The effects of temperature and photoperiod on times from sowing to flowering (f) were investigated in medium-and late-maturing pigeonpea (Cajanus cajan). Twelve genotypes were sown in two seasons at seven sites in Kenya, covering latitudes 0–4°S and a wide range of altitudes (50–2000 m), as well as under polythene enclosures constructed at six sites to create warmer temperature regimes (a total of 26 environments). The same genotypes were also sown at monthly intervals and in an artificially extended photoperiod (in the open as well as under polythene) created by incandescent lamps suspended above the plots at Katumani (1°30′S).

Times from sowing to flowering varied from 70 to more than 300 days and were associated with variations in mean pre-flowering values of temperature and photoperiod which ranged from 15.2° to 32.7°C and from 12.6 to 15.0 h d−1. Genotypic variation in f in the most inductive regimes (a mean pre-flowering temperature of 24.3°C for the medium- and 20.8°C for the late-maturing genotypes, combined with a mean pre-flowering photoperiod of 12.6 and 12.8 h d−1) ranged from 70 and 76 days and from 85 to 112 days, respectively. There were no photoperiodic effects on f over the range from 12.6 to 13.1 h d−1, but the artificially extended day delayed flowering, especially in the late-maturing genotypes.

The relation between l/f and the mean pre-flowering temperature was linear below and above an optimum temperature, To. The genotype-specific parameters derived from these thermal linear rate models based on flowering responses in 26 environments closely predicted l/f and therefore f in an independent sequence of monthly sowings. It was thus responses to temperature below and above To and not responses to daylength which modulated flowering throughout the wide range of natural environments tested within this vast country, even in the late-maturing and most photoperiod-sensitive genotypes.

The physiological basis for differences in yields from well-watered and draughted plants of four contrasting clones of tea was studied in terms of light interception, dry matter production and partitioning at a high altitude site in Southern Tanzania where there are marked seasonal variations in rainfall and temperature. The plant dry weights, including roots, were measured eight months after field planting and subsequently at intervals of three to four months, corresponding to the different seasons, during the following two years. Fully irrigated plants of one clone (S15/10) were also harvested after four years in the field. Clones differed in the rates of canopy spread and hence in their capacity to intercept solar radiation. The ‘radiation use efficiency’ (the net total dry matter production per unit of intercepted short-wave radiation) was similar for the four well-watered clones and ranged from 0.40 to 0.66 g MJ−1, which corresponds closely to values reported for other woody tropical plants. A 16-week drought treatment imposed two years after planting reduced the mean light interception of the four clones by about 25% and the mean radiation use efficiency by 78% to 0.09 g MJ−1. Clone S15/10, a cultivar from Kenya which produces large yields, partitioned a greater proportion of dry matter to leaves and harvested shoots than the other clones, and correspondingly less to large structural roots. This resulted in a maximum harvest index of 24%, substantially greater than other values reported in the literature. There were seasonal differences in partitioning, with more dry matter being diverted to roots and less to shoots during the cool season. Although the drought treatments had no significant effect on root growth, the amount of dry matter partitioned to leaves, stems and harvested shoots declined by 80–95%. The roots of all four clones extended in depth at similar rates (about 2 mm d−1), those of Clone S15/10 reaching 2.8m after four years. The results are discussed in terms of appropriate field cultural practices and possible selection criteria for high yielding clones.

The base (Tb), optimum (To) and ceiling (Tce) temperature for 50% seed germination and seedling emergence, and the thermal time at sub-optimal temperatures (θ1), were examined in experiments with a range of cowpea genotypes from different habitats and latitudes. There was substantial genotypic variation in seed germination for Tb and To, but not for Tce. There was no evidence that genotypes from semi-arid habitats had higher values of To or Tce, but estimates of Tb for seed germination were related to latitude and were highest in genotypes originating from close to the equator. The significance of these responses as a component of developmental plasticity in relation to the adaptation of cowpeas is discussed.

In the last 35 years, the population of sub-Saharan Africa has increased nearly threefold and is expected to reach 681 million by the year 2000, with nearly 50% of the population living in urban centres. Such population pressures, exacerbated by a range of social and political factors, have already resulted in widespread land degradation in areas of high population densities and the expansion of agriculture on to marginal and sloping land. Declining soil fertility and soil erosion are increasingly threatening the sustainability of small scale farming systems throughout Africa, and affordable external nutrient inputs are seldom available to farmers. In addition, shortages of wood for construction and fuel and high-quality dry-season fodder for livestock are widespread and serious constraints to farm productivity.

Agroforestry, the deliberate integration of woody perennials into crop and livestock systems, has the potential to mitigate many of these constraints through both the service and production functions played by trees. In recent decades much agroforestry research has been undertaken in sub-Saharan Africa. In this review we focus specifically on research which addresses the potential of agroforestry systems to enhance soil fertility, prevent soil erosion, provide high-quality dry-season fodder or generate much needed income through the production of high-value goods.

Much emphasis has been placed on a wide range of agroforestry systems for the maintenance of soil fertility and the prevention of soil erosion losses, and encouraging results, both in technical performance and farmer enthusiasm, have occurred. However, it is clear that agroforestry solutions to land degradation are always likely to be location-specific in their relevance, performance and farmer acceptability. It is essential that farmers are included as research partners to determine what is appropriate for their conditions.

Good progress has also been made on identifying fast-growing leguminous trees and shrubs for high-quality livestock fodder supplements. Where livestock enterprises, such as peri-urban milk production, are market-oriented the adoption and impact of such systems have been high. Given population and urbanization projections, it is likely that fodder trees and shrubs will have a major role to play in meeting future feed demands for both milk and meat production. Research on the potential of high-value indigenous and exotic trees to generate income has been less extensive in Africa, although the huge potential of this approach has been clearly demonstrated by farmers in south-east Asia. We suggest that there is a need for increased research emphasis on the domestication of high value indigenous trees, and their integration into more sustainable, diverse and intensive land use systems.

We conclude that, although good progress has been made in agroforestry research in Africa and farmer adoption is occurring, future population projections pose a clear challenge. Agroforestry systems which provide solutions for today's land degradation problems will need to evolve in both diversity and intensity if they are to remain relevant and effective for tomorrow's Africa.

Biophysical and socio-economic variability have several impacts on farmers' cropping strategies in dryland areas of Africa. This study examines data from a communal area in southern Zimbabwe over a seven year period. Variation in the output of different crops, in the pattern of crop mixes, in the diversity of cropping patterns and the level of crop sales is explored. Through contrasts between ecological zones and household wealth status, the analysis helps to explain the range of farm-level risk management strategies employed. Finally, the importance for research and extension approaches of understanding variability, through a differentiated analysis of cropping strategies, is emphasized.

Sorghum landraces from contrasting districts of northern Nigeria were grown during the 1990 rainy season to describe their physiological and morphological features. Changes in their dry matter production and yield could be predicted from thermal time to flowering (based on response to a fixed daylength) and partitioning indices. Many other morphological and physiological features, such as light-extinction coefficient, light-use efficiency, plant height, leaf area and leaf number, were either stable or varied systematically with time to flowering. Grain yields of early maturing lines were limited by low light interception from flowering to physiological maturity and those of later maturing lines by highly site-specific drought stress.

Simulations of the dynamics of the seed bank of Orobanche crenata Forsk. under different crop rotations are presented. Rotations studied involved four host species, lentil (Lens culinaris Medik.), chickpea (Cicer arietinum L.), faba bean (Vicia faba L.) and woolly-pod vetch (Vicia villosa subsp. dasycarpa (Ten.) Cavill.), and non-host species. Simulation showed that the Orobanche seed bank dynamics in three-course crop rotations would result in a high Orobanche seed population and hence in low yields of the respective crops. Replacing the susceptible by resistant legumes such as woolly-pod vetch in some of the cycles of the rotations would keep the Orobanche infestation at a low level without reducing the proportion of legumes. A three-course crop rotation with faba bean would have to be changed to a 12-coursc rotation in which faba bean was grown every twelfth year but was replaced by woolly-pod vetch or other resistant legumes in seasons 3, 6, 9; 15, 18, 21; and so on. In the three-course rotations with chickpea or lentil, these susceptible legumes would be grown every ninth year but would have to be replaced in seasons 3 and 6; 12 and 15; 21 and 24; and so on, thus changing these three-course rotations into nine-course rotations.

Twenty-nine diverse genotypes of cowpea (Vigna unguiculata) were grown in approximately 30 photothermal environments in Nigeria and Niger, with mean temperatures ranging from 19° to 30°C, photoperiods from 10 to 16 h d−1 and saturation deficits from 0.5 to 3.1 kPa. Twelve of these genotypes were insensitive to photoperiod. The time of flowering of these photoperiodinsensitive genotypes showed a similar response to temperature. Time to flowering was also delayed by mean pre-flowering saturation deficits greater than 1.5 kPa. The implications of these responses to temperature and saturation deficit for phenological adaptation in West Africa are discussed.

Over several decades agronomists have developed a mineral nutrition management method for perennial oil crops based on leaf analysis combined with reference field trials. This method has been used in a coconut plantation on the peats of the east coast of Sumatra to optimize mineral nutrition in a very short time, at minimum cost and with maximum reliability, through successive adjustments of the fertilizer schedules. Two reference trials were set up to study a wide range of mineral nutrients assumed to be deficient on this area. Further experiments were set up as new nutritional problems occurred at the plantation, and as soon as the first results were obtained from these two trials. In this way, the iron and copper deficiency problems at the plantation were dealt with satisfactorily taking an experimental approach. New experiments were set up recently to test the hypothesis of a silicon deficiency. The fertilization schedules for the commercial plantation were developed by successive adjustments based on the trial results. Leaf analysis data showed that the coconut palms in the commercial plots always complied with the experimental models. In addition to being rapid and reliable, this method was also cost-effective, especially when larger areas were involved.

Preparation of small catchment areas on rainfed valley floor soils in highland Balochistan, Pakistan, is a low-cost method of generating run-off and increasing crop yields within the cropped areas. The effect of different proportions of water catchment area to cropped area were investigated by comparing a control treatment with the entire area planted to the crop (traditional rainfed agriculture); a 1:1 treatment, with one half of the area used for water catchment and one half for planting; and a 2:1 treatment, with two thirds of the area used for water catchment and one third for planting. Results from six seasons of trials using wheat (Triticum aestivum) showed that the 1:1 treatment had 23% higher net benefits than the control, with a 19% reduction in the coefficient of variation. The 2:1 treatment had 29% lower net benefits than the control and reduced the variation in net benefits by 8%. By contrast, four seasons of trials using barley (Hordeum vulgare) showed that the l: l treatment yielded 25% lower net benefits than the control but increased by 4% the variation in net benefits. Treatment 2:1 had 36% lower net benefits than the control and 18% more variation. Even though the gross revenue from wheat under the 1:1 treatment was less than that from the control, the reduction in total costs in the 1:1 treatment resulted in larger net benefits than the control. Water-harvesting in the valley floors is not a net yield-increasing technology. Land suitable for cultivation is limited and the increases in yields in the cropped area resulting from water-harvesting are offset by the opportunity costs of the catchment area. However, wheat grown in a 1:1 ratio of cropped to catchment area can increase farmers' income and decrease its variation. For barley, farmers are better off using their traditional management practice than giving up part of their cropped area to create water catchments.

Seventeen photoperiod-sensitive genotypes of cowpea (Vigna unguiculata) were grown in approximately 30 photothermal environments in Nigeria. Photoperiods ranged from 10 to 16 h d−1, mean temperatures from 19° to 30°C and times from sowing to flowering (f) from 32 to 140 d. Rate of progress towards flowering (1/f) was related to mean pre-flowering values of temperature and photoperiod using simple linear rate models comprising one, two or three planes (thermal, photothermal and insensitive). There were no significant differences (p > 0.25) among genotypes in response to temperature within the thermal plane and the common base temperature was estimated to be 7.6°C. Photoperiod-sensitivity varied by a factor of 15 among genotypes, and the critical and ceiling photoperiods varied from 12.2 to 13.4 and from 13.8 to more than 16 h d−1 at a mean temperature of 27°C, respectively. These simple models satisfactorily predicted f in an independent data set (R2 = 0.62) for plants grown in the main cowpea growing seasons at latitudes between 7° and 13°N. The utility of photothermal models and methods to screen for photothermal responses are discussed.

The grass Leptocloa fusca is very useful on salt-affected soils as it can tolerate extremely saline and alkaline conditions, Since its growth is not affected by gypsum application, planting with Leptochloa is a good biological method for the reclamation of alkaline soils. It is also well adapted to the waterlogging encountered on saline and alkaline soils, and improves the soil's physical, chemical and biological properties so that within two or three years many commercial and forage crops can be grown. Leptochloa excretes salts through specialized glands and is therefore reasonably palatable to farm animals. Because of its vigorous growth on alkaline soils it does not allow satisfactory growth of companion trees, especially in the initial years of soil reclamation.

Weeds are generally considered to be undesirable plants as they can cause yield losses but fodder weeds are traded as a valuable feed in Niger, Vest Africa. In vitro analyses of fodder weeds indicated that they are a suitable crude protein supplement to low quality millet stover, a major feed during the dry season. Dry matter production of fodder weeds increased with a crop residue mulch. When weeds are used as feed, farmers may generate additional income through an increased availability of energy for animal traction or for dairy and beef production. Species with the highest crude protein content, usually the most limiting factor in feed for animal production, had the highest market prices. The sale of fodder weeds may provide farmers with cash. Agricultural research programmes should be encouraged to consider the use of fodder weeds to improve both the quality and quantity of feed in the West African Sahel.

Ninety five cocoa clones were tested in Sabah, Malaysia at planting densities of 1096 and 3333 plants ha−1. Analysis of dry bean yields up to 60 months after planting showed that vigorous clones performed better at the lower planting density, some clones of intermediate vigour performed well at both planting densities and some weak clones performed well at high planting density. The conclusion that optimal planting densities are highly variable has significant implications for procedures in clone selection and suggests that the relation between the yield of a clone and its general combining ability will become clearer once planting density is taken into account.

Seedling tubers of three true potato seed (TPS) crosses raised in a seedbed soil mixture of clay, sand and dry leaves produced about 3.1 kg of seedling tubers m−2 in the growing season of 1988–89 and between 4.3 and 5.1 kg m−2 in 1989/90. The seedling tubers produced were stored and their use as seed tubers was subsequently compared with the locally popular variety Alpha. The mean tuber yields of the hybrids ranged between 9.4 and 11 t ha−1 in 1990/91 and between 21.9 and 22.9 t ha−1 in 1991/92. Mean tuber yields of cultivar Alpha were 14.3 t ha−1 in 1990/91 and 24.1 t ha−1 in 1991/92, showing that tuber yields comparable to those of imported seed tubers could be obtained from seedling tubers of TPS origin.

The composition and control of the grass weed flora of India and Pakistan are discussed, and the losses caused by the weeds are reviewed. The most important perennial weeds are Cynodon dactylon and Sorghum halepense and the most important annual weeds Echinochloa crusgalli and E. colona in the summer (kharif) crops and wild oats (Avena spp.) and Phalaris minor in the winter (rabi) crops. Losses caused by these weeds can be considerably reduced by the use of selective herbicides but these may be too expensive for many farmers. The problems and control of Phalaris minor are considered in detail.

The role of straw yield and quality in farmers' decisions about the adoption of pearl millet cultivars in western Rajasthan was examined using farm household surveys. Information on the relative importance of grain versus straw yield, the perceived risks associated with the use of new cultivars under variable climatic conditions and perceptions of straw quality indicated that improved cultivars of pearl millet were not adopted primarily because of poor grain yield in years with low rainfall, though poor straw yield in such years was also important. These results are particularly pertinent in the light of farmers' perceptions of the likelihood of experiencing drought or low rainfall. If new cultivars of pearl millet are to replace the traditional ones, they must perform better under conditions of limited rainfall.

Soil erosion and the productivity of cassava (Manihot esculenta Crantz) cropping systems, including forage legume intercropping, were monitored under natural rainfall conditions on Inceptisols in southwest Colombia. Trials were conducted over two cropping periods on moderate slopes. The best erosion control and cassava fresh root yields were achieved by sole cropping on contour ridges. Erosion was greater with forage legume intercropping than with cassava sole cropping in the first cropping period but once well established and uniformly distributed, undersown legumes controlled erosion effectively, though yields of cassava were depressed by 40% or more. Dense stands of grass contour barriers decreased erosion to tolerable levels. Lower cassava yields due to reduced cropping area and competition from the grasses could be compensated for by the production of valuable forage and by increasing cassava planting density.

A field experiment was conducted to assess the effect of competition between a leucaena hybrid and maize (Zea mays L.) when planted simultaneously in an alley cropping system. The leucaena hybrid (a cross between L. diversifolia and L. leucocephala) was planted at hedgerow spacings of 3 and 5.25 m, while maize was planted in rows 75 cm apart between the hedgerows. The spacing between the leucaena hedgerow and maize was varied by removal of 0, l or 2 rows of maize to give three spacing treatments of 37.5, 75 or 112.5 cm between leucaena and maize. A control plot of leucaena alone was also included in the treatments. The growth and yield of individual maize rows were virtually unaffected by the presence of leucaena, but maize had a significant influence on the growth and yield of leucaena. At full maize canopy development, photosynthetically active radiation reaching the leucaena was reduced in all treatments, resulting in a 75% yield reduction in leucaena at the closest spacing. Overall, maize grain yield reached 10.3 t ha−1 in the continuous maize plots (37.5 cm treatment). This was reduced by up to 40% after removal of two maize rows in the closest leucaena row spacing treatment. The implications of these results for the practical establishment of leucaena hedgerows with a maize crop are discussed.

Nitrogen efficiency from Azolla microphylla or Sesbania rostrata green manure, rice straw, and inorganic fertilizer-N was compared in two long-term experiments with irrigated lowland rice (Oryza sativa L.). Treatments included a control and each nitrogen source alone or in combinations that provided 50% of the total applied nitrogen from an organic and inorganic nitrogen source. All nitrogen sources were applied at equivalent nitrogen rates to 19–22 consecutive rice crops. Residual effects were assessed in two subsequent cropping seasons at one site. Lower grain yield, agronomic efficiency (Δgrain per kg total applied nitrogen), and apparent nitrogen uptake were obtained from green manure and rice straw nitrogen as sole or dual nitrogen sources rather than from a standard split application of prilled urea. Compared to prilled urea, residual effects from green manure or rice straw included a significant increase in soil organic carbon and total nitrogen, and greater extractable soil nitrogen in the vegetative growth period. After panicle initiation there was no residual effect on the rate of crop nitrogen accumulation, and final grain yields were similar regardless of previous nitrogen source. Recycling of rice straw appeared to have greater potential for reducing fertilizer-N requirements than use of green manure because rice straw is often a wasted resource in irrigated rice systems of the humid tropics, the efficiency of rice straw nitrogen in combination with prilled urea is comparable to green manure nitrogen, and the increase in soil nitrogen from rice straw was 50–150% greater than from green manure.