Abstract
To meet the growing rice demand in Africa, gaps between actual and attainable yields have to be reduced. In Tanzania, this particularly concerns smallholder rain-fed production systems in the floodplains. After quantifying the existing yield gaps, key contributing factors need to be analyzed to improve site-specific management. Field experiments were conducted for three years and in three pedo-hydrological environments (fringe, middle, and center positions) of the Kilombero floodplain to evaluate: (1) The grain yield under farmers’ management (actual yield), (2) yield with the best-recommended management (attainable yield), and (3) the non-limited yield simulated by the APSIM model (potential yield). In the field, we additionally assessed incremental effects of (1) field bunding and soil levelling, (2 and 3) additionally applying of 60 kg N ha−1, as urea or as farmyard manure (FYM), and (4 and 5) incorporating in-situ-grown leguminous green manures. Attainable yields were determined with mineral N application at 120 kg ha−1, additional PK fertilizer and supplemental irrigation. On average across years and positions, the potential, the attainable, and farmers’ actual yields were 11.5, 8.5, and 2.8 t ha−1 indicating a high total yield gap. About 16–38%, 11–20%, and 28–42% of this gap could be attributed to non-controllable yield-reducing (i.e., pest and diseases), yield-limiting (i.e., water and nutrient deficiencies), and yield-defining factors (i.e., poor soil and crop management), respectively. Results indicate a closure of the exploitable yield gap (differences between attainable and farmers’ actual yields) by up to 6.5 t ha−1 (nearly 60% of the potential yield). This exploitable yield gap was larger in 2016 than in 2017. Also, the gap was larger in the water-limited fringe and middle than in the frequently submerged center positions. Simple field bunds combined with land levelling could close 15–35% of the exploitable yield gap, depending on field positions and year. FYM or green manures were less effective than mineral N; however, in 2017 and in the wetter middle and center positions, they reduced the yield gap by >50%. We conclude that yield gaps in rainfed rice in Kilombero floodplain are large, but that a site- and system-specific adaptation of crop management can close much of the exploitable yield gap and increase grain yields by 0.7–4.8 t ha−1. Similar benefits may be obtained in other hydrologically variable floodplain environments of the region and beyond.
Highlights
In many countries of sub-Saharan Africa, rice increasingly replaces traditional staple food crops such as maize and cassava in both daily diets and in dominant agro-production systems
We considered a new indicator focusing on the percentage share of individual sequentially applied management practices on the exploitable yield gap
The yield variability within hydrological positions increased from the fringe with 3.3 ± 0.8 t ha−1 to the center position with 2.6 ± 1.3 t ha−1 (Figure 2)
Summary
In many countries of sub-Saharan Africa, rice increasingly replaces traditional staple food crops such as maize and cassava in both daily diets and in dominant agro-production systems. With about 2 t ha−1 , grain yields of rain-fed lowland rice in Africa are far below the global average of Agronomy 2020, 10, 1135; doi:10.3390/agronomy10081135 www.mdpi.com/journal/agronomy. Rice supply gaps in most countries are caused by low yields in combination with high demographic growth and are, in many instances, further exacerbated by land scarcity [2]. The extent of the yield gaps and the effectiveness of technology options to close them largely differ by crop species, production environments and farmers’ ability to adopt technologies [6]
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