Ideally each block of an experiment should be made up in the same way with respect to treatments, that is, the design should be ‘orthogonal’. In practice that can be difficult to achieve, especially if the blocks have been chosen to fit the fertility pattern of the field. Sometimes it is impossible, in which case each block will have to contain its own selection of treatments. A number of simple and useful possibilities exist.

Whatever non-orthogonal design is chosen some of the contrasts of interest (perhaps all of them) will be evaluated less efficiently, but that can be compensated by the smaller error mean-square given by a better blocking system. Also, where blocks do differ in their content, comparing their means will provide additional information about treatment effects. Sometimes the information may be worth the trouble of recovery.

Special attention is given in this paper to total balance (including balanced incomplete block designs), supplemented balance, square and rectangular lattices and alpha-designs. The reinforcement of a design is explained and the advantages considered.

Problemas de diseño en la experimentación con cultivos. III

Common bean (Phaseolus vulgaris L.) is perhaps the most important grain legume in sub-Saharan Africa (SSA) smallholder systems for food security and household income. Although a wide choice of varieties is available, smallholder farmers in western Kenya realize yields that are low and variable since they operate in risky production environments. Significant seasonal variations exist in rainfall and severity of pests and diseases. This situation is worsened by the low and declining soil fertility, coupled with low capacity of farmers to purchase production inputs such as fertilizers, fungicides and insecticides, and land scarcity. The objective of this study was to investigate whether growing multiple-bean varieties instead of a single variety can enable farmers enhance yield stability over seasons and ensure food security. Five common bean varieties were evaluated in multiple farms for 11 seasons at Kapkerer in Nandi County, western Kenya. Data were collected on grain yield, days to 50% flowering and major diseases. In addition, daily rainfall was recorded throughout the growing seasons. The five varieties were combined in all possible ways to create 31 single- and multiple-bean production strategies. The strategies were evaluated for grain yield performance and yield stability over seasons to determine the risk of not attaining a particular yield target. Results indicated that cropping multiple-bean varieties can be an effective way for reducing production risks in heterogeneous smallholder systems. Yield stability can be greatly enhanced across diverse environments, leading to improved food security, especially for the resource-poor smallholder farmers operating in risk-prone environments. Although the results show that some of the single-bean variety strategies were high yielding, their yield stability was generally lower than those of multiple strategies. Resource-poor risk averse farmers can greatly increase the probability of exceeding their yield targets by cropping multiple-bean varieties with relatively low yields but high grain yield stability. Trading-off high grain yield for yield stability might be an important strategy for minimizing bean production risks.

Shifting cultivation of dry rice on steep hill-sides continues to be widely practised in Sarawak, Malaysia. Fertilizer experiments have shown that economic increases in yield can be obtained by mixing 0·9 kg. (2 lb) of mono-ammonium phosphate pellets (11 per cent N: 21 per cent P) with 2·4 kg. (5·3 lb) of seed just before dibbling. In certain areas additional increases might be gained by further increasing the rate of application of phosphorus, from the use of potassium, and from top-dressing with nitrogen. Closer spacing (30·5 cm. or 12 in. square) than that traditionally used should also increase yields per unit area.

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.

Cultural standards and the husbandry techniques used on small farmers' fields are seldom the same as those practised on research and experimental Stations in the developing countries. The need for extensive agronomic experiments to be sited on small farmers' fields is emphasized. This method has been quite widely used with annual crops, but could with advantage be more generally adopted in perennial crops. Some of the advantages of this approach in agronomic investigations are illustrated by examples from work carried out in East Africa.

The effects of intercropping roselle with blackgram, cowpea, soyabean, groundnut and sesame under single, double and triple row planting patterns and at three different intercrop sowing dates were studied from 1983 to 1985. The fibre yield of roselle was reduced by intercropping. The reduction was least with groundnut and greatest with cowpea but was compensated for by the yield of the intercrops. Double and triple row planting systems favoured the productivity of both roselle and the intercrops, increasing the Land Equivalent Ratio (LER) of the cropping system. The best yields of roselle were obtained when the intercrops were sown 15 days after the roselle.

In three field experiments, peat pH increased in parabolic relation to liming at the rate of 0.0544 pH unit/t/ha between 0–40 t/ha lime, but only 0.0224/t/ha between 40–72 t/ha. Groundnut and sorghum seed yield and crop residue also showed parabolic responses to peat pH and liming, but groundnut seed yield correlated poorly with soil pH (r2 = 0.38, P = 0.05), compared with its crop residue as well as with sorghum seed and residue (r2 = 0.75, 0.76 and 0.79, respectively, P = 0.01). Groundnut was generally more tolerant of peat acidity (pH 3.35–3.65) than sorghum, giving higher plant survival in unlimed plots, optimum seed and residue at pH 4.60 compared with 5.70 for sorghum, and reduced responsiveness to pH increase. Seed and residue of both crops decreased with continuous cropping, probably because of increasing pest and disease incidence and depletion of available soil nutrients.

A comparison has been made of drip and conventional check basin methods of irrigation, using onions and ladies finger as test crops. Significant increases in yield and water use efficiency in drip irrigated over conventionally irrigated plots resulted from increased availability of soil moisture at low tensions and reduced surface evaporation losses. Reduced soil strength in drip irrigated plots was also a factor resulting in increased yield of onions.

A technology for improving the productivity of short fallows using herbaceous legumes as cover crops was evaluated with farmers in the forest and savannah transition zones of Ghana. Legumes were relayed in maize 5–8 weeks after planting maize, depending on the legume species. An economic analysis of the technology compared with natural fallow over two cropping seasons (2001–2002) showed that the legume fallows were more profitable under the scenarios tested. Farmer assessment of the biological performance of the technology revealed the benefits of weed suppression and soil moisture conservation potential of the legume biomass as well as an improvement in the yield of the succeeding maize crop. However, farmers realized that planting legumes at close spacing and weeding before and after relaying are essential in the development of an appreciable biomass cover. The technology is suitable for farmers of all strata, in several major ecological zones of Ghana, and for planting on land with poorly secured tenure. However, at least two years of tenancy is required for the landless to derive some benefits from the legume fallow.

Mechanical disintegration seems to be the most effective way of damaging the cells in potato tubers so that their juice can be expressed. If disintegration is too intense the pulp is intractable; a grater makes a uniform pulp with suitable texture. With existing continuous equipment it is necessary to press in two stages to get to 40% dry matter and further batch pressing is needed to get to 50%, but experience suggests it should be possible to make a press that would press to > 50% in one operation. Much less energy is needed to press out water rather than evaporate it, but about one-sixth of the tuber dry matter and more than one-half of the N is in the juice.

No natural population of Rhizobium japonicum specific for soyabeans (Glycine max L.) could be found in any soil type in Israel. A crop of 5450 kg seed/ha was obtained in a field of sandy soil, inoculated with commercial granular inoculant, by maintaining soil at a moisture level near to field capacity during the growing season. In another field, on heavy soil inoculated with different strains of R. japonicum, the highest yield was obtained with strain 31-SM, a mutant derived from strain 61A75. Nitrogen fixation, as measured by the acetylene reduction method, was highest with strain 31-SM. Under local conditions soyabean plants snowed 100% nodulation when inoculated by the slurry method, by granular inoculant or by spraying bacteria into the furrow.

Seven maize cultivars and seven soyabean cultivars were grown in monocultures, and in all possible 49 mixtures of maize and soyabean, at two locations to assess the intercropping advantages of mixtures. On average, intercrops produced 6% more dry matter and 17.5% more protein than the monocultures of maize, the most productive component. Both the dry matter and protein yields of intercrops were closely correlated with the yield of the maize component, but not with that of the soyabean component, indicating that there was no need for a specific breeding programme for maize to be used in intercropping systems. Although the average intercropping advantage, the monoculture yields of maize cultivars and, to a lesser extent, the monoculture yields of soyabean cultivars differed between locations, there was no evidence that the yield of various intercrop combinations differed between locations.