Photosynthetic photon flux density (PPFD) has variable effects on cotton (Gossypium hirsutum) plant growth, development and lint yield, but little is known about the responses of yield, yield components and fibre quality to low PPFD at different developmental stages. Field studies were conducted in 1993–95 to determine the effects of an 8-d period of shade (63% reduction in PPFD) imposed at different developmental stages (pinhead square (PHS), first flower (FF), peak flower (PF) and boll development (BD)) on cotton growth, lint yield, yield components and fibre quality. Shade for 8 d at the early square stage did not affect cotton growth and yield. Shade during FF, PF and BD increased fruit abscission, and decreased specific leaf weight, dry matter accumulation, lint yield, fibre micronaire, and fibre strength compared with the unshaded control. The detrimental effect of shade on yield increased with later growth stages. Shade at the FF, PF and BD stages decreased lint yield by 18, 34 and 52% in 1993, and by 18, 21 and 29% in 1994 respectively compared with the unshaded control. Shade caused the greatest decrease in the dry weight (DW) of fruits (squares + flowers + bolls) and the smallest decrease in leaf DW among leaves, stems and fruits. Of the three yield components (boll number, boll weight and lint percentage), boll number was the most sensitive to shade. Shade during squaring and early flowering had little effect on average boll weight, and did not affect lint percentage. Decreased yield of cotton shaded during FF and PF appeared to be related to decreased boll retention. Shade during BD decreased both boll retention and average boll weight. Under shade conditions, fibre micronaire and fibre strength were decreased, while other fibre properties were unaffected. These results help explain the yield response of field-grown cotton to low PPFD stress.

Wheat (Triticum aestivum) production using no-tillage is becoming an increasingly accepted management technology. Major obstacles to its adoption in Morocco, however, are exportation of wheat straw from the field and stubble grazing. Among pertinent solutions is the control of these practices. A four-year field study was conducted to determine the effect of residue level under no-tillage on wheat grain and total dry-matter yields, water use and water-use efficiency, and to compare this with conventional tillage systems. The aim was to evaluate whether all the straw produced is needed for no-till cropping or whether partial removal of straw from the field is possible without any adverse effect on production. No-tillage and deep tillage with disk plough performed equally well and subsurface tillage with an off-set disk produced the lowest yields. Both bare and full no-tillage covers depressed wheat production. Uo to 30% of straw produced under no-tillage can be removed without jeopardizing wheat crop performance.

The effect of tillage and crop rotations can only be seen over many crop years. Crop yield and soil results are evaluated from two long-term trials, established in 1978–79 and 1985–86 to investigate various forms of tillage and the timing of such operations in various wheat (Triticum aestivum)-based rotations on a Calcixerollic Xerochrept in northern Syria. In a tillage systems trial involving two wheat–legume–watermelon (Citrullus lanatus) rotations, deep tillage showed no advantage over a shallow sweep-tillage system, either for soil moisture storage or yield increase of any crop. The zero-till system suited legume crops but gave lower productivity in wheat due to a build up of grassy weeds, and was not suitable for watermelon. Minimum tillage, with its higher energy-use efficiency and yield levels equal to or even slightly above those of deep-tillage systems, appears promising for the lowland areas of West Asia and North Africa. In a tillage timing trial, wheat in a wheat–lentil (Lens culinaris) rotation yielded best after conventional deep disc-plough tillage, but lentil yields were higher in a zero-till system.

There is a lot of interest in the contribution that agroforestry can make to reverse land degradation and create resilient multifunctional landscapes that provide a range of socio-economic benefits. The agroforestry research agenda has been characterized by approaches that promote a few priority tree species, within a restricted set of technological packages. These have often not spread widely beyond project sites, because they fail to take account of fine scale variation in farmer circumstances. New methods are needed to generate diverse sets of agroforestry options that can reconcile production and conservation objectives and embrace varying local conditions across large scaling domains. Here, we document a novel approach that couples local knowledge acquisition with structured stakeholder engagement to build an inclusive way of designing agroforestry options. We applied this approach in the eastern part of the Democratic Republic of Congo (DRC) where armed conflict, erratic governance and poverty have resulted in severe pressure on forests in the Virunga National Park, a global biodiversity hotspot. Around the park, natural resources and land are severely degraded, whereas most reforestation interventions have consisted of exotic monocultures dominated by Eucalyptus species grown as energy or timber woodlots mainly by male farmers with sufficient land to allocate some exclusively to trees. We found that structured stakeholder engagement led to a quick identification of a much greater diversity of trees (more than 70 species) to be recommended for use within varied field, farm and landscape niches, serving the interests of a much greater diversity of people, including women and marginalized groups. The process also identified key interventions to improve the enabling environment required to scale up the adoption of agroforestry. These included improving access to quality tree planting material, capacity strengthening within the largely non-governmental extension system, and collective action to support value capture from agroforestry products, through processing and market interventions. Integrating local and global scientific knowledge, coupled with facilitating broad-based stakeholder participation, resulted in shifting from reliance on a few priority tree species to promoting tree diversity across the Virunga landscape that could underpin more productive and resilient livelihoods. The approach is relevant for scaling up agroforestry more generally.

Mixtures of legumes and cereals in various proportions (seed ratios) were evaluated in Northern Syria for forage production and quality and for yield of barley grain in the following year. Both a vetch-cereal and pea-cereal mix were tested in proportions of 0:100, 33:66, 50:50, 66:33 and 100:0. The study was carried out between 1982 and 1985, covering two seasons for forage production and two for grain production. Dry matter yields of the mixtures exceeded the yield of either one or both components of the mixture grown as pure stands, depending on the rainfall. The maximum overall yield and quality of forage was recorded for the 66:33 legume-cereal combination. The greatest benefit to subsequent barley grain yield was realized when the legume was grown in a pure stand or formed a high proportion (66:33) of the mixture. Herbage quality was better in forages containing vetch than in those containing pea.

When a leaf absorbs radiant energy, only a small fraction is stored chemically in photosynthesis. In sunlight, this fraction is at most one-fifth of the energy in the visible spectrum, decreasing with increasing light intensity because of the finite resistance to the diffusion of carbon dioxide through the leaf to the chloroplasts. Energy absorbed but not stored in photosynthesis is dissipated by transpiration and convection.

The potential or maximum photosynthesis of a crop canopy can be estimated from a set of six parameters describing the photosynthesis-light curve of single leaves, the arrangement of leaves in the canopy, and radiation climate. Comparing estimates of potential photosynthesis with measurements of carbon dioxide exchange over a field of sugar beet, the estimated rate of respiration was 2 gm carbohydrate per m2 leaf area per day, equivalent to 44 per cent of gross photosynthesis over the whole growing season. Over the season, the foliage lost 34 per cent of incident radiation by transmission to the soil.

The potential rate of transpiration can be found from Penman's formula assuming values of external (aerodynamic) and internal (mainly stomatal) resistance for the canopy as a whole. In south-east England, the energy for potential transpiration is almost equal to net heat H in summer and is therefore about half the energy of incoming solar radiation. For a real crop of grass subject to moisture stress, transpiration was less than the potential rate at about 0·8 H on average and 0·3 H in very dry weather.

During the summer, cumulative photosynthesis increases linearly with cumulative transpiration to give a production ratio (gross photosynthesis/transpiration) of 1/100 in the Thames Valley and 1/200 in the Sacramento Valley. The production ratio is expected to change with crop type as well as with climate.

Integrated pest management (IPM) in rice has been mainly applied to irrigated, lowland paddy rice ecosystems which generally use more inputs and have higher yields ha−1 than rainfed ecosystems. Large scale implementation of IPM was stimulated in the 1970s in several Asian countries by pest resurgences resulting from indiscriminate insecticide use. Legislation has been enacted in at least five countries to support IPM and by 1992, approximately 0.05% of Asian farmers had received training in IPM. In at least two countries, adoption of IPM as national policy has resulted in large savings from reduced importation of pesticides with no reduction in total rice output or productivity. The inter-country programme on rice IPM of the Food and Agriculture Organization (FAO), the United Nations, the International Rice Research Institute and national programmes have all contributed to promoting rice IPM. Host plant resistance, deployed with location-specific cultural practices that conserve natural enemies, remains the basis for sustainable, low-cost rice IPM. Thresholds, monitoring and forecasting systems have little or no impact on farmer practice in the tropics; farmer group learning appears to be the most effective way towards IPM adoption.

Manejo integrado de plagas en el arroz

A framework is proposed for the classification of farm systems, which are defined as decisionmaking units comprising farm household, cropping and livestock systems that transform land, capital and labour into products for consumption and sale. Two general principles underlying the classification are outlined. First, since farm systems are embedded in a hierarchical structure, the classification is based on the characteristics of the underlying systems and their interactions. Secondly, ecological factors, i.e. physical and biological parameters, are the primary determinants of farm systems. Changes in farm systems, at least in the foreseeable future, depend on the development of socio-economic variables. The classification is summarized in a set of comprehensive tables.

L. O. Fresco y E. Westphal: Una clasificación jerárquica de sistemas agrícolas

The response of tea shoots to temperature was studied in the field and in controlled environments. Shoot extension stopped below about 12.5°C. Shoots of mature tea took 491 day degrees above the base temperature of 12.8°C to grow to a harvestable size of 15 cm.

Initial results of long-term trials in northern Nigeria indicate that deficiencies of potassium and trace elements, and soil acidity, may become important under continuous cultivation on some soils, in addition to the more common deficiencies of nitrogen and phosphorus.

There are many prerequisites for potato production to meet its full potential as a food security crop for subsistence farmers in Sub-Saharan Africa. One of these is the introduction of improved varieties. Traditionally, the introduction of new varieties is by government agencies or Non-Governmental Organizations (NGOs). To understand the diffusion of seed tubers (seeds) of new potato varieties in farmer networks, we analysed social factors influencing the sharing of seed of improved potato varieties amongst farmers in Chencha, Ethiopia. We interviewed 166 farmers and analysed 146 seed transactions. We used seed flow mapping to visualize seed sharing amongst farmers. We found that the social networks of farmers are differentiated by wealth, gender and religion, and that this differentiation affects the dispersion of new varieties through the farming communities: Wealthier farmers shared seed tubers most frequently and poor farmers did not share seed at all. Seed sharing was influenced by, but not restricted to, gender and religion categories. Most sharing was with relatives (as gifts) and neighbours (in exchange for labour). There weas no equal access to seed for all households because of (i) the targeting of the better off farmers by the NGO (ii) differences in frequency of sharing and (iii) terms and motivation of the transactions. Our results show that wealthy farmers most effectively multiply and share the seed of new varieties with medium wealthy and poor farmers. This study shows that for the introduction of new technologies into a community, its dynamics of social differentiation need to be understood.

Selenium (Se) is an essential micronutrient for humans, yet its dietary intake is low, mostly due to the low bioavailability in soils and therefore in edible plant tissues. To overcome Se deficiency, the breeding approach (i.e., genetic biofortification), namely in rice, is largely dependent on available Se pools. To ensure the success of genetic biofortification with Se, agronomic biofortification can be accomplished through foliar Se application. Considering this background, the main hypothesis of this work was centered in the foliar application of Se to attain agronomic biofortification of rice crops. This study also aimed to assess the full potential for increasing grain Se concentrations during rice filling, as well as the types of nutrients deposition. An experimental design applying two foliar fertilizers (sodium selenite and sodium selenate) was developed. As test systems, four rice genotypes (Ariete, Albatros, OP1105 and OP1109) were used and the kinetics of micro- and macro-nutrients accumulation and deposition were assessed. Biofortification was performed in field trials for two years with foliar fertilization ranging between 0 and 300 g Se ha−1. At the end of the plant cycle, selenite applications triggered 427- to 884-fold increases in grain Se concentrations among rice genotypes (Albatros > OP1105 > OP1109 > Ariete). The application of selenate also prompted 128- to 347-fold increases in grain Se concentrations in rice crops (Albatros > OP1105 > Ariete > OP1109). Regardless of the foliar fertilizer applied, Se deposition among genotypes occurred throughout the grain without relevant inhibitory effects on yields. In each genotype, micro and macronutrients varied among crop tissues.

Nodulation and N2-fixation of groundnut were investigated in sole and intercropping systems. Intercropping with pearl millet, maize, or sorghum reduced nodulation and N2-fixation. This effect is ascribed to the shading of groundnut by the cereal component and the consequent decrease in photosynthesis of the legume canopy.

Sweet potato (Ipomoea batatas L.) virus disease (SPVD) causes severe disease symptoms of various combinations of leaf strapping, vein-clearing, puckering and stunting. The disease is widespread, especially in Africa, and yield is adversely affected, though there are no figures to quantify this effect nor is it known how yield is affected. Loss of tuberous root yield due to SPVD was studied in sweet potato showing symptoms of the disease. Slips were obtained from plants with and without symptoms and two yield trials in 2 years were conducted by planting them alternately in rows. Yield of fresh tubers was reduced by 78% in plants showing SPVD symptoms but the reduced chlorophyll level of diseased leaves did not appear to affect yield.

On-farm trials were conducted in the northern Guinea savanna of Nigeria. The objective was to compare integrated Striga hermonthica control measures (soyabean or cowpea trap crop in the first year followed by maize resistant to Striga in the second year) with farmers' traditional practices (cereal-based cropping systems) under farmer-managed conditions.

Integrated control proved to be highly effective in terms of reducing Striga incidence both in terms of reduced seed density in the soil and decreased infection in maize. Resistant maize following the soyabean trap crop yielded 1.58 t ha−1 of grain and out-yielded local maize following traditional practices by more than 80%. Similarly, the overall productivity over the period of the experiment was highest with the integrated control treatment using soyabean. Conversely, resistant maize after the cowpea trap crop yielded only 0.92 t ha−1 (possibly due to the poor performance of the cowpea crop in the first year), and maize yields were similar to those obtained with farmer practices. Initial Striga seed density in the soil was negatively correlated (r = −0.33) with soil nitrogen, but nitrogen-fertilizer application rates did not seem to affect the level of Striga infection in maize.

Many public-sector breeding programmes do not use explicit techniques to orient their programmes close to their clients' needs. Participatory techniques can be used to achieve high client orientation but these techniques do not have to involve farmers making selections during the segregating generations. This particularly applies when a sound initial market survey has been made or the learning from a participatory varietal selection (PVS) programme provides feedback to scientists. However, some published results on selection by farmers in the segregating generations (collaborative selection) indicate that it can produce appropriate varieties more effectively than less collaborative research. There is also evidence, from the few cases reported in the literature, that it is cost-effective. Alternative, less collaborative, approaches are also effective. Consultative forms of farmer participation, i.e. where farmers evaluate material grown by scientists, to aid selection in the segregating generations are more widely applicable because they demand fewer resources than collaborative methods. For more time-consuming tasks, such as in the selection of aromatic rice in segregating material, the most appropriate from of participation is contractual i.e., farmers are paid for their work. Mainly using examples from our research in Nepal, we present the particular circumstances in which the involvement of farmers in selection in the segregating generations is desirable or essential. These include: the occurrence of market failure (where the usual mechanisms of supply and demand have failed so there is no incentive to breed new varieties) and supply can only be met by actively involving farmers in the breeding process; when there are cost advantages to involving farmers – this is determined by the particular resources available to the institute undertaking the plant breeding research; when grain quality is both important and determined by a complex set of factors that are difficult to measure in the laboratory; when the objective is to learn in more detail about farmers' selection criteria to better orient the breeding programme to client needs; and when the goal is to empower farmers.

Four genotypes of groundnut grown with limited irrigation during the post-rainy season in Central India produced similar amounts of dry matter per unit of intercepted solar radiation (e) before pod-filling, although different e values were observed during pod-filling. The relation between cumulative transpiration and intercepted radiation was similar for all genotypes. When drought became severe, fractional radiation interception (f) was reduced by folding of leaves, with little decrease in leaf area (L). The ratio f/√L was used as an index of the degree of leaf folding and was correlated with leaf water potential. The degree of folding varied with genotype and may have contributed to the observed differences in e and the dry matter:water ratio (q). The genotype EC76446(292) had the smallest q and largest f/√L ratio (the poorest radiation avoidance), while Kadiri 3 had the largest q and smallest value of f/√L.

Four genotypes of groundnut grown with limited irrigation in a medium depth Alfisol in Central India transpired similar total amounts of water (220–226 mm) over the season, but produced different amounts of shoot dry matter (390–490 g m−2). The extraction front of Kadiri 3 moved most rapidly down the soil profile which may have enabled it to maintain the fastest rates of transpiration when soil water depletion was greatest. Tap root extension rates of Kadiri 3 in the first 32 days after sowing were also the fastest. NC Ac 17090 was more efficient than the other genotypes in extracting water immediately after irrigation from the upper 40 cm of the soil, but this had little value in determining the pattern of water availability in this experiment. Differences in the water extraction characteristics of these genotypes explain little of the variation in dry matter:water ratio, and do not account for the major variation in harvest index associated with drought.

Green revolution technologies of the 1960s and 1970s have led to the emergence of rice (Oryza sativa)–wheat (Triticum aestivum) (R–W) as a major cropping system in the Indo-Gangetic Plains (IGP). Expansion of irrigation through a network of canals and privately owned tube wells was one of the key factors in the success of the new technologies. Since canal water supplies are not demand-driven, farmers increasingly relied on the use of ground water by installing shallow tube wells. The strategy of ground water development and its use in conjunction with canal supplies paid rich dividends in control of waterlogging and secondary alkalization and in increasing the pace of alkali soils reclamation programmes in areas underlaid with fresh quality aquifers. Rice–wheat systems were the preferred choice of farmers. Adoption of the R–W system, however, changed the salt and water balance in north-western parts of the IGP. In the north-east of Punjab and Haryana states it led to receding water tables and waterlogging, and in south-west parts to associated problems of secondary salination. In eastern parts of the IGP in West Bengal, development of ground water aquifers to meet irrigation water requirements for the intensification and diversification of agriculture has led to serious problems of ground water contamination due to dissolution of arsenic-bearing minerals under altered soil moisture regimes, conducive to oxidation of pyritic sediments. These experiences suggest that there is an urgent need for a change in the way food is produced in the IGP for sustainability of the natural resource base. This will depend on the ability to predict long-term consequences of intensification and diversification of agriculture on the farm and regional scale. Whereas issues of favourable regional salt and water balances need attention in the north-west of the IGP, rain-water management and alleviating drainage congestion can facilitate the adoption of agricultural technologies and improve productivity of the R–W systems.

A consideration of the apparent lack of general success of minor pulse crops can lead to a better appreciation of the factors which have made the major pulses successful. The relevant factors are both biological and economic. High productivity based on an efficient physiology, amenability to local agronomic practices and a satisfactory range of ecological tolerance are prime prerequisites for success. Aspects of quality, palatability, lack of toxic factors (or ease of detoxification), ease of preparation and cooking and the more imponderable questions of consumer preference are equally important. If any of these prerequisites are not adequately met success in cultivation is improbable. At the present time the lack of more extensive success of the winged bean appears to be due to some deficiency in ecological tolerance, and of the horse-gram and jack and sword beans to a lack of palatability; the Hausa groundnut (Kersting's groundnut) suffers from a probably inefficient physiology while limited consumer acceptance has possibly restricted production of pigeonpea and hyacinth bean. The rather limited evolutionary advance achieved by this group of pulses is probably directly related to their somewhat restricted production and the relatively low level of selection pressures to which they have been subjected.