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.

The response of the latest wheat cultivars to irrigation regimes was studied between 1970 and 1975 in a heavy soil of the Yaqui Valley of north-west Mexico. Yield showed greatest sensitivity to water shortage in the period 65–110 days after seeding (spike emergence around 90 days), due largely to responses in grains/m2. More frequent irrigation increased yields 5–10% over the average of 7 t/ha obtained with the commonly-adopted five irrigation regime. Various irrigation criteria were tested: potential evapotranspiration calculations seemed the most useful. Measurement of leaf permeability (with an air flow porometer) showed more promise than the use of plant water potential (measured with a pressure chamber).

Combining ability analysis was carried out on diallel crosses of 20 yellow maize varieties in four environments. General combining ability variance (σ2g) was of greater importance than specific combining ability (σ2s) in the inheritance of all traits except grain yield and ear length, where the reverse was true. Interaction components (σ2ge, σ2se) were greater than the respective main components (σ2g, σ2s) for grain yield. The study brought out the prominent role of genotype-environmental interactions. Heritability in the broad sense was very high for all traits except grain yield and grain moisture, and narrow sense heritability was also high for all traits except grain yield and ear length.

The effect of one, two or three applications of monocrotophos at 1120 g ai/ha at weekly intervals was determined on cowpea var. Ife Brown. All regimes were equally effective in grain production in early 1974 but spray applications 35 and 42 days after planting gave the best result in the late crop. Grain production from two applications was not significantly different from three applications in 1975. The control plots in the late season of both years failed, due to moisture stress and lack of pest control measures. It is considered that two sprays at flowering is adequate to forestall a crop failure in this variety.

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.

Effectively nodulated, pot-grown cowpea plants were irrigated with nutrient solution containing 25 ppm 15N, and the relative contributions to total plant N status of inorganic and nodule-fixed N were determined. Maximum rates of N assimilation occurred during pod-fill, with nodules contributing ten times more nitrogen than the applied source. Symbiotic fixation supplied over 80% of total plant N throughout growth, and contributed significantly to seed N during late pod-fill, when nutrient N assimilation was negligible. Vegetative N content was greatest at mid pod-fill, and mobilization from this 'pool' was equivalent to 44% of total seed N. The implications of these (and previously reported) data are discussed.

It is suggested that quite a small computer, in association with a minimal program for the analysis of variance, can be used to calculate quantities of use to the agronomic research worker beyond what he usually obtains. For example, without further programs it is possible to calculate an analysis of covariance, in which adjustment is made for some disturbing factor. It is also possible to find how much of the error variance arises from a particular plot, and to deal with situations in which data are incomplete or the yields from two plots have become mixed.

In short clonal rows of sugarcane the mean weight of adjacent rows contributed significantly to the variance of row weight, mainly because some clones could increase their cane number at the expense of slow-tillering clones. Weight per cane and sugar content were unaffected by competition. Significant correlations were found between sugar content and (a) the rate of leaf appearance, (b) the ratio of fresh weight of leaf blade to fresh weight of joint, and (c) fibre as a percentage of cane fresh weight.

Experiments conducted on saline-sodic soils during 1972 and 1973 showed that delay in transplanting medium-duration dwarf high-yielding varieties like Jaya beyond 20 July caused substantial reductions in grain yield, but there was no depression in yield of such varieties even if 55 day old seedlings were used for transplanting at normal times. Older seedlings gave very high yields compared with younger counterparts at late transplantings. Fifty-five day old seedlings took about two weeks less to mature compared with ones planted when 25 days old.

The sulphur content in the sixth leaf from the apex of coconut palms was found to be the most sensitive index to sulphur treatments. Although the S concentration in this leaf correlated negatively with applied levels of S, the response curve was concave. Sulphur treatments increased the total yield of fruits and weight of copra, but decreased the weight of kernel per nut. The derived curve for yield versus S concentration in the tissue was C-shaped, attributed to growth dilution effects caused by enhanced initial growth following the restoration of a nutrient in minimum.

By comparing flowered and non-flowered plants of the same age (where flowering was inhibited by photoperiod control), cane yield gains due to prevention of flowering were estimated to be 56·6% in a plant crop and 33·8% in a ratoon crop. Sugar gain due to prevention of flowering was 69·1% in the plant and 35·4% in the ratoon crop. A simple negative linear relation was found between cane (and sugar) yield per plant and the proportion of canes that flowered. There was no significant difference in fibre content of flowered and non-flowered canes. The influence of loss of growth, side-shoot production and pithiness on the yield of flowered canes are discussed.

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.

An experiment is described in which the productivity of maize–bean and maize–potato mixtures in alternate rows was compared with pure stands of the component species in four cropping seasons. In seasons with low rainfall, yield from the mixtures fell short of that from pure stands, but in one exceptionally wet season a yield advantage was found for maize–bean mixtures. The poor performance of mixtures in low rainfall seasons appeared to result from large reductions in maize yield attributable to competition from the potato or bean. In the one high-rainfall season, no such reduction in maize yield occurred. An implication of these findings for farming systems with mixed cropping is discussed.

Studies to determine the effect of planting unshelled pods, using groundnut varieties with different shell thicknesses, indicated that emergence and yield were suppressed by planting unshelled pods. Depth of sowing, and soaking pods in water prior to planting, had limited effects.

The properties are discussed of a number of relatively simple equations that describe non-linear relations of the type which frequently occur in biology. Most of the equations can be linearized by applying simple transformations (e.g. logy, log (a–y), I/y) to either y or x, or both. Some approximate methods of fitting the equations to experimental data are given, but statistical methods are not discussed in any detail.

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.

A small bench-mounted seed thresher for vegetables has been developed for the use of plant breeders. The construction and operation of the thresher is described.

An experiment at the Kabete Field Station of the University of Nairobi compared maize–bean mixtures with pure stands of the two crops at three plant densities. Although the mixtures gave an apparent yield advantage over pure stands, this could be explained solely by the increased population pressure in the mixtures. The implication of this finding for other published work with cereal–legume mixtures is discussed, and a distinction is drawn between those sites in East Africa where a conclusive yield advantage from mixed cropping has been found and those where any apparent advantage might be explained by the higher population pressure.

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.

The results of eight fertilizer experiments with cotton, conducted in several areas of Macedonia, were evaluated by leaf analysis to establish possible relations between leaf N, P and K and yield, as well as to estimate the adequacy ranges of those elements under local conditions. Such a relation between leaf N and yield does exist, and leaf N within the range 4·4–4·9% of DM is adequate. On the other hand, leaf P and K within the ranges of 0·20–0·29 and 1·5–2·6%, respectively, also seem to be adequate. These findings are in agreement with general experience in this region, where cotton seldom benefits from application of fertilizers other than nitrogenous.