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To determine the most important production constraints and associated yield losses for six major food crops in 13 farming systems with high poverty in SubSaharan Africa, South Asia and East Asia, surveys were conducted with 672 experts representing a diversity of backgrounds and experience. Respondents reported large gaps between highest achieved crop yield on smallholder farms and average yield on farm. Yield gaps were smallest for rice (about 60% of current average smallholder farm grain yields), mid size for wheat and cassava, and larger (sometimes double current farm yields) for sorghum, cowpea and chickpea. Gaps were also smaller in the high input and yield farming systems of East Asia and largest in the marginal, drier systems, particularly in Sub-Saharan Africa. Four categories of production constraint (abiotic, biotic, management and socio-economic) were considered important contributors to yield gaps. A diversity of specific constraints was reported for the crops in the different systems. The most severe and widespread specific constraints for wheat involved the deficiency, high cost and poor management of N fertilizer, and problems associated with drought stress at grain filling, mid season drought and irrigation management. Those for rice included N fertilizer problems, soil fertility depletion, various leaf, stem and head pests and diseases, weed competition and inadequate water management. Striga and weed competition, soil resource degradation, poor soil fertility management, and drought were the most severe specific constraints for sorghum. Insect pests of pod, leaf, stem and flower and the high cost of their control dominated the constraint set for cowpea. Helicoverpa pod borer, Botrytis grey mould and control costs were the most severe for chickpea. Unsuitable varieties/poor seed, soil infertility and fertilizer constraints were also widespread with the legumes. Marketing problems and lack of finance were concerns for cassava along with weed competition, African cassava mosaic virus and poor varieties/planting materials. The findings can help to inform priority setting for international agricultural research and development activities on important food crops in major farming systems occupying areas of high poverty.

Two trials were conducted at the Mexican National Institute of Agricultural Research (INIA) experimental station, Yaqui Valley, Sonora, Mexico, to determine the genetic yield potential of bread wheat (Triticum aestivum L.) cultivars released in northwest Mexico over the period from 1950 to 1982, with emphasis on progress since 1970. Nonlimiting levels of fertility and moisture, a preventative pest and disease program, and netting to prevent lodging were used. Grain yield, yield components, and rates of phytomass production and grain filling were determined. Yield data on selected genotypes grown in 4 yrs of the International Spring Wheat Yield Nursery (ISWYN) at the same station were also examined. The grain yield potential of cultivars, successively released since 1950, has risen at an estimated (from a regression slope) average of 59 kg/ha/yr of release, or about 1.1%/yr. Although yield potential may have plateaued in the early 1970s, cultivars released since 1979, i.e., modern genotypes (e.g., ‘Ciano 79’, ‘Genaro 81’, ‘Glennson 81’, and ‘Seri 82’) have improved yield potential at an estimated rate similar to that prior to 1970. Improvements in grain yield were associated with increases in grain number per unit area (r = 0.74, P < 0.01), which has risen by about 34% in modern genotypes compared to pre‐1970 cultivars, and grain number per spike (r = 0.51, P<0.05). The 1000‐grain weight, was reduced slightly in the modern high grain number cultivars (r = −0.76, P<0.01). Harvest indices for modern genotypes were lower than those of the landmark cultivar ‘Yecora 70’, but the modern genotypes had, on average, 16% greater phytomass than pre‐1970 cultivars. However, only 43% of the variation in grain yield was attributable to phytomass. The improvements in yield potential are mainly the result of empirical selection for grain yield. Indications were that rates of grain filling and phytomass production in modern genotypes were similar to those in Yecora 70. The higher grain yield and phytomass in modern genotypes compared to Yecora 70 was probably due to the formation and survival of a larger grain sink (more grains/m2) and greater C assimilation during a longer preanthesis phase.

Many environmental factors constrain the production of major food crops in Sub-Saharan Africa and South Asia. At the same time, these food production systems themselves have a range of negative impacts on the environment. In this paper we review the published literature and assess the depth of recent research (since 2000) on crop x environment interactions for rice, maize, sorghum/millets, sweetpotato/yam and cassava in these two regions. We summarize current

One of the biggest challenges in the tropics is to develop organic matter technologies which are adopted by the farmers. Technologies must be effective within farmer resource constraints, increase food production, reduce risk and enhance the soil fertility. Results from on-farm participatory research were used to quantify the effects of agronomic practices on soil resources. Agricultural productivity is primarily nitrogen (N) limited throughout Malawi, and sub-humid Zimbabwe. Tightening economic constraints faced by farmers in the region have reduced inorganic fertilizer inputs and necessitate increased reliance on biologically-®xed N and N cycling. Three components of organic matter technology were evaluated: (1) the effects of residue quality; (2) the role of deep rooting systems; and (3) tradeoffs between legumes grown for grain versus soil regeneration. Perennial systems investigated include improved fallows, intercropping, and biomass transfer. Annual systems include intercrops and rotations of cereals with legumes. The most promising non-food legumes were Tephrosia and Sesbania. Interestingly, high quality residues of perennial legumes were most effective at supplying N in the short to medium term, whereas low quality residues immobilised N. Low quality residues were problematic for smallholder farmers who need immediately available N. Challenges to adoption of perennial system technologies include establishment costs, resource competition and delayed bene®ts. Farmer adoption of annual grain legumes is promoted by the simultaneous production of food; however, those species which have a high N harvest index add little to no net N to the soil. Species that combine some grain yield with high root and leaf biomass, thus a low N harvest offer a useful compromise of meeting farmer food security concerns and improving soil fertility. Promising genotypes include Arachis, Cajanus, Dolichos and Mucuna spp. On-farm N budgets indicate that legumes with high quality residues and deep root systems are effective ways at improving nutrient cycling. Areas of future research priority for smallholder farms in southern Africa were identi®ed, including technologies which combine inorganic and organic fertilizer and improve legume growth and establishment on degraded soils.