Forgoing the fallow in Bangladesh's stress-prone coastal deltaic environments: Effect of sowing date, nitrogen, and genotype on wheat yield in farmers’ fields
Introduction
Sustainable intensification (SI) has been widely proposed as an important agricultural development policy goal (Godfray et al., 2010). While SI is widely contested and definitions of what constitutes SI differ (cf. Tittonell, 2014), most advocates argue that increased crop production should come from currently arable land, rather than through expansion into frontier forests, wetlands, or other natural areas. In contrast to intensification strategies that focus on boosting yield alone, SI also aims to limit agriculture's environmental footprint through management practices that increase or maintain resource quality and use efficiencies as yields are increased, thereby minimizing tradeoffs. In South Asia, most potentially arable land is already in production, and future food needs can only be met through intensification (FAO, 2014). Two potential pathways for SI in this region include (i) increasing yield on currently intensively cultivated lands (‘crop intensification’) using better-bet agronomy, stress-tolerant crop genotypes, and resource conserving management practices, and (ii) by encouraging farmers to move from single into double cropping on seasonally fallowed lands (i.e. ‘system intensification’) where only monsoon ‘aman’ season rice is currently grown, also using the above approaches (Rawson et al., 2007). Double cropping on currently under-utilized lands could substantially benefit smallholder farmers and make considerable contributions to regional food security objectives, though there is uncertainty about what management options are most appropriate in these environments.
With South Asia's densest population at more than 1000 people per km2, and over 100 million living in rural areas, Bangladesh makes a useful case study for these issues. Urbanization has generated a 10% reduction in agricultural land area availability in the last 30 years (FAO, 2014, Hasan et al., 2013). Yet at the same time, considerable amounts of land in southern Bangladesh remain seasonally fallowed following monsoon season rice, with estimates ranging from 240,000 to 800,000 ha, depending on the year and measurement method (cf. BADC, 2010, Poulton, 2011).
Wheat (Triticum aestivum) is commonly grown in South Asia's rice (Oryza sativa)–wheat systems during the dry winter ‘rabi’ season from November through April. It is the region's second most important food security crop after rice, including in Bangladesh (Timsina and Connor, 2001). Average yields in Bangladesh are 67 kg ha−1 below the regional mean of 2.75 t ha−1 (FAOSTAT, 2014), though estimated attainable yields for the country, the southern coastal region, and southern saline zones are higher, at ∼6, 5, and 3 t ha−1, respectively (Carberry et al., 2011, Rawson et al., 2011). Currently, per capita wheat demand is 17.3 kg year−1, approximately 20% of rice consumption. With 3% more protein than rice, wheat makes an important contribution to protein intake at 4.3 g day−1 (FAOSTAT, 2014).
While Bangladesh's wheat production decreased by 34% in the last decade, aggregate demand jumped by 13% in the same period. Bangladesh's wheat imports are currently ∼3.12 mt annually (FAOSTAT, 2014), costing $0.67 billion in foreign currency reserves in 2013 (MoF, 2014). In response, the Government of Bangladesh has called for an additional $7 billion of donor investment to facilitate SI in the coastal delta. In particular, wheat's moderate tolerance to salinity and low-irrigation requirement may make it well-suited to the delta, where accesses to fresh irrigation water and soil salinity are concerns. Dalgliesh and Poulton (2011) concluded that yields of more than 3 t ha−1 can be obtained with one irrigation, and 2–2.5 t ha−1 without irrigation, where wheat is sown before mid-December in Southern Bangladesh. Modeling studies indicate that yields ≥3 t ha−1 can be achieved where soils are non-saline, when groundwater tables are shallow, and a single irrigation is applied at crown root initiation (Carberry et al., 2011).
However, the process of system intensification in the delta of southern Bangladesh is not without challenges, particularly with respect to soil salinity, irrigation water availability, and flooding. The coastal areas of Bangladesh are affected by varying degrees of soil salinity (MOA and FAO, 2012). Yield begins declining after 6 dS m−1 with >50% loss expected at 10–13 dS m−1 when salinity remains consistent from sowing to maturity (Ayers and Westcot, 1989, El-Hendawy et al., 2005). But soil salinity is also temporally dynamic, starting at low levels following rice harvest and rising in concert with increasing temperatures and evaporative demand before the subsequent monsoon rains flush salt from the root zone (Dalgliesh and Poulton, 2011). Farmers can therefore escape the detrimental effects of salinity by early planting and harvesting, though in practice sowing is often delayed due to the late harvest of long-duration monsoon season rice varieties that precede dry season rabi cropping. Late vacating floodwater can also push sowing into December, forcing the crop into maturity in late March or early April when minimum temperatures are high (Poulton and Rawson, 2011). The consequent heat stress can accelerate leaf senescence, lower spikelet density, and cause spikelet abortion, resulting in lower yields (Al-Khatib and Paulsen, 1984, Pfeiffer et al., 2005), though little is known regarding the severity of yield loss resulting from late planting in the unique environment of the southern Bangladesh delta. Adapted and stress-tolerant genotypes and management strategies are required to optimize the productivity of late-planted wheat while reducing the economic risks posed by sub-optimal management – a crucial consideration in the relatively impoverished coastal zone of southern Bangladesh. Knowledge of how crop and nutrient management are affected by sowing time and genotype, and their interactions with environmental gradients – especially where salinity is concerned – would assist the development of appropriate management practices for South Asia's coastal areas.
To assess and refine such practices for wheat in the southern delta, we examined the yield responses of two elite wheat genotypes with salinity and heat-tolerant traits compared to an established cultivar as a local check. All three cultivars were grown in 64 farmers’ fields across a range of sowing dates (grouped as ‘early’ and ‘late’), under differing N rates. Trials spanned eight different production environments chosen with varying levels of soil salinity in coastal southwest Bangladesh. Our objectives were to confirm earlier on-farm study results (e.g., Carberry et al., 2011, Rawson et al., 2007), indicating that wheat can be used to achieve system intensification objectives in this stress-prone region, and to help establish efficient recommendations for N management as a function of sowing date and genotype in saline-affected deltaic environments that are common in Bangladesh, but which may also have applicability in similar environments such as in West Bengal, in India.
Section snippets
Experimental sites and field characteristics
Farmer-managed experiments were conducted during the 2011–2012 and 2012–2013 winter rabi season in four upazillas (sub-district administrative units) spanning two districts (Satkhira and Khulna) in south-western Bangladesh. This region composes what has been referred to as a ‘stress-prone’ production environment for rabi crops, with relatively limited quality groundwater availability, high minimum temperatures, and soil salinity (MOA and FAO, 2012). Farmers were selected based on their
Combined analysis for yield and yield attributes under early and late sowing
Highly significant (P ≤ 0.001) main effects of environment, sowing group, and N rate for grain and straw yield, and yield components were observed when all 0 and 100 kg N ha−1 data were combined across early- and late-sowing groups, although genotypic effects were observed for yield components only (each P ≤ 0.001). Significant Environment (E) × Sowing (S), E × Nitrogen (N), S × N and E × S × N interaction effects (each P ≤ 0.001) were found for grain and straw yields and yield components, excluding sterility (
Discussion and conclusions
Most research findings and management recommendations for wheat in South Asia come from favorable environments. Research in less-favorable environments – for example those located in coastal and deltaic areas with environmental stresses – is less frequent, as is work conducted in farmers’ fields, where bio-physical and socio-economic diversity typically necessitates site-specific management strategies (e.g., Erenstein et al., 2008, Rawson et al., 2007). In southern Bangladesh, we assessed
Acknowledgements
This research was conducted under the Cereal Systems Initiative for South Asia Expansion in Bangladesh (CSISA-BD) project funded by the USAID Mission in Bangladesh. The contents and opinions expressed herein are those of the author(s) and do not necessarily reflect the views of USAID or the United States Government and shall not be used for advertising or product endorsement purposes. We appreciate partnership with the Bangladesh Meteorological Department that provided access to climactic data.
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