The tubers of yam, principally those of Dioscorea rotundata (white Guinea yam) and D. alata (water or greater yam), are important staple foods and sources of carbohydrate in West Africa. Yams are grown in diverse environments – from the high-rainfall forest zone on the coast to the seasonally arid savannas of West Africa, that is in situations in which the duration and the timing of the onset of the growing season vary appreciably. Dormancy in both underground and aerial tubers of the Dioscoreaceae is an important adaptive mechanism that helps to maintain organoleptic quality during storage and also ensures that tubers germinate at the start of the growing season. Plant breeders are especially keen to manipulate the duration of the dormant period in order to synchronize growth periods and, therefore, to produce more than one generation per year. The control of tuber dormancy, however, is poorly understood. This review examines critically those factors that affect tuber initiation, dormancy and sprouting, and makes recommendations for future priorities in research.

The results of 86 replicated experiments on soybean (Glycine max) cultivation practices in eight provinces – planned, conducted and analysed by farmers – were examined to identify local differences in the effects of cultural practices on yield, and to explore the potential for farmers to improve their practices. Plant spacing consistently improved yield relative to broadcast seeding. The comparison between a moderate and a high dose of N, and between local spraying practices and integrated pest management, also had a relatively consistent effect on yield. Conversely, the effects of straw mulch, of moderate doses of N compared to zero N, and of weeding, were highly location-specific. A change in cultural practices influenced yield and economic benefits more strongly in low- than in high-yield situations. Modifications of current cultivation practices often resulted in a significant yield increase. This suggests that farmers need training in how to improve their cultural practices and that it should utilize locally conducted field experiments. Training in experimental skills made farmers less dependent on external measures and advisers, and enabled them to become ‘experts’ who utilize science.

The study of interactions between cacao (Theobroma cacao) and coconut (Cocos nucifera) in Lampung, Indonesia, examined different combinations of age, plant lay-out, planting chronology and choice of planting material under changing environmental conditions. Four coconut-cacao intercropping trials were used to assess the performance of each intercrop under limiting or non-limiting environmental conditions. In intercropping experiments with young cacao trees and young coconut palms, delayed cacao tree development and reduced yields were observed. When coconut palms were aged five years or over, coconut and cacao growth were satisfactory under virtually normal environmental conditions; death rates remained reasonable and yield percentages differed little from those of the monocultures for each crop. The performance of both plants, however, changed when water became a limiting factor.

Farmers' Participatory Varietal Evaluation (PVE) was conducted on sorghum (Sorghum bicolor) crops in the Kile-Bisidimo plains of eastern Ethiopia for three consecutive years, 1995–1997. The study aimed at providing farmers with alternatives to their landrace to enable them to overcome crop losses and to identify farmers' varietal selection criteria for inclusion in future breeding work. In 1995 constraints and opportunities in sorghum growing and farmers' varietal matching characteristics were identified through an informal survey. This was followed by a search for varieties and acquisitions of seed. Subsequently, eight varieties were evaluated of which five varieties were released and three were at the pre-release stage.

In 1996 and 1997 farmer-managed on-farm trials were conducted and farmers evaluated the performance of the experimental varieties against a locally grown cultivar. Through pairwise and matrix ranking, farmers' selection criteria were listed, preferred varieties identified and initiatives taken to diffuse the varieties through the local seed system. Out of the eight varieties that research workers considered the best, farmers selected only three. The study negated the generally accepted view that farmers in lowland areas of eastern Ethiopia are reluctant to grow short-duration varieties. The selection and introduction of three new varieties into a farming system where farmers were growing only one local variety reconfirmed PVE to be a means for enhancing adoption and increasing genetic diversity. The study also confirmed that increasing farmers' access to their preferred varieties would result in a faster rate of diffusion through farmer-to-farmer seed exchange.

Intercropping of pearl millet (Pennisetum glaucum) with clusterbean (Cyamopsis tetragonoloba) is a common practice in the arid zone of northwestern India. Field experiments were conducted for two years to examine the performance of two contrasting genotypes of each of these crops grown both as sole crops and as intercrops using all possible genotypic combinations of the pearl millet with the clusterbean. The seed yields of both crops were always lower in intercropping than in sole cropping, though the degree of reduction in the seed yield of each intercrop component was influenced greatly by the genotype of its companion crop. The seed yield of both clusterbean genotypes was reduced more when intercropped with the tall and late-maturing pearl millet, MH 179, than with the medium-statured and early-maturing HHB 67. On the other hand, the two clusterbean cultivars had similar effects on intercropped pearl millet. As a result, the highest land equivalent ratios (LERs) were obtained with intercrops containing pearl millet HHB 67 rather than MH 179, while the genotype of clusterbean had little overall effect on LER. The results obtained are discussed in the context of developing cultivars for intercropping.

Gas exchange, leaf carbon isotope discrimination, growth, yield and beverage quality were evaluated for two Coffea arabica cultivars (Catuai and Mundo Novo), grafted on to C. canephora and C. congensis progenies growing in open fields. During the years 1994 to 1997, grafting resulted in an average increase in bean yield of 151 and 89% for Catuai and Mundo Novo respectively. As analysed by sensory analyses and by the ratio between the mono-isomers and di-isomers of caffeoylquinic acid, beverage quality of the C. arabica was not altered by grafting. Shoot growth was significantly greater in grafted plants, showing an increase of 52% in total leaf area compared with the non-grafted plants. Under conditions of water excess in the soil there was little difference in the transpiration and stomatal conductance rates between the grafted and non-grafted plants, but the net photosynthesis was higher in grafted plants. With an accentuated water deficit in the soil in the dry period, the grafted plants showed significantly higher transpiration and stomatal conductance rates than the non-grafted plants, and similar values to those of C. canephora. Carbon isotope discrimination was greater in the grafted plants, suggesting greater root hydraulic conductance. The results suggest that the better performance of the grafted plants during the dry period was due to the greater capacity of the root system of C. canephora to provide water to the shoot thereby maintaining greater gas exchange in the leaves and consequently a greater carbon gain.

Under rainforest vegetation, the central Amazonian Ferralsols are characterized by relatively high availability of N in relation to other nutrients. After forest clearing, several tree crops also have not shown yield responses to N fertilizer. To elucidate the mechanisms of this apparent N sufficiency, the mineralization of soil N was measured under three tree crops and a leguminous cover crop (Pueraria phaseoloides) in a multi-strata agroforestry system at two fertilizer input levels on a Xanthic Ferralsol in central Amazonia. In situ incubations of topsoil (0–10 cm) were carried out using the buried-bag method on five occasions over ten months. The highest mineralization rates were found under the cover crop, intermediate rates under rubber trees (Hevea brasiliensis) where the soil was also covered by the cover crop, and lowest rates under peach palm (Bactris gasipaes) and cupuaçu (Theobroma grandiflorum) with no cover crop. The increased N mineralization under the cover crop was due to more total N in the soil, higher soil moisture and, presumably, a larger pool of readily mineralizable soil N compared with the soil under the tree crops. Other fertility parameters also differed significantly between sampling positions within the plots, but this had no major influence on net N mineralization. Also, the input of NPK fertilizer and dolomite had no significant influence on N mineralization, indicating that N mineralization was not nutrient-limited. High total N mineralization rates in the soil (approximately 350 kg ha−1 a−1 at 0–10 cm depth) explained earlier observations of nitrate leaching into the subsoil under multi-strata agroforestry at this site. Considering the spatial patterns of N mineralization with maximum values under the cover crop, the exploration of the soil volume by crop roots should be maximized to increase the uptake of mineralized soil N by the crops and reduce nitrate leaching. Appropriate measures are narrow tree spacing, use of annual and semi-perennial intercrops and encouragement of the lateral root development of the trees. In addition, the mineralization of soil N close to the tree crops can be influenced through the management of the cover crop. In view of the high total N mineralization rates in the system and unclear yield responses of tree crops to N fertilizer, the application of N fertilizer to tree crops with well-developed root systems and a well-managed cover crop may often be unnecessary on this soil type. This may facilitate the further development of tree crop agriculture in the region.