schemes is determined by successive sedimentation of suspended material in floodwater (Verheye, 1995). In consequence, while soils are generally deep with high water holding capacity, texture and drainage depends upon elevation in the floodplain. Soils are heavy and drain slowly in the lowest (known as ‘‘hollaldé’’) and light and fast in the highest areas (known as ‘‘fondé’’). ‘‘Hollaldé’’ soils are suited to rice production while ‘‘fondé’’ soils are preferred for irrigated sorghum production in order to avoid water logging after irrigation or rainfall events.

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Soils of the five schemes were sampled to determine organic matter content, P, pH and electrical conductivity (0–30 cm) and texture (0–30, 30–60 and 60–90 cm). Composite samples were prepared from 5 to 9 sites per plot according to size. Organic C was analyzed according to Walkley and Black (1934) and converted to soil organic matter with the multiplier 1.72, P content was analyzed according to Olsen and Sommers (1982), pH (H2O, 1:2.5) with a pre-calibrated glass electrode, and electrical conductivity (1:2 TDPS/distilled water) using a conductivity metre. Texture percentages were determined by the hygrometer method. Water content at saturation, field capacity, wilting point and saturated hydraulic conductivity for rice and sorghum soils were taken from Mateos et al. (2010) and Verheye (1995). Daily weather data (solar radiation, wind speed, air temperature, and relative humidity) were obtained from the airport at Kaédi (16°090 N, 13°300 W), the Gorgol capital. Reference evapotranspiration (ETo, mm) was calculated using the Penman–Monteith equation (Allen et al., 1998). When solar radiation data were unavailable, they were estimated from maximum and minimum temperatures (Allen et al., 1998). One rain gauge was installed at each scheme to monitor daily rainfall, except at Wabounde and Dagveg in 2008, for which rainfall measured at nearby Boghé (16°40 N, 15°450 W) was applicable. 2.2. Research approach The study used methods of conventional on-farm research supported by community-based Participatory Learning and Action Research (PLAR) (Defoer et al., 2009), including Rapid Rural Appraisal (RRA) (Chambers, 1994a, 1994b). Each community was the group of farmers sharing the assets of a small irrigation scheme managed by a village cooperative. The actors were: (i) the cooperative board; (ii) collaborating farmers; and (iii) researcher/extension agents. Other farmers participated in RRA and in farmer-to-farmer and end-of-season discussions and evaluations. Research was structured in the following stages: Stage 1: Community awareness and scheme characterization. RRA started with an introduction of the study to representatives of the cooperative board followed by semi-structured interviews. These interviews sought details about soil types, cropping patterns, pump characteristics, and water distribution rules. Next, the interviewers (researchers) and the cooperative board walked a transect (Defoer et al., 2009) of the irrigation scheme accompanied by the irrigation organisers who explained critical issues with respect to soil type and location in the network. Stage 2: Farmer/plot selection. A direct result of the Stage 1 was selection of six farmer/plots per scheme and year, three for rice and three for sorghum. Selection was based on the following criteria: (1) active farmers; (2) plots with suitable soil for sorghum or rice cropping; (3) minimum risk of uncontrolled water filtration from neighbouring plots; and (4) reliable water supply. Soil sampling described in the previous section took place in the selected plots. Stage 3: Participatory on-farm research. Before each growing season, collaborating farmers were trained on irrigated sorghum cropping. Additionally, they were advised by researchers at every crop operation to assure good crop establishment and subsequent development and growth. Seed, fertilizer and insecticide were provided to collaborating farmers to ensure comparability. Fermers’ perceptions and justification of actions were discussed with researchers while data were collected following structured protocols in order to compare adaptation, development, yield and water use of both crops. Details about cropping practices and specific measurements are given in following sections. Stage 4: Participatory evaluation. Field days, to which neighbouring communities were invited, were organized after flowering of

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sorghum crops. Collaborating farmers presented their experiences regarding sorghum cropping (farmer-to-farmer discussion). At the end of the cropping season, all actors met to discuss results, evaluate the cropping season, identify and rank agronomic constraints, and plan the research strategy for next season. A list of 20 constraints potentially occurring during three periods of sorghum cropping (pre-sowing, from sowing to flowering and grain filling) was presented to farmers in order to identify those they had faced and at what level of importance on a scale of 1–5 (1 = not important, 5 = very important). Initially, it was hoped that farmers would participate during two consecutive years but some did not continue after the first season. Failure of motor pumps at the start of 2008 in Bélinabé prevented rice cropping that year. In Dagveg, two farmers had jobs in town and were not fully committed and withdrew. They were replaced the second year. Consequently, while data were collected for 2 years at all sites, the plots and farmers involved were not identical in both years. 2.3. Cultivar and cropping practices Cultivar used were the medium-duration indica type ‘Sahel 202’ (syn. ITA 306), for rice, and ‘IRAT204’ (syn. CE 151-262) and ‘93B1057’, for sorghum. ‘Sahel 202’, considered the best yielding locally adapted cultivar, was introduced in 1996 by the Africa Rice Center (AfricaRice, former West Africa Rice Development Association, WARDA). Both sorghum cultivars were provided by the Institute of Agriculture Research in Senegal (Institut Sénégalais de Recherches Agricoles, ISRA) and have similar average cycle length of 90–95 d and attainable yield around 5 t ha1. The two were used to overcome seed shortage in 2007 and meet preferences of some growers for ‘93B1057’ in 2009. More characteristics of the two sorghum cultivars are described in Ba et al. (2010). Specific recommendations concerning crop management were given to farmers prior to the start of each season. For rice, they were local standard protocols provided by the national extension service (Société Nationale pour le Développement Rural, SONADER), together with AfricaRice. Irrigated sorghum is a new crop to Mauritania so management was based on experience in preliminary research trials conducted at the National Agriculture Research Center in Kaédi (Centre National de Recherche Agronomique et de Développement Agricole, CNRADA). Plots for both rice and sorghum were prepared using a disk plow after pre-irrigation. Plots were levelled manually if necessary and berms along the outer limits were rebuilt for control of flood irrigation. Water distribution in poorly levelled sorghum plots was facilitated by constructing small internal berms or distribution channels conducting water to highest zones. Rice was transplanted from nurseries: a target density of two seedlings per hill at 0.20  0.20 m (50 plants m2) was maintained even if transplanting was delayed. Sorghum was sown manually, using guiding ropes, at 3–4 seeds each in holes spaced at 0.20 m in rows 0.70 m apart. Then, 15 days after planting (DAP) plants were thinned to two per hole. In general, sorghum was sown before the beginning of the rainy season in order to avoid water logging during crop establishment and other problems associated with later sowings: nutrient leaching and weed pressure with first rains, and increasing bird damage during late grain filling (Dingkuhn et al., 2006; Kouressy et al., 2008). An initial application of 100 kg ha1 of triple super phosphate (TSP) was made to each experimental plot. Urea was applied at 300 kg ha1 in two or three splits for rice, and at 250 kg ha1 in two equal applications for sorghum. At Rindiaw–Silla, 200 kg ha1 of potassium sulphate (K2SO4) was applied to sorghum plots in 2009. Weeds were removed by hand on one or two occasions in all plots in both years. No herbicide was applied. A pyrethroid

insecticide (beta-cyfluthrin) was applied at a rate of 20 kg ha1 to sorghum fields to prevent attack by stem borers. 2.4. Crop establishment, development and yield components Number of plants per m2 was determined at 10–12 sampling sites (0.7 m2) in sorghum plots 10 d after emergence and just after the thinning operation. Sorghum ground cover and crop height were observed throughout the crop cycle. Rice and sorghum maturity was recorded when 50% of plant population attained that stage. Then, grain yield (GY) and yield components of either crop were determined from 10–12 random samples (1.4 m2), depending on plot size. Panicles were classified fertile or infertile, counted and evaluated for bird damage (and stem borer damage in the case of sorghum). Plant height was measured in five plants per sample taken at random. Grain and straw were separated, oven-dried (3 days at 70 °C) to zero moisture and weighed. Thousand kernels per sample were separated and weighed. Harvest index (HI) was calculated as the relationship between the dry grain production and total above-ground dry matter (AGDM). 2.5. Water balance and water productivity For each plot, applied irrigation water was measured using rectangular thin-plate weirs 0.25 m wide (Kindsvater and Carter, 1957; Walker and Skogerboe, 1987). Hydrographs were determined from periodic measurements of water depth above the weir crest. Total applied volume per irrigation was obtained by integrating the corresponding hydrograph. Average irrigation interval was determined for each plot without including pre-irrigation. Seasonal crop evapotranspiration (ETc, mm) was estimated based on a daily water balance. ETc was estimated from ETo and crop coefficients using FAO methodology (Doorembos and Pruitt, 1977; Allen et al., 1998). Crop coefficients were derived using the dual approach (Allen et al., 1998) that treats crop transpiration and evaporation from the soil surface separately. A basal crop coefficient curve was constructed for each crop from periodic observations of ground cover and plant height. The stress coefficient was assumed to be unity when the root zone soil water content was greater than a characteristic proportion (0.2 and 0.55 of the soil root-zone water-holding capacity, for rice and sorghum, respectively) below which transpiration is restricted, and reduced linearly to zero at the root zone water content corresponding to wilting point (Allen et al., 1998). Drainage (mm) was estimated as the excess of water on the day of rainfall or irrigation that could not be held in the soil root-zone. In the case of sorghum, soil rootzone water-holding capacity was calculated as the difference between field capacity and wilting point. In the case of rice, this capacity was the difference between saturation and wilting point plus a free water layer of 100 mm. Drainage is assumed to occur either by deep percolation or through temporarily opened breaches in the plot berms to evacuate the excess of water into open drainage ditches. Berms surrounding the plots prevented non-intentional runoff. Relative evapotranspiration deficit of sorghum, determined as 1  ETc/ETm, where ETm is unstressed maximum evapotranspiration, was calculated for the total growing cycle and for three component growing periods (Doorenbos and Kassam, 1979) derived from the basal crop coefficient curve: vegetative (21 days), stem elongation until flowering (45 days) and flowering to physiological maturity (35 days). For each plot, irrigation water productivity (IWP, kg m3) was calculated as the ratio of grain yield to applied irrigation water. Productivity of evapotranspired water (WPET, kg m3) was calculated as the ratio of grain yield to ETc. In addition, fuel productivity

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(FP, kg MJ1) was also calculated as the ratio of grain yield to fuel energy (caloric value of diesel: 39 MJ l1). For the latter calculation, volumes of water and fuel consumed were recorded as described in García-Bolaños et al. (2011). 2.6. Crop profitability Gross margin (UM ha1), in Mauritanian local currency (Ouguiya, UM), was calculated for each plot as incomes minus production costs as recorded by collaborating farmers. Family labour was not included in the calculation. Prices of seed, fertilizer and insecticide were the same for all experimental plots because these inputs were bought from the same source and at the same time. Cost of land preparation and pumping water for irrigation, however, differed between the schemes. Irrigation cost was related to plot surface except in Bélinabé scheme where the irrigation fee in sorghum plots was set at half that for rice. Local market prices for crop products (paddy rice and sorghum) at the time of harvest were used in calculations. Exchange rate to convert ouguiya into euro was: 1 € = 350 UM. 2.7. Statistical analysis Differences in GY, AGDM and HI with scheme/year combinations (n = 10) were tested by analysis of variance separately for each crop. Differences in IWP, WPET, FP and gross margin were tested by analysis of variance, considering only cases in which both crops (n = 2) coincided in the same scheme/year combination (n = 8; all cases except Bélinabé 2009 and Rindiaw–Silla 2007 in rice and Bélinabé 2008 and Rindiaw–Silla 2009 in sorghum). Differences between plots with GY below the first quartile (lowest yielding plots) and those above the third quartile (highest yielding plots) were tested by analysis of variance. Significant statistical differences were established by the Tukey test at p < 0.05. Statistical analyses were carried out using Statistix 9 software (Analytical Software, 2008) with results expressed as mean values. 3. Results 3.1. Weather Monthly weather data representative of the two regions are presented in Table 1 as maximum and minimum temperatures, rainfall and ETo for Kaédi (8 km east from Bélinabé and Rindiaw– Silla) during the experimental years together with their long-term (50-y) averages. Rainfall is concentrated in July, August and Sep-

tember but amounts are variable and well below ETo, particularly from October onwards. Irrigated cropping starts with the rainy season and lasts until December or January. In 2007, total rainfall (mm) was 300 in Bélinabé, 247 in Rindiaw–Silla, 234 in Bakaho and 232 in Wabounde. In 2008, the amounts were 289, 275, 230 in the same first three schemes, respectively, and 219 in Wabounde/Dagveg. In 2009, the amounts were 432 in Bélinabé, 423 in Rindiaw–Silla and 232 in Dagveg. In general, rainfall decreased westwards from Bélinabé and Rindiaw–Silla in the Gorgol Region to Wabounde, Bakaho and Dagveg in Brakna. In Gorgol, rainfall was around the 50-y average (288 mm) during the study period, although above in 2009. Long-term rainfall averages are not available for Brakna Region. 3.2. Plot selection The RRA and transect walks provided sketch maps of distribution of the two soil types. The heavy textured ‘‘Hollaldé’’ soils dominated most schemes because they had been selected for rice production. In three of the five schemes, there were only dispersed patches of ‘‘fondé’’ soil, but in Bélinabé and Rindiaw–Silla the proportion was estimated as one third of total area. Sampling in the selected plots revealed low fertility and deep, light-textured Eutric fluvisols (IUSS/FAO, 2007). Globally, the soils had low organic matter (average 0.65% in top 30 cm) and available phosphorous (average 4.1 mg kg1 in top 30 cm). Average soil pH (H2O) and soil electrical conductivity were 6.7 and 0.11 dS m1 (both in top 30 cm), respectively. Soils were deep and homogeneous with no significant difference in texture among the three analyzed layers (0–30, 30–60 and 60–90 cm). Soil texture analysis corroborated farmers’ perception: soils of plots selected for rice had higher clay and lower silt and sand content (32%, 19%, 49% clay, silt, and sand) in top 90 cm than those selected for sorghum (22%, 27%, 51% clay, silt and sand). In Bakaho, Wabounde and Bélinabé, plots with ‘‘fondé’’ soil selected for sorghum were located towards heads of secondary canals, near the main canal. This is not surprising since the main canals in irrigation schemes typically follow the highest contour lines to facilitate irrigation. In Rindiaw–Silla, however, ‘‘fondé’’ soils were located on slightly elevated areas at the tails of three consecutive secondary canals, where rice production was impractical. Finally, at Dagveg, the few ‘‘fondé’’ patches were scattered throughout the scheme, and therefore surrounded by rice plots, which increased the risk of water logging, making it difficult to adapt the irrigation schedule to the water needs of such different crops.

Table 1 Average monthly maximum and minimum temperature, rainfall and reference evapotranspiration (ETo) during 2007–2009 at Kaédi compared to long-term 50-year average. Month

Temperature (°C) 2007

January February March April May June July August September October November December

1960–2009

Rainfall (mm)

Max

Min

2008 Max

Min

2009 Max

Min

Max

Min

2007

2008

2009

1960–2009

ETo (mm) 2007

2008

2009

1960–2009

32 35 38 41 43 42 38 35 36 39 37 33

20 23 25 27 29 27 26 26 26 26 24 20

31 36 40 44 43 41 36 35 37 38 37 33

18 23 26 29 29 28 26 25 26 27 25 20

30 35 37 41 44 40 38 36 35 38 36 35

17 21 22 26 29 28 27 26 26 27 24 22

32 35 38 40 42 41 37 35 36 38 36 33

17 20 22 25 27 27 26 25 25 25 22 19

0 0 0 0 0 0 28 120 128 0 0 0

0 3 0 0 0 42 76 152 30 12 0 0

0 0 2 0 0 52 32 160 135 2 0 0

2 1 0 0 1 17 73 102 76 14 1 1

199 218 269 270 264 267 228 178 148 174 175 169

197 197 251 256 271 239 202 175 162 181 194 190

201 201 261 286 287 251 225 180 148 191 197 199

201 209 263 275 293 259 215 173 155 176 180 190

E. García-Ponce et al. / Agricultural Systems 115 (2013) 72–82

0.13 435 86 335 27 October 17 July

0.18 0.18 424 448 92 74 368 269 28 October 5 November 17 July 25 July

Mean

12 July

32

7 December

601

100

731

0.10

2008 2009 0.23 0.17 651 729 76 101 512 506 24 November 2 December 31 30 2008 2009 Dagveg

6 July 1 July

0.05 0.13 418 439 70 67 372 269 8 October 29 October 4 July 24 July 2007 2008 0.00 0.09 716 722 95 92 666 646 18 November 7 December 27 33 2007 2008 Bakhaw

1 July 20 July

0.03 0.26 419 329 79 49 525 155 4 October 17 October 2 July 19 July 2007 2008 0.08 0.07 638 827 79 87 601 784 3 November 18 December 15 28 2007 2008 Waboundé

4 July 24 July

0.17 0.09 440 490 117 115 426 179 17 November 20 October 3 August 7 July 2008 2009 0.10 0.20 725 754 138 115 554 490 2 January 8 January 46 42 2007 2008

17 July 29 July

0.09 0.16

Rindiaw-S

ETc (mm)

469 472 99 98

D (mm) I (mm)

463 323 28 October 14 November

Harvest date Sowing date

7 July 4 August 2007 2008

Year

0.02 0.06 759 785 108 110 753 497 8 December 5 December 43 27 2 July 15 July 2007 2009

Sorghum

Sowing date

Age (d)

Harvest date

I (mm)

D (mm)

ETc (mm)

1  ETc/ETm Rice

Year

Bélinabé

1  ETc/ETm

3.3. Agronomic and water management

Schemes

Table 2 Average sowing and harvest dates, age of transplants, seasonal applied irrigation (I), average applied depth per irrigation event (D), seasonal crop evapotranspiration (ETc) and the relative evapotranspiration deficit (1  ETc/ETm) by crop, year and irrigation scheme.

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A summary of key management parameters is presented in Table 2. In general, rice plots were established and managed according to protocols provided, except for age of transplants. Rice nurseries were sown (on average per scheme) between 1st July and 29th July and transplanted between 20th July and 9th September. At all schemes, except Wabounde in 2007, farmers used seedlings older than the recommended 20 d (Wopereis et al., 1999), while in two schemes seedling age exceeded 40 d. The causes were breakdown of irrigation pumps in Bélinabé and Rindiaw–Silla in 2007 and a general delay in 2008 while farmers awaited release of Government credit. Sorghum plots were sown between 2nd July and 4th August (Table 2). Late sowing of sorghum was mostly due to non-availability of tractors for land preparation, e.g. Bélinabé 2008, or to general delay in release of credit in 2008. Further, some cooperatives tended to postpone start of irrigation until after the first significant rains (e.g. Bakaho 2008) to reduce pumping cost. Late crops established after rains were especially problematic regarding weed control due to difficulty of working on wet soil. Additionally, timely weeding in large plots (0.5 ha) as in Rindiaw–Silla was not always possible. Done by hand, this operation required 12 person-d per hectare when carried out properly early in the season but up to 20 if delayed. In general, weeding was carried out by individual families, except in Rindiaw–Silla in 2009 where farmers weeded all plots collectively. Unnecessary confusion was introduced by farmers who waited, as in traditional rainfed sorghum culture, until seedlings were well developed and so more easily distinguished from weeds. Rice received more irrigation than sorghum in every scheme and year. The overall comparison was 601 vs. 335 mm, i.e. a ratio of almost 2:1 (Table 2). Seasonal ETc ranged from maximum values of 827 mm in rice and 490 in sorghum, to minimum values of 638 and 329 mm, respectively. Irrigation delivery schedules (times and amounts of water for each plot) were fixed by the cooperatives so in schemes designed for rice monoculture, schedules may not satisfy irrigation requirements of sorghum. On average, applied depth per irrigation event was greater for rice than sorghum (100 vs. 86 mm) in order to maintain free water on rice plots. Moreover, average irrigation interval was smaller for rice than for sorghum (22 vs. 30 d). In Rindiaw–Silla, organization of irrigation was flexible enough to deliver water to sorghum plots even though they were located at the tails of secondary canals. In Dagveg, however, irrigation of sorghum was given lower priority than rice and the average interval in sorghum plots was 16 d longer than in rice plots. Relative evapotranspiration deficit (1  ETc/ETm) revealed that rice plots in Bakaho and Bélinabé 2007 and sorghum plots in Wabounde 2007 met full water requirements. In contrast, rice and sorghum crops experienced stress in Rindiaw–Silla 2008, Dagveg 2008 and 2009, and sorghum in Bélinabé and Wabounde in 2008, the latter with the highest deficit (0.26). In Dagveg, a slight underestimation of ETc occurred due to uncontrolled water filtrations from neighbour rice plots. These major constraints to sorghum cropping observed during on-farm research (research Stage 3) coincided with those identified and ranked for the 30 plots (15 per year) in research Stage 4 ‘‘Participatory evaluation’’ (Table 3). The most cited constraint was water logging (67% of plots with an average importance of 3.1 on a scale of 1–5) resulting from poor plot levelling and drainage. Poor weed control was also identified as a major constraint (57% of plots and average importance of 3.7), particularly in large plots. Poor maintenance of motor pump and irrigation canals at the start of the growing period was relatively common (40% of plots and average importance of 3.4), often indirectly caused by late availability of credit (30% of plots and average importance of 4). One pump

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failed throughout the 2008 cropping season in Bélinabé and was evaluated with maximum relevance (10% of plots and average importance of 5). The precarious irrigation system made rice cultivation impossible that year. Minor constraints according to farmers’ perception were late thinning of plots, depredations of termites and insects, and bird damage during grain filling in late sowings. Agricultural inputs (quality seeds and fertilizers) were provided, thus avoiding related constraints to production.

Table 3 Major constraints faced by farmers cultivating sorghum expressed as percentage of plots where constraint was observed and average perception of its importance (1 = not important, 5 = very important). Constraint Pre-sowing Delay in credit concession Lack of title deed Poor maintenance of pump and irrigation canals Late start of rainfall season Service provider delay in soil preparation Poor soil preparation by service provider Poor soil levelling Sowing to flowering Damage due to ants/termites during establishment Damage due to locusts during establishment Water logging/poor drainage Poor weed control Delay of fertilizers application Delay in thinning Damage due to termites and other insects Brake of pump/drought Water got into plot from neighbour rice plots/ canals Grain filling Damage due to insects or diseases Drought Poor bird control Damage by wandering animals

Plots (%)

Importance (scale 1–5)

30 0 40 0 30 17 67

4

2.0 3.8 2.7

20 30 67 57 10 37 23 10 23

1.8 1.7 3.1 3.7 3.0 2.8 1.9 5.0 1.4

37 10 30 0

1.8 5.0 2.9

3.4

3.4. Grain yield and yield components Measurements of grain yield and total above-ground biomass (including grain) at harvest are presented in Table 4. Average rice yields ranged from 4.0 t ha1 at Rindiaw–Silla to 7.3 t ha1 at Wabounde. Overall average for sites and years was 5.6 t ha1. Sorghum yields were smaller than rice, ranging from 1.7 t ha1 at Dagveg to 3.2 t ha1 at Rindiaw–Silla and Bélinabé. Overall average yield was 2.5 t ha1. Although rice and sorghum differed in average grain yield between schemes, there were no differences in average above-ground dry matter at harvest; 12.4 and 11.0 t ha1 for rice and sorghum, respectively (Table 4). Average performance parameters of the highest and lowest yielding plots are presented according to quartiles (Table 5). The top rice plots all yielded more than 6 t ha1. By comparison, the bottom group yielded 4.1 t ha1 and all three components, kernels per panicle (KNPAN), number of productive panicles per m2 (NPAN) and kernel weight (KNWT), were smaller than in the top group (76%, 80% and 89%, respectively). In addition, in the bottom

Table 4 Average grain yield (GY), above-ground dry matter (AGDM) and harvest index (HI) by crop, year and irrigation scheme. Schemes

Rice

Sorghum

Year

GY (t ha1)

AGDM (t ha1)

HI

Year

GY (t ha1)

AGDM (t ha1)

HI

Bélinabé

2007 2009

5.2 bc 6.0 ab

10.3 bcd 12.5 b c

0.46 ab 0.45 ab

2007 2008

2.2 bcd 1.5 d

9.5 b 6.4 b

0.18 ab 0.22 ab

Rindiaw-Silla

2007 2008

4.6 bc 3.4 c

10.5 bcd 7.0 d

0.39 abc 0.47 ab

2008 2009

1.6 cd 4.8 a

10.9 ab 17.1 a

0.15 b 0.27 a

Wabounde

2007 2008

6.7 ab 7.9 a

14.3 abc 15.1 ab

0.42 abc 0.48 a

2007 2008

3.0 bc 3.4 ab

12.3 ab 13.2 ab

0.20 ab 0.24 ab

Bakaho

2007 2008

5.6 b 5.4 bc

18.0 a 9.9 cd

0.29 c 0.51 a

2007 2008

2.7 bcd 2.6 bcd

10.1 ab 12.0 ab

0.21 ab 0.20 ab

Dagveg

2008 2009

6.0 ab 5.0 bc

13.9 abc 12.9 bc

0.41 abc 0.34 bc

2008 2009

1.9 cd 1.6 cd

8.3 b 10.5 ab

0.21 ab 0.17 ab

5.6

12.4

0.42

2.5

11.0

0.20

Mean

Results in a column followed by a common letter are not significantly different according to Tukey’s HSD test (P < 0.05).

Table 5 Average grain yield (GY) and yield components (panicles m2, NPAN; kernels per panicle, KNPAN; kernel weight, KNWT), plant height, plot surface area, and age of transplants of rice seedlings and sowing date of sorghum of the best and poorest plots according to grain yield, and global crop average (n = 30 plots). Crop

GY (t ha1)

NPAN (m2)

KNPAN

KNWT (g)

Height (m)

Plot surface (m2)

Age of transplants (d) or sowing date (J day)

Rice Top mean Range Bottom mean Range

7.2 a (6.6–8.5) 4.1 b (2.1–4.9)

273 a (239–310) 219 b (131–298)

100 a (85–108) 76 b (63–89)

27 a (25–28) 24 b (23–26)

93 a (85–102) 71 b (51–97)

2670 a (1194–5000) 4541 b (3433–5150)

25 a (15–35) 42 b (27–59)

Sorghum Top mean Range Bottom mean Range Rice mean Sorghum mean

3.9 a (3.0–5.6) 1.5 b (1.1–1.6) 5.6 A 2.5 B

16 a (12–20) 9b (6–12) 242 A 12 B

1057 a (641–1686) 963 a (653–1465) 88 B 1041 A

25 a (19–30) 19 b (16–23) 25 A 22 B

151 a (142–158) 131 b (111–144) 83 B 141 A

2287 a (1868–3896) 4462 b (2387–6220) 3957 A 3479 A

191 a (182–206) 207 b (193–222) 32 199

Means for a given crop and parameter followed by the same lower-case letter are not different at P < 0.05. Crop means for a given parameter followed by the same capital letter are not different at P < 0.05.

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Low NPAN was not related to the water deficit calculated for the vegetative phase (0.17 average relative evapotranspiration deficit for this period) but mostly to water logging and late weeding. Among the bottom group there were four plots that suffered recurrent flooding and only had 11 plants per m2 after plant thinning (data not shown). Additionally, the bottom group also had shorter plants and 17% of unfertile panicles compared to 5% in the top group, even though they experienced practically no water deficit during stem elongation. Water deficit was more severe later in the season and resulted in smaller KNWT. When all plots were considered, KNWT decreased around 25% with an increase of 60% in relative evapotranspiration deficit during grain filling (Fig. 1). Compared to the top group, the bottom group had 76% of KNWT.

plots, average seedling age was 42 d compared to 25 d in the top group. Within the top group of sorghum plots (Table 5), five produced more than 3.5 t ha1 but only two achieved the average potential yield of 5 t ha1 according to the cultivar description. On average, sowing date in the top group was 10th July compared to 26th July in the bottom group, well after the start of rainy season. The major difference in yield components was found in NPAN that in the bottom group was 56% of the top group (9 vs. 16), i.e. well below that expected from the target plant density of 14 plants m2.

35

KNWT (g/1000)

30

3.5. Water and energy productivity

25 20

Productivity of evapotranspired water (WPET) was significantly greater in rice than in sorghum (Table 6). However, average irrigation water productivity (IWP) was similar for rice (0.96 kg m3) and sorghum (0.87 kg m3), although maximum values were observed in sorghum (2.6 kg m3 in Wabounde 2008 and Rindiaw–Silla 2009). As for IWP, average fuel productivity (FP) did not differ statistically between crops (1.93 vs. 1.71 kg MJ1) but did vary among plots, particularly for sorghum. Maximum FP for rice was observed in Dagveg 2008 (3.77 kg MJ1) and for sorghum, in Wabounde 2008 (5.76 kg MJ1). Differences were due to variations in both

15 10

y = -12.14x + 25.404 R2 = 0.446

5 0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

1-ET/ETm (grain filling) Fig. 1. Relationship between sorghum kernel weight (KNWT) and relative evapotranspiration deficit (1  ET/ETm) during the grain filling period.

Table 6 Average irrigation water productivity (IWP, kg m3), productivity of evapotranspirated water (WPET, kg m3) and fuel (irrigation energy) productivity (FP, kg MJ1) for the irrigation schemes and year where sorghum and rice crops coincided. Scheme

Year

Bélinabé Rindiaw–Silla Wabounde Wabounde Bakaho Bakaho Dagveg Dagveg Mean

2007 2008 2007 2008 2007 2008 2008 2009

Bélinabé Bélinabé Rindiaw–Silla Rindiaw–Silla Global mean

2008 2009 2007 2009

IWP (kg m3)

WPET (kg ET m3)

Rice 0.69 0.70 1.15 1.05 0.84 0.88 1.33 1.04 0.96

b b b b b b b b A

Sorghum

Rice

0.48 0.39 0.58 2.68 0.76 0.98 0.52 0.60 0.87

0.68 0.46 1.04 0.96 0.78 0.76 0.93 0.69 0.79

b b b a b b b b A

FP (kg MJ1)

abcde de a abc abcd abcd abc abcde A

0.47

Sorghum

Rice

0.46 0.37 0.71 1.02 0.66 0.60 0.44 0.36 0.58

1.04 0.88 2.48 2.25 0.85 0.90 3.77 2.95 1.93

de e abcde ab bcde cde de e B

0.31

1.24 0.82 2.69 1.02

0.98 0.72 1.24 5.76 0.77 1.00 1.47 1.72 1.71

bc c bc a c bc bc bc A

0.95

0.77 0.63

0.97

Sorghum bc bc bc bc bc bc ab abc A

2.51 1.04 0.97 0.59

0.77

1.90

3.38 1.80

Means for a given parameter followed by the same lower-case letter are not different according to Tukey’s HSD test (P < 0.05). Means values by crop within each parameter followed by the same capital letter are not significantly different.

Irrigation

Inputs + Land preparation

Grains

Fodder

Gross margin

Sorghum Rice Sorghum Rice

-300

-200

-100

0

100

200

300

400

500

600

700

kUM ha-1 (kUM = 103 UM) Fig. 2. Average variable costs (irrigation, solid grey; inputs/land preparation, diagonal lines), income (grain, blank; fodder, vertical lines) and gross margin (solid dark) for rice and sorghum.

E. García-Ponce et al. / Agricultural Systems 115 (2013) 72–82

yield and amount of fuel consumed to pump water: 0.009 l m3 in Dagveg, 0.012 l m3 in Wabounde, 0.013 l m3 in Bélinabé, 0.020 l m3 in Rindia-Sylla and 0.025 l m3 in Bakaho. 3.6. Crop profitability Production costs were similar for both crops but with different compositions (Fig. 2). While irrigation was the main cost in rice plots, the most important cost in sorghum was land preparation and inputs of seed (350 UM kg1 vs. 140 UM kg1) and insecticide (627 UM kg1), because of their limited availability in the local market. In general, irrigation cost was related to plot surface. In Bélinabé, however, the cooperative distinguished between the two crops by setting the irrigation fee in sorghum plots at half of that for rice. Average income for sorghum was greater than for rice (Fig. 2), due to higher prices for both grain (up to 200 UM vs. 95 UM kg1) and straw (20 vs. 10 UM kg1). Price of rice grain corresponded to paddy rice (approximately half price of processed rice). As a result of incomes and costs, average gross margin for sorghum was significantly greater than for rice (410,256 vs. 275,839 UM ha1) (Fig. 2) although it varied widely among schemes, between 260,934 and 603,034 UM ha1 (746 and 1723 € ha1). This variation was mainly due to variability in grain yield but also to cost of irrigation, as in Bélinabé scheme. 4. Discussion 4.1. Performance of sorghum Results from rice were used as a benchmark to compare performance of sorghum and to establish the potential and difficulties of growing the crop in the small-holder irrigation schemes of Mauritania. Except for Rindiaw–Silla, rice yields obtained by collaborating farmers were satisfactory with a global average similar to the 5.7 t ha1 obtained by Mauritanian farmers following integrated rice management recommendations developed by AfricaRice and partners (Haefele et al., 2000). Furthermore, the average rice yield of the top performing plots was relatively close to the 8–9 t ha1 estimated yield potential using crop models locally calibrated in Mauritania (Haefele et al., 2001; Van Asten et al., 2003; Poussin et al., 2006). In this study, low yield in the bottom group of plots was associated with smaller KNPAN, NPAN and KNWT (Table 5). Since densities of transplanted rice seedlings were presumably similar for all plots, explanation for low KNPAN and NPAN can be found in use old-seedlings or in nutrient and/or water stress during tillering or before anthesis (Wopereis et al., 1999; Haefele et al., 2001). For example, low yield at Rindiaw–Silla (4.0 t ha1) was due to delayed transplanting of 40-day-old seedlings (Table 2) caused by breakdown of motor pumps and lack of irrigation water early in the season. In the bottom group, average seedling age was 42 days compared to 25 days in the top group (Table 5) and the locally recommended age of 20 days (Wopereis et al., 1999). Small KNPAN and NPAN did not result in bigger kernels, rather to the contrary, plants in the bottom group had smaller grains than in the top group. In other studies in Mauritania, low rice yields have been attributed to sub-optimal timing of weeding and N fertilizer application (Kebbeh and Miezan, 2003; Poussin et al., 2003, 2006) and inadequate use of fertilizers (Haefele et al., 2004). The global average yield of sorghum was 2.5 t ha1 (at 0% moisture), well below the expected 5 t ha1 according to the cultivar description provided by ISRA, Senegal, where the crop has been widely tested. On the other hand, global average yield (2.8 t ha1 at 10% moisture content for comparison purposes) was greater

79

than both 1.9 t ha1 reported from farms in Burkina Faso (Fox and Rockström, 2000) and 1.5–2.0 t ha1 reported in Niger (Mossi Maïga et al., 2010), although less than the research station yield (3.8 t ha1) obtained in Sudan (Farah et al., 1997). In our study, average grain yield of the highest yielding sorghum plots (3.9 t ha1) was more than double the average yield in the bottom plots, revealing the existence of a substantial yield gap among farmers. Improved sorghum cultivars were cultivated in Mauritanian irrigation schemes for the first time within the framework of this study. Although a recommended crop management protocol was available and awareness activities with the cooperatives were undertaken before the growing season, farmers were reluctant to follow it strictly because of habits developed with traditional sorghum cropping in rainfed and flood-recession systems. In particular, weeding was often delayed until seedlings were well developed even though plants under irrigation were sown in rows facilitating identification for early weeding. Some cooperatives, e.g. Bakaho, were also reluctant to start irrigation before the rains, preferring to wait as they do in rainfed cropping or in irrigated rice cropping. Progress came unevenly after the first year. Farmers at Rindiaw–Silla and Wabounde managed to increase average grain yield in the second year but not those at the other sites (Table 4). Even so, conflict with traditional habits or with rice cropping was not recognized by farmers as a constraint to sorghum production during the evaluation in research Stage 4. Farmers identified irrigation problems at the start of the growing period, poor plot levelling and drainage-related water logging and poor weed control as the major constrains to sorghum cropping (Table 3). As reported for rice in similar conditions (Comas et al., 2012), it appears that there was no shortage of family labour when irrigated sorghum was combined with traditional systems. NPAN was the most important component of sorghum grain yield (Table 5), in agreement with Mason et al. (2008) but in contrast with Maman et al. (2004) and Farah et al. (1997) who reported greater influence of KNWT. As sorghum plants have limited tillering, and because little pest damage was observed, low NPAN mostly reflected problems during early crop stages, particularly the effect of water logging, the most common constraint identified by farmers (Table 3). Low NPAN, plant height and grain yield corresponded also to late sowing dates (Tables 2 and 5), carried out after the start of rains and, therefore, when flood probability was high and weeding operations were more difficult and less effective. To avoid yield penalty, sorghum fields should be weedfree from 20 days after emergence (Everaarts, 1993) but this was not achieved in many cases. The relationship between yield and successful manual weed control was reflected in the association between highest yields and a small plot surface area (Table 5). According to farmers, weed control was a major constraint (Table 3) and the best managed sorghum plots were 0.25 ha or smaller. At Rindiaw–Silla, the best performing scheme, farmers organized effective group weeding operations in 2009. Mechanization with 2-wheel tractors would help intensification of irrigated systems in Mauritania, as has occurred in Asia (Rawson et al., 2007). Late sowing also extends crop cycles longer after the rainy season when irrigation demand is highest. In some schemes, large water demand at that time, places sorghum at water deficit because irrigation priority is given to rice. Fig. 1 indicates that some crops did suffer water deficit during grain filling as reflected in small kernel size, although weed pressure would also contribute to small KNWT through competition for nutrients and water (Limon-Ortega et al., 1998). According to farmers, late-sown crops also required more work days for bird control during late grain filling, particularly in medium-size plots and when surrounding natural vegetation has dried. Total above-ground dry matter at harvest was similar for both crops but harvest index (HI) was smaller in sorghum than in rice

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(0.20 and 0.42, respectively). According to Farah et al. (1997), irrigated sorghum crops in Sudan could produce 14.2 t ha1 total above-ground biomass with HI of 0.27. In the top yielding plots here, the average grain yield, total above-ground biomass and HI were 3.9 t ha1, 14.5 t ha1 and 0.24, respectively (Table 5). These yields are considered readily attainable by farmers in the region, provided crops and irrigation are managed properly, but the required improvement in management will take time and continuing advice. Distribution of sorghum yields (Fig. 3a) reveals that most collaborating farmers achieved low yields (positively skewed distribution) compared to greater yields (negatively skewed distribution) of rice plots (Fig. 3b). These two patterns agree with the conceptual framework proposed by Laborte et al. (2012) for the evolution of farmers’ yield distribution in response to introduction of new technologies. The distribution of sorghum yields corresponds to the initial step (subsistence level) in which few farmers achieve high yield. As farmers become skilled, the distribution should shift to the right (commercial production level), comparable to that of rice. While farmers master sorghum cropping, harvest index could probably be increased using better adapted cultivars or hybrids, perhaps to 0.4 or beyond, as reported in Northeast Spain (Farré and Faci, 2006; Berenguer and Faci, 2001) and in the USA (Tolk and Hoell, 2003; Garrity et al., 1982). However, any sorghum cultivar introduced in the region must satisfy morphological and grain quality traits preferred by local population. A serious constraint is that the commercial seed sector in Mauritania remains practically nonexistent for crops other than rice. 4.2. Water and energy productivity and crop profitability

Frequency (number of plots)

Average productivity of evapotranspired water (WPET) in rice plots (0.82 kg m3) was within the range reported by Zwart and Bastiaanssen (2004) for rice (0.6–1.6 kg m3) in their review of crop water productivity of irrigated crops, and greater than values reported by Raes et al. (1992) in the Senegal River Delta (0.53–0.64 kg m3) or those reported by Mdemu et al. (2009) in semi-arid Ghana (0.55 kg m3).

14

(a) Sorghum

n = 30 average = 2.5 skewness = 1.2

12 10

For sorghum, average WPET (0.58 kg m3) was well below values found in other regions: 1.5 kg m3 in USA (Tolk and Howell, 2003, 2008) and 1.3 kg m3 in southern Italy (Mastrorilli et al., 1995) and northern Spain (Farré and Faci, 2006). However, the maximum value found in this study (1.02 kg m3 in Wabounde 2008) was close to those results. No data are available in the Sahel region for comparison but average WPET in our sorghum plots was greater than for irrigated maize in Burkina Faso (0.11–0.34 kg m3), Senegal (0.24 kg m3) and Cameroon (0.12 kg m3), as reported by Mdemu et al. (2009). In general, low water productivity in sorghum was mostly due to low yield and harvest index. Yield losses were caused in part by water stress as shown by the relationship between kernel weight and relative evapotranspiration deficit during grain filling (Fig. 1). Design and current operation of these schemes developed for rice cropping makes it difficult to match irrigation requirements for both rice and sorghum with one rotation schedule. Irrigation interval was longer in sorghum than rice plots, particularly at Dagveg; however, sorghum cropping in poorly drained plots with relatively heavy soil requires irrigation depths and intervals that avoid soil water saturation, particularly during the vegetative phase. Location of plots suitable for crops other than rice within the channel network is critical to successful crop diversification. Nevertheless, as Fig. 4 shows, most yield decreases in relation to evapotranspiration deficit for the whole cycle were scattered well below the yield function provided in Doorenbos and Kassam (1979). This indicates that most yield reductions were caused by factors other than irrigation in accordance with the positively skewed yield distribution typical of a newly introduced crop (Fig. 3a). IWP was similar for all rice plots but more variable in sorghum (Table 6). For sorghum, the greatest value (2.7 kg m3) was obtained at Wabounde 2008 and Rindiaw–Silla 2009 (Table 6). This is twice the highest value obtained in rice (Dagveg 2008). In Wabounde 2008, high IWP was the result of low applied irrigation depth and relatively high grain yield despite evident crop water stress. In Rindiaw–Silla 2009, irrigation depth was also low but rainfall was high and well distributed during the season. Additionally, in the last case, plots were the best managed in the study and had the highest grain yield. High IWP attained in sorghum identifies opportunities for increasing production with less water resources. Furthermore, rice profitability at plot level, consistent

8

1-ET/ETmx

6

0,5

0,4

4

0,3

0,2

0,1

0,0 0,0

2 0 0

1

2

3

4

5

6

7

8

9

0,2

10

0,4 14

(b) Rice

12

n = 30 average = 5.6 skewness = - 0.1

10

1:1 0,6

1-GY/GYmx

Frequency (number of plots)

Grain yield (t ha-1)

8 6 0,8

4 2 0 0

1

2

3

4

5

6

7

8

9

10

Grain yield (t ha-1) Fig. 3. Distribution of sorghum and rice plot yields obtained during the study. Vertical lines refer to global average yields.

1,0 Fig. 4. Relationship between relative evapotranspiration deficit (1  ET/ETm) and relative yield decrease (1  GY/GYm) for sorghum plots, where GYm (5.6 t ha1) is the highest plot grain yield obtained during the study. Solid line is the yield function according to Doorenbos and Kassam (1979).

E. García-Ponce et al. / Agricultural Systems 115 (2013) 72–82

with results of other studies in Mauritania (Poussin et al., 2006), was significantly lower than that of sorghum (Fig. 2). Irrigated sorghum grain is more valuable than rice because it is harvested (October–November) before traditional flood recession sorghum, that is then in short supply. Sorghum price may decrease 20–25% later when traditional sorghum is harvested (January–February). Sorghum straw is also more valuable than rice straw being preferred for greater sugar content and nutritional value for animal fodder. Complications in cost comparisons arise, however, because of different pricing decisions for water. For example, irrigation fees in most schemes were set according to plot surface area and not to volume of water used. In contrast, at Bélinabé fees for sorghum plots were simply set at half the amount for rice. Real costs of rice production in similar Mauritanian schemes were estimated by Comas et al. (2012) and expressed equivalent to 4 t paddy ha1. This included direct costs, 15% credit charge, and amortization costs (45,250 UM ha1 for the pumping system and 48,000 UM ha1 for the scheme construction). Adding these costs in our study would require production of 1.8 t ha1 sorghum grain to cover all costs. If sorghum grain price were reduced by 25%, as at harvesting time of traditional sorghum, the amount of sorghum grain needed to cover costs would rise further to 2.5 t ha1. Thus, early establishment and harvest of sorghum are needed to obtain high yields (Table 5), the benefit of high sorghum price to assure profitability, and maintenance of the scheme and pumping system. Farmers themselves now acknowledge benefits of producing sorghum and also management limitations when grown in riceoriented irrigation schemes. They are aware of the potential for greater sorghum productivity with improved crop and water management. Furthermore, the short growing season of sorghum enables them to follow with ratoon (sorghum re-growth) crop or a second short cycle crop in the same field. With this strategy, they would benefit from residual moisture and nutrients in the soil and continuing supply of irrigation water in the scheme required to finish the longer season rice crops. The double cropping alternative, e.g. sorghum followed by cowpea, is expected to result in greater gross margin with low additional irrigation and costs (Connor et al., 2008). The potential of sorghum ratoon for cropping intensification has not being studied in Mauritania but it could be attractive to farmers as source of animal forage during the severe winter shortage.

5. Conclusions Previous studies have highlighted low productivity of rice in Mauritania (Haefele et al., 2001; Van Asten et al., 2003; Poussin et al., 2006) and Sahel (Wopereis et al., 1999; Haefele et al., 2000), but limited information is available on profitability of alternative cereals. This paper focuses on the role of sorghum as a potential contributor to productivity of irrigated cropping along the Mauritanian banks of the Senegal River. On-farm research revealed several limitations to irrigated sorghum cropping, many related to characteristics of rice schemes (drainage system, irrigation delivery schedules and heavy soil types) and to farmers’ habits. Main constraints identified were: (i) poor crop establishment because of late sowing and water logging; (ii) inadequate weed control; and (iii) inadequate irrigation timing. However, the low irrigation requirement, the high average profitability of sorghum crops, and the large yield variation among collaborating farmers revealed considerable scope for commercial sorghum production in the area, particularly if harvested when sorghum is in short supply. In order to pass from initial subsistence to commercial levels of irrigated sorghum production, most farmers will require assistance

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to change habits established in traditional sorghum production. This will take time, however, because farmers in the area have limited resources, lack agricultural policies to support crops other than rice, and a weak national extension service generally hinders agricultural diversification and intensification (Aune and Bationo, 2008). To support adoption of irrigated sorghum cropping, local structures should account for differences in levels of resource endowment among farmers (Kebbeh and Miezan, 2003) and provide support to farmers for accessing agricultural inputs, particularly fertilizers and appropriate cultivars (Ahmed et al., 2000). A programme to develop sustainable mechanization should also favour intensification and productivity. Furthermore, national policies and interventions should include possibilities for crop diversification in design of new or rehabilitation of existing irrigation schemes. Our study concludes that schemes (or sectors within schemes) developed on relatively light soils and designed specifically to grow crops other than rice offer opportunities for profitable crop diversification. This is already occurring on the opposite bank of the river in Senegal, where tomato cultivation is expanding notably and, although mostly controlled by large companies, small holder farms are also participating and benefiting of this expansion (Maertens et al., 2011). Acknowledgements The authors express their gratitude for the participation and contributions of collaborating farmers. Our thanks also go to the local staff for their technical support. This study was supported by a project funded by the Spanish Agency for International Cooperation for Development and the Ministry of Rural Development in Mauritania. References Ahmed, M.M., Sanders, J.H., Nell, W.T., 2000. New sorghum and millet cultivar introduction in Sub-Saharan Africa: impacts and research agenda. Agric. Syst. 64, 55–65. Allen, R.G., Pereira, L.S., Raes, D., Smith, M., 1998. Crop evapotranspiration: Guidelines for computing crop water requirements. Irrigation and Drainage Paper 56, FAO, Rome. Analytical Software, 2008. Statistix 9. User’s Manual. Analytical Software, Tallahassee, Florida. Aune, J.B., Bationo, A., 2008. Agricultural intensification in the Sahel – the ladder approach. Agric. Syst. 98, 119–125. Ba, K., Tine, E., Destain, J., Cissé, N., Thonart, P., 2010. Étude comparative des composés phénoliques, du pouvoir antioxydant de différentes variétés de sorgho sénégalais et des enzymes amylolytiques de leur malt. Biotechnol. Agron. Soc. Environ. 14, 131–139. Berenguer, M.J., Faci, J.M., 2001. Sorghum (Sorghum bicolor L. Moench) yield compensation processes under different plant densities and variable water supply. Eur. J. Agron. 15, 43–55. Blum, A., 2005. Drought resistance, water-use efficiency, and yield potential: are they compatible, dissonant, or mutually exclusive? Aust. J. Agric. Res. 56, 1159– 1168. Chambers, R., 1994a. The origins and practice of participatory rural appraisal. World Dev. 22, 953–969. Chambers, R., 1994b. Participatory rural appraisal (PRA): analysis of experience. World Dev. 22, 1253–1268. Comas, J., Gómez-Macpherson, H. (Eds.), 2007. L’agriculture et l’élevage dans les petits villages de la vallée du fleuve Sénégal en Mauritanie. Options d’amélioration et de diversification dans les petits périmètres irrigués. AECID. Comas, J., Connor, D., Isselmou, M., Mateos, L., Gómez-Macpherson, H., 2012. Why has small-scale irrigation not responded to expectations with traditional subsistence farmers along the Senegal River in Mauritania? Agric. Syst. 110, 152–161. Connor, D., Comas, J., Gómez-Macpherson, H., Mateos, L., 2008. Impact of smallholder irrigation on the agricultural production, food supply and economic prosperity of a representative village beside the Senegal River, Mauritania. Agric. Syst. 96, 1–15. Defoer, T., Wopereis, M.C.S., Idinoba, P., Kadisha, K.L., Diack, S., Gaye, M., 2009. Curriculum for Participatory Learning and Action Research (PLAR) for Integrated Rice Management (IRM) in Inland Valleys of Sub-Saharan Africa: Facilitator’s Manual. Africa Rice Center, Cotonou, Benin.

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