An experiment has its origin in the need to find answers to stated questions. From the start great care is given to its correct conduct (e.g. the application of treatments and the method of recording) as well as to statistical design, always with the original questions in mind. The analysis of its data is the climax of a long process and the questions to be answered must dominate all else. It is not enough to feed data into a computer package in the hope that it will provide an automated path to a true interpretation.

Where the treatments have been chosen with care to answer specific questions, the statistical way of designating purpose is to declare ‘contrasts of interest’, each corresponding to a degree of freedom between treatments. They derive solely from the reasoning behind the selection of treatments. If possible the questions posed should be equal in number to the degrees of freedom and should admit of separate study because no one can give a single answer to several diverse questions.

This paper shows how to define a contrast of interest and how to isolate it in the analysis of variance. Attention is given both to its contribution to the treatment sum of squares and to its variance (i.e. the precision with which it is estimated). Independence of estimation is also considered. Algebraic formulae are given for a restricted though important range of designs, which includes those that are completely randomized, in randomized blocks or in Latin squares, all treatments having the same replication. The methods can, however, be generalized to cover all designs. With these formulae it is possible both to test the existence of an interesting effect and to set confidence limits round its estimated value.

The effects of component population, rhizobial inoculation and applied nitrogen on the efficiency of phosphorus utilization in maize/mungbean intercropping were examined in two experiments. The efficiency of phosphorus absorption decreased by between 5 and 43% in maize and by between 31 and 58% in mungbean as a result of intercropping. The overall efficiency of phosphorus absorption in intercropping decreased with increasing nitrogen level but a parallel increase in overall conversion efficiency maintained a constant and large land equivalent ratio (LER) up to the highest level of nitrogen. The increase in LER over unity, however, was due primarily to the greater total absorption of phosphorus by maize and mungbean together in intercropping compared with that when they were grown in pure stands.

Field experiments were conducted on very alkaline calcareous soils to investigate the effects of two proprietary foliar micronutrient fertilizers, Wuxal Suspension Polymicro and Bayfolan 11-8-6, containing iron, zinc, manganese and boron, on the seed cotton yield of Gossypium barbadense and G. hirsutum. Two applications of foliar fertilizer, one at flowering and one three weeks after flowering, increased yield by 40–68%. The foliar fertilizers enhanced earliness and minimized nutrient depletion from the leaves, and thus increased reproductive efficiency.

A mobile sampling method was used to measure the amount of photosynthetically active radiation (PAR) transmitted through the canopies of coconut palms in two long term field trials. PAR transmissions were measured at approximately monthly intervals for one year. Fractional interception of light was found to vary considerably between months, and to be associated with frond shedding caused by dry season water stress. Palms receiving potassium fertilizer intercepted more light and replaced leaf area lost during the dry season more rapidly than those receiving no potassium. Palms planted at a higher density intercepted more light and carried more fronds per palm, but intercepted less light per frond than those planted at a lower density. The ratio of moles of carbohydrate yield to moles of intercepted PAR quanta was calculated for six plots of hybrid palms, giving a dimensionless light efficiency index (LEI) comparable to harvest index. Mean LEI for all plots was 1.275 × 10−4 and was found to increase with increased light interception, results suggesting that there is a basal level of light interception below which no yield is produced. The potential for using LEI in agronomic experimentation on sole cropping and multicropping and as a criterion for genetic selection of coconut palms is discussed.

Beans and bananas were grown in sole crop and intercrop systems for three seasons in 1989 and 1990 to determine the effects of cropping system on diseases, insects and land-use efficiency and the roles of certain bean plant physiological traits on bean yield in the two systems.

Bean yield in the intercrop system was 52% of bean sole crop yield. Shading reduced yield, but stem starch levels, chlorophyll content and the chlorophyll a:b ratio were not related to seed yield in either system. Leaf area index was linearly related to yield in the intercrop system, suggesting that a higher plant density may result in higher yields. Nutrient concentration levels in the foliar tissues indicated that low potassium and high manganese availability constrained intercrop bean yield. Leaf manganese levels were higher in beans grown under bananas, reaching levels normally considered toxic for beans. Competition for other nutrients did not appear to affect bean yield. Apparently, interspecies competition for soil moisture did not much reduce bean yields as moisture levels did not differ with cropping system. Disease and insect pest levels were similar for the two bean production systems.

Banana yields were associated with potassium levels in the soil. Bananas appeared to be more competitive than beans in the intercrop system. The land equivalent ratio of the banana and bean intercropping system over the three seasons was 1.60.

Interactions between upland rice and three phenologically distinct pigeonpea cultivars were examined on a medium deep Vertisol. The productivity of each intercrop component and its respective sole crop was determined in terms of a crop performance ratio (CPR). The extra-early pigeonpea cultivar recorded the largest partial CPR of grain followed by early and medium genotypes. Spreading genotypes had a larger partial CPR than semi-compact genotypes. However, the CPR of intercropped rice was less (0.65–0.69) with spreading pigeonpeas but exceeded unity with compact types. The canopy structure of pigeonpea appeared to be more important than differences in phenology. A large range of light transmission coefficients (K) existed in pigeonpea (from 0.45 to 0.78) but it is argued that a further reduction in K may not be necessary since intercropped rice yield was unaffected even with a K of 0.64. The relative height of intercropped pigeonpea and upland rice may also determine competitive ability since rice is very sensitive to low light and shading, particularly during the reproductive phase.

The traditional crops of Oceania are asexually propagated and are losing their positions in cropping systems because of their genetic vulnerability. This paper reviews genetic reasons explaining the deterioration of their agronomic performance, using information mostly derived from surveys of genetic resources conducted in more than 50 Pacific islands, coupled with genetic investigations. More than 2000 cultivars have been analysed for isozyme variation. Cultivars and wild forms showed greater isozymic variation in Melanesia than in Polynesia and Micronesia but the genetic base for many of these crops was much narrower than morphological and chemical variation had suggested. No correlations were found between zymotypes, chemotypes, and morphotypes. Cultivars exhibited variation in qualitative traits as a result of human selection of somatic mutations. Accumulation of viral particles in propagules and build-up of pest and pathogen populations are often responsible for poor agronomic performance. If breeding of these traditional crops is to have any future, it is important to exchange genotypes to broaden the base of existing programmes.

Data recently published in this journal on grain yields of new rice varieties, grown under rainfed conditions on-farm in two villages in Uttar Pradesh, were analysed. The new varieties were spectacularly better than the landraces with which they were compared and showed a high regression slope (>2) against the landraces. If supported by further studies, such experiments could provide an appropriate, but hitherto unexplored, approach to plant breeding for poor, small farmers operating with low inputs.

Rapid initiation and growth of nodal roots (NR) are critical for the establishment of post-rainy season (rabi) sorghums grown in drying soil. The growth of NR was studied in sorghum in the field when sown at different depths in order to vary the crown depth, when different levels of irrigation water were applied to create different moisture contents in the topsoil, and in a number of cultivars to assess genotypic variability. Variation in sowing depth did not result in differences in NR growth, partly because of the capacity of sorghum to vary the mesocotyl length, which brought the crown to approximately the same depth. Different soil moisture levels in the field affected the elongation rate of NR, but not their initiation. Genotypic variation existed in the thermal time required for NR initiation and rate of elongation. The variation in NR growth was independent of the variation in shoot growth. The genotypic variability in NR growth offers an opportunity for combining root-related traits with useful agronomic characters.

The best available varieties of barley, durum wheat, rapeseed, safflower and sunflower were compared for their yield of grain and oil and other traits under rainfed conditions and with supplementary irrigation in the relatively dry Mediterranean climate of Cyprus. The grain yields of the oilseed crops safflower and rapeseed when grown under conditions of plentiful rainfall or with supplementary irrigation were 3–5 t ha−l. Considering the high prices of oilseeds on the world market, the income from oil crops compares favourably with that from barley and wheat. However, under conditions of limited rainfall (300 mm) and without supplementary irrigation the yields of oilseed crops were extremely poor, at the most 0.6 t ha−1. Grain and oil yields of irrigated safflower and rapeseed were larger than those of sunflower. Among the oilseed crops, sunflower received most irrigation water and rapeseed the least. Under rainfed conditions, with precipitation less than 300 mm, cereals were the only successful crops. It is concluded that in the high rainfall areas of West Asia and North Africa, or with supplementary irrigation, including treated municipal wastewater, oilseed crops can be successfully grown.

Production possibility frontiers contribute much to an economic evaluation of yield advantages from intercropping. The difficulty with estimating a production frontier empirically from experimental data is one of ascertaining that the fitted curve corresponds with the frontier. This problem has been overcome by deriving the frontier from a priori knowledge of the biological processes that determine the outcome in intercropping. The hyperbolic relationship between biomass yield and plant density, and the parameters that characterize the degree of intra-and inter-specific competition in intercropping are used in this paper to derive production possibility frontiers. The method is illustrated with data from three intercropping studies. A brief review of the two main methods used by researchers to evaluate the results of intercropping, and their limitations, is also presented.