Trials to determine the optimal rate and time of nitrogen application to cotton were conducted at three sites for three seasons. The results indicate that nitrogen is used most efficiently when applied at relatively low rates, with at least half the application delayed until flowering. Early application leads to the incorporation of a greater proportion of the nitrogen into the vegetative structures which, at lower rates, may leave insufficient for fruit development. With late application a greater proportion of the nitrogen is utilized in reproductive growth, although full utilization may not be achieved at higher rates.

The discrepancy between farmers' performances and research-based recommendations in the control of cotton pests in Southern Malawi is discussed. Analysis of the former in two seasons revealed that the farmers' low levels of insecticide application approached the economic optimum, whereas several factors of economic importance were omitted from the research-based justification of high spraying levels. Recommendations for improving the applicability of research recommendations to smallholder farming are made, with particular reference to the farming risks that arise from uncertain rainfall, the wide range of farming ability encountered in practice, and the dangers of atypically high levels of management in research experiments.

In experiments conducted with three rice varieties at different plant populations and N levels, the non-lodging varieties responded to 150 kg N/ha. The highest yield in non-lodging varieties were at the highest transplanted density, but at the lowest density in the lodging variety. The percentage of total milled rice of all varieties was not significantly affected by N application, which improved head-rice recovery in chalky IR-8, but had no significant effect on non-chalky varieties. Competition for nutrients could be a major factor limiting head rice return at high densities.

This paper attempts to determine the relations between weather and cotton yields in the Gezira Scheme in the Sudan, and to establish a prediction function with climatic factors as variables. The precision of the function established to forecast the final yield in April will as yet be low, when based on the information available at the end of October, November or December. A significantly better yield forecast can be made at the end of January. Variables of the prediction function are temperature, humidity, radiation and evapotranspiration. A strategy is proposed to improve early forecasts step by step, by making further forecasts at later stages.

Concentration of tea yields in Malawi into regular successive peaks within the main growing season is the cause of severe economic problems. A model of the causes of these peaks has been developed and its explanatory power is confirmed in the light of empirical tests of its component parts and predictions. A computer version of the model is presented and its predictive power demonstrated. Use of the model to explore relations between weather and yields is discussed, with indications of its limitations for this purpose.

There have been conflicting reports of the effects of different plucking (harvesting) policies on yields of tea in Malawi. An experiment is reported which separates the effects of plucking round length from those of plucking standard, and is compared with the results of simulating those plucking policies. Between the experiment and the simulation the conflict is resolved, and the validity of the experiment and simulation model are thereby enhanced. The immediate practical implications of the work are limited because of the unquantified relations between plucking standard and quality and grade out-turn of made tea.

Crop yield response to soil moisture deficiency varies for different crops and also depends on the time of its occurrence in the growth cycle. Many attempts have been made to derive a single relationship between total water consumption and yield for various crops, but this has proved of limited use, because the effect of time was omitted from such production functions. Jensen (1968) derived two expressions, for determinate and indeterminate crops, bringing the time element into his expressions indirectly by a parameter (λi) which defines the relative sensitivity of the crop to soil moisture stress at different growing stages. The usefulness of this approach depends on the accuracy with which this parameter can be determined. The aim of this paper is to derive λi for a number of crops from available experimental data and subsequently to find a way of computing the quantitative contribution of each single irrigation application to the crop yield. This should lead to a more rational use of irrigation water resources.

Increments of fibre, sucrose and reducing sugars per cane were estimated for 12 varieties of sugarcane for successive periods in plant and ratoon crops. Sucrose production increased only as that of fibre and reducing sugars fell. Dry matter production per unit fresh weight of leaf blade was negatively correlated with an estimate of gram days of leaf per gram fresh weight of cane. In a further study the net assimilation rate of one variety was found to remain fairly constant over a period of six months at 0·55g/dm2/week. It is suggested that breeding should seek to reduce leaf area ratio while maintaining sucrose percentage cane.

A diallel cross study on Indian mustard revealed a preponderance of additive gene action for seven characters out of the nine studied. Plant height and seeds per siliqua seemed to be controlled by both additive and non-additive gene action. Days to flower, number of secondary branches, length of main branch, number of siliquae on the main branch, length of siliqua and yield per plant were mainly controlled by additive gene action, whereas oil content was determined mainly by non-additive gene action. Heterosis was prevalent and the maternal effect was not generally apparent in the crosses. Rank correlations established a positive correlation between performance per se and GCA, and between heterosis and SCA.

Practical solutions are described to problems incurred in growing cultivars of cowpea, soyabean, lima bean and pigeon pea under simulated tropical conditions in controlled environments, namely (a) ‘Saxcil’ growth cabinets in which daylength, day and night temperatures, light intensity and quality, relative humidity and CO2, concentration are precisely controlled and can be varied over time; (b) compartmentalized glasshouses and adjoining dark compartments where daylength, and day and night temperatures are automatically controlled and night-break illumination can be provided, and (c) heated plastic film houses in which daylength-insensitive tropical legumes can be grown to maturity during the UK summer months since the environment of the wet tropics can be simulated in all respects other than daylength.

Forty-six cultivars of linseed oil (Linum usitatissimum L.) were grown under wet and dry season conditions in the Sudan Savanna of Nigeria. Only during the dry season was production good and cv Dunes consistently outyielded all other cultivars in terms of seed and oil content. Trials of sowing dates gave maximum seed yield and oil content from plots sown during late October. In row spacing trials, rows 15–22 cm apart and seed rates of 58–78 kg/ha were found to be the most practicable. There was a linear relation between increasing rates of nitrogen and seed yield, maximum production being obtained at 55–88 kg N/ha, but oil content was slightly depressed at levels above 33 kg N/ha.

Dodoma Edible outyielded Natal Common groundnut, due to varietal effects associated with differences in pod number, leaf area duration, crop growth rate, number of grains/pod and m2, and weight of individual grain. Increase in plant density led to an increase in grain yield. There was a close and positive relation between stem and pod dry weight; grain yield and leaf area duration; and crop growth rate, grain yield and grain number/m2 of land surface. A greater proportion of total dry matter was diverted into the stems of Dodoma Edible than Natal Common.