A framework is proposed for the classification of farm systems, which are defined as decisionmaking units comprising farm household, cropping and livestock systems that transform land, capital and labour into products for consumption and sale. Two general principles underlying the classification are outlined. First, since farm systems are embedded in a hierarchical structure, the classification is based on the characteristics of the underlying systems and their interactions. Secondly, ecological factors, i.e. physical and biological parameters, are the primary determinants of farm systems. Changes in farm systems, at least in the foreseeable future, depend on the development of socio-economic variables. The classification is summarized in a set of comprehensive tables.

L. O. Fresco y E. Westphal: Una clasificación jerárquica de sistemas agrícolas

That experimental design is fundamentally more important than the formal analysis of variance of results is emphasized. Attention is drawn to the many weaknesses of split plot designs, and the case for adopting other types of confounding whenever possible is stated. Split plots are sometimes necessary but rarely desirable. The need for randomization is stressed, with a warning against abandoning this principle before fully considering the dangers. The error in analysing repeated observations as though they are from split plots is discussed at length. After brief comment on variance heterogeneity, a final section looks at how measures of the precision of treatment means should be presented. The two main points made are that standard errors of treatment means should always be included in tables of results, and that existing multiple range significance test procedures have little or no practical utility.

Shoot and root growth and soil water depletion were studied in mungbean and soyabean grown at three plant populations after non-flooded rice (Oryza sativa) during the post-monsoon dry season in the Philippines. The site had a shallow fluctuating water table (1-2 m) but rooting depth was restricted to 0.8 m by a volcanic tuff layer. Soyabean had a longer duration (89 days) than mungbean (68 days) and intercepted more solar radiation, but from 61 days after sowing was severely stressed and accumulated little dry matter. Mungbean avoided severe water stress due to its short duration. Despite this stress, grain yield (1.0 t ha-1) was similar for the two species and soyabean yielded more grain nitrogen but less straw nitrogen than mungbean. Higher plant population achieved by narrower row spacing increased cumulative light interception and both grain and forage yields in both crops.

K. D. Shepherd, P. J. Gregory, T. Woodhead, R. K. Pandey y E. C Magbujos: Crecimiento de la soja (Glycine max) y del frijol mungo ("Vigna radiata) en la estacidn post-monz6n despuis del arroz de secano.

Breeding for high and stable yields is a major objective for cassava breeding programmes. Owing to the simultaneous formation of steins and leaves (source) and storage roots (sink), competition for assimilates implies that there is an optimum leaf area index for yield. This has led to the use of the harvest index as a practical criterion for selection. The optimal harvest index and leaf area index are discussed for selection for stressful environments. A successful strategy for developing high yielding varieties for favourable conditions is presented. Recent information has shown that selection for a slightly larger than optimal leaf area index, and hence greater biomass, can lead to stable yield in both favourable and stressful environments. Moreover, yield levels can be increased by selecting for moderate harvest indices (e.g. 0.5-0.65) and increased biomass. Longer leaf life is an integral part of this selection strategy. These concepts are discussed in detail.

James H. Cock y Mabrouk A. El-Sharkawy: Caracteristicas fisiológicas para la selección de la mandioca.

Experiments on okra to determine the economic loss caused by insect damage to the leaves or by harvesting young leaves as a vegetable are described. Leaf harvests at 1-weekly intervals yielded 30-50% more fresh leaf than harvests at 2 or 3-weekly intervals. Removing a quarter of each leaf or single leaves from the upper or lower parts of the plant at 3-weekly intervals did not affect marketable fruit yield, but yield was reduced by about 30% when one young leaf was harvested at 1 or 2-weekly intervals or when two or three leaves were harvested at S-weekly intervals. Removing more than one quarter of each leaf or two or three fully expanded leaves from the lower three-quarters of the plant at 3-weekly intervals also caused up to 40% yield loss.Harvesting single leaves at S-weekly intervals is therefore recommended if okra is to be grown for both leaves and pods. The plant can tolerate at least 25% defoliation, as can happen with insect damage, before any yield loss occurs.

F. O. Olasantan: Efecto de la deshojadura sobre el crecimiento y rendimiento del gombo (Abelmoschus esculentus,) y su importancia para modelos de cosecha de hojas y dano causado porplagas.

Root and tuber crops make a major contribution to the food supply in many developing countries. Most of the relatively limited research effort on tropical root crops has focussed on improved production technologies, especially with regard to genetic improvement. Interest in post-harvest technologies and crop use has increased recently with the growing recognition of their importance in the marketing of these perishable crops. This paper reviews recent advances in storage research for three of the principal root crops: cassava (Manihot esculenta C), yam (Dioscorea spp.) and the edible aroids (Colocasia spp.).

R. D. Cooke, J. E. Rickard y A. K. Thompson: El almacenamiento de tubérculos tropicales - mandioca, ôame y aráceas comestibles.

Line sowing of jute has been advocated by some workers for the past 40 years as being more productive than the traditional broadcast method. But most cultivators still prefer broadcast sowing. A comparative study employing identical management practices showed that there is no significant difference in fibre yield per unit area between the two sowing methods. Under similar population pressures self-thinning is greater in line sown than in broadcast crops. Thus, provided similar seed rates and cultural conditions are maintained, farmers may use either sowing method.

P. Palit, A. C Bhattacharyya y B. K. Samanta: Re-evaluación de los efectos de la siembra al voleo y en lineas sobre el rendimiento defibra del yute.

Maize yields where the nitrogen source was sulphur-coated urea combined with ordinary urea were significantly greater than those where an equivalent amount of ordinary urea was applied at identical times. Urea blended with neem cake was less effective.

K. P. Prabhakaran Nair y P. B. Sharma: Eficacia comparada de la urea normal y revestida y urea tratada con inhibidores de la nitrificación como fuente de nitrógeno para el maiz

The measurement of leaf water and osmotic potential using thermocouple psychrometry was investigated in field grown stands of sunflower and lucerne. For the chamber size and tissue dimensions used, vapour pressure equilibrium was achieved after 4 to 6 h for leaf water and osmotic potential samples. In lucerne, as the time delay between excising the leaf and sealing it in the chamber was increased from 30 s to 8 min, water apparently evaporated from the leaf sample and leaf water and osmotic potential markedly decreased. For routine psychrometer determinations of leaf water and osmotic potential in lucerne (and presumably other species) leaf samples should therefore be sealed in the chambers as quickly as possible to minimize rapid water loss and associated errors in water potential determination. Vapour pressure equilibration for sunflower capitulum and root water and osmotic potential samples was achieved only after 6 and 24 h, respectively.

G. C. Wright y D. M. Whitfield: Medición de los potentiates totales y osmóticos en tejidos de alfalfa y girasol usando psicrómetros termopar.