This paper documents the yield gains attributable to the breeding and distribution of new cocoa varieties in Ghana, using data from a 2002 survey of 192 fields in the country's key cocoa producing regions. We find that planting the more recently-released varieties is associated with at least 42% higher yields, and that genetic improvement accounts for much but not all of the observed correlation between tree age and cocoa yield. Fertilizer use is also very important, being associated with 19% higher cocoa yield per 50 kg bag of fertilizer. We find no evidence that varieties differ in their response to fertilizer, pesticide use or labour, and no evidence of a decline in the yield advantage of new varieties over the 17-year age span observed in the sample.

Maize and soybean are commonly intercropped in the drier zones of the western mid-hills in Nepal, but farmers report that productivity of soybean has been declining in recent years. Two researcher managed on-farm field experiments were conducted in the mid-hills environment of Nepal during 2001 and 2002, and one glasshouse experiment at the University of Wales, Bangor during 2003, to determine whether varying densities of maize and soybean influenced productivity of the system and to what extent soybean exhibited adaptation to shade. In neither season was maize yield affected by the presence of soybean, but grain yield of soybean was reduced in mixture by means of 59 and 53% during 2001 and 2002 respectively. Biomass and grain yield of maize were greatest at 53×103 plants ha−1 and least at the lowest density, whilst conversely biomass and grain yield of soybean increased. With increasing maize density, rates of accumulation of dry matter and leaf area index also increased, the latter resulting in decreasing transmission of light to the intercropped soybean. Soybean exhibited no photosynthetic adaptation to shade, but the specific leaf area was greater in artificially shaded and intercropped plants. Land equivalent ratios of all intercrops were greater than unity (1.30 to 1.45), indicating higher efficiency of intercropping compared to sole crops. Given the low plasticity in response of the maize canopy to variations in density, it is suggested that soybean could be better grown under maize by increasing between-row spacing of maize from 0.75 to 1.0 m to improve light transmission to the understorey, resulting in higher overall productivity of the intercropping system, and also that soybean germplasm be screened for adaptation to shade.

Recommendations of crop varieties are usually done on the basis of grain yield. However, there are other important traits related to quality and agronomy that need to be considered. This study has clearly shown that grain yield is often not related to some important traits of the crop. Under such circumstances, selection based on yield can lead to the loss of these important characters. Recent research has also indicated the need for a multi-trait approach to recommending crop varieties. However, such approaches, as suggested in the literature, are subjective and do not have a sufficient statistical basis. This study proposes a methodology for varietal recommendations by taking account of all important traits. It is a multivariate approach considered to be an improvement on the univariate method previously proposed. Data from rice varieties of 3, 3½ and 4½ month maturity groups, cultivated across diverse environments, over two seasons in Sri Lanka were used to illustrate the proposed methodology. The results suggest that the new method will be appropriate for taking in to account all important traits along with yield.

Potatoes are an important cash crop for small-scale producers worldwide. The move away from subsistence to commercialized farming, combined with the rapid growth in demand for processed agricultural products in developing countries, implies that small-scale farmers and researchers alike must begin to respond to these market changes and consider post-harvest treatment as a critical aspect of the potato farming system. This paper presents and assesses a low cost potato-grading machine that was designed explicitly to enable small-scale potato growers to sort tubers by size for supply to commercial processors. The results of ten experiments reveal that the machine achieves an accuracy of sort similar to commercially available graders. The machine, which uses parallel conical rollers, has the capacity to grade different tuber shapes and to adjust sorting classes, making it suitable for locations with high potato diversity. Its relatively low cost suggests that an improved and adapted version of this machine might enhance market integration of small-scale potato producers not only in Peru, but in other developing countries as well.

In Mozambique, the sweet potato (Ipomoea batatas) is grown both as a leafy vegetable, the terminal shoots or vines being progressively harvested during the season, and as a root crop. It is produced in the dry season, in areas with a high water table or with irrigation, and in the rainy season. This paper reports the results of measurements made during the 1995 dry season to evaluate the effects of water availability and the frequency of vine harvesting on the processes of radiation interception, dry matter production and partitioning. An irrigated crop (cv. TIS 2534) with a single, end of season vine-harvest (H1) intercepted 71% of the total solar radiation, reducing to 52% with weekly (H4) harvests. The corresponding values for a rain-fed crop were 33 and 20% respectively. When the leaf area index (L) exceeded 3–4, virtually full interception of photosynthetically active radiation (PAR) occurred, except when frequent vine-harvests modified the architecture of the leaf canopy. The extinction coefficient (k, PAR) was highly variable: at a given leaf area, interception was greatest earlier in the season, with infrequent harvests and with irrigation. Total dry matter production (vines, foliage and storage roots), over a period of 161 days, reached 23 t ha−1 in the single harvest (H1), irrigated crop, less when the vines were harvested weekly (H4). The corresponding value for the rain-fed crop, which was able to resume active growth immediately after a rainfall event, following a prolonged dry period, was 7 t ha−1. The efficiency of conversion of PAR into dry matter (εs) was 2.74 g MJ−1 in the irrigated treatments combined, decreasing to around 73% of this without irrigation. The value of εs appeared not to be influenced by dry air or high temperatures. The reduction in εs due to drought was less than the corresponding reduction in intercepted PAR. Increasing the frequency of vine harvests increased the partitioning of assimilates to vines. Irrigation had similar effects early in the season, but afterwards it encouraged preferential growth of the storage roots. The end of season harvest index (h) for vines increased with harvesting frequency from, for example, 0.08 (H1) to 0.38 (H4) in the irrigated crop. By comparison, irrigation increased ‘h’ for storage roots from 0.24 (rain-fed) to 0.57 in the H1 treatment. The total harvest index (vines plus roots) increased with the number of vine harvests from 0.32 (H1) to 0.55 (H4) in the rain-fed crop, and from 0.62 (H1) to 0.72 (H4) in the irrigated crop. Clearly, ‘h’ is not a conservative parameter in the sweet potato, but is sensitive to crop management practices and to soil water availability.

The poor fertility of sandy Sahelian soils remains one of the major constraints to pearl millet (Pennisetum glaucum) production in West Africa. On-farm trials under farmers' management were conducted in two rainfall zones of Niger in 1996 and 1997 to evaluate the risk characteristics of six soil fertility restoration options. Stochastic dominance analysis was used to compare the fertilizer treatments tested. Results showed that the farmers' traditional method (no fertilizer control), Tahoua phosphate rock (PRT) alone applied at 13 kg P ha−1 broadcast, and a combination of PRT broadcast at 13 kg P ha−1 and single super phosphate (SSP) hill-placed at 4 kg P ha−1 had the most desirable risk characteristics and were acceptable to risk averse decision-makers in both rainfall zones. At current input–output price ratios, most fertilizer-using farmers would choose the combination of PRT broadcast and SSP hill-placed. If the availability of SSP was limited, some farmers would use PRT alone. The demand for risk efficient alternatives could significantly increase if farmers could bear less than half the fertilizer costs at the current output price, although further research is required to say if a fertilizer subsidy could be justified on broader economic or social grounds.

Farmers themselves are allowed to manage on-farm variety adaptability trials (VATs), to represent farmer management practices in preference to research management. In this approach, within-trial uniformity and thus the scientific value of VATs will be lost. A study was conducted using rice to ascertain whether representation of farmer management in VATs is necessary and to test an approach that represents farmer management without losing within-trial uniformity and the scientific value of VATs. In the proposed approach, both the representation of farmer management and within-trial uniformity are reasonably assured through implementation of site-specific farmer practices by research personnel. This approach was compared with complete research management using recommended practices in a VAT conducted with five rice varieties in six farmers' fields over two seasons. When farmer management was implemented by research personnel, popular varieties among farmers were judged most adaptable. However, when VATs were managed using recommended practices, popular varieties among farmers were judged least adaptable. This provided direct evidence to show the usefulness of representation of farmer management and the validity and practical feasibility of the proposed approach in managing VATs.

Factorial experiments are described and the importance of interactions emphasized with suggestions about how they should be interpreted. A distinction is made between factors introduced only to see if they will provoke an interaction and those actually under study. Split-plot designs receive special attention. Factorial experiments often involve a large number of treatments and ordinary block designs may be ineffective in controlling environmental variation. If factors have few levels, as in exploratory experiments, the usual device for reducing block size is confounding, which is explained along with single-replicate experiments, partial replication and hidden replication. Alternatively, non-orthogonal designs and analysis of data by nearest-neighbour and spatial methods might prove useful. The need for randomization and the role of significance are discussed. It is pointed out that interactions can sometimes be avoided by transformation of the variate.

Leaf scorch decline (LSD) is a disorder of coconut (Cocos nucifera) palms in Sri Lanka. The etiology of the disorder is not yet known. This study aimed to compare and quantify physiological and biochemical characters of LSD-affected palms with healthy palms. Thirty-year-old apparently healthy and LSD-affected (mild, moderate and severe) palms (variety typica) were compared at Bandirippuwa Estate, Coconut Research Institute, Sri Lanka. In LSD-affected coconut palms, the functional leaf area of the canopy, rates of photosynthesis and transpiration, leaf water potential, leaf chlorophyll and Zn contents were appreciably reduced with the development of the disorder, whereas stomatal diffusive resistances and abscisic acid concentrations in the xylem sap increased. The volume of inflorescence sap collected from LSD-affected palms was reduced compared to healthy palms, but the composition of the sap remained unaltered. Since altered stomatal behaviour might affect gas exchange and related processes, such as water movement, photosynthesis and the assimilate partitioning pattern, these results support the hypothesis that stomatal closure in LSD-affected palms may influence LSD symptoms. However, more research is needed to come up with conclusive evidence of its cause.