Increasing transplanting costs in southeast Asia have encouraged farmers to adopt labour-saving crop establishment techniques for rice, such as wet seeding. This practice has been accompanied by an increase in weed problems and a shift in the dominant species to grassy weeds. Other problems are encountered with wet seeding but weeds are the most severe and most widespread constraint. It is impossible to produce rice economically without a well-planned weed control programme. The problems caused by weeds in wet-seeded rice and ways of controlling them by preventive, ecological, managerial, physical and chemical means are discussed in this paper. Herbicides are the most cost-effective weed control method in wet-seeded rice but there is a need to reduce the almost total reliance on them for weed control; cultural practices need to be integrated with judicious herbicide use. Worldwide concerns over environmental issues also need to be addressed.

Control de malezas en el arroz de siembra húmeda

Fertilizer use in the semi-arid tropics is considered risky because of the unreliable rainfall pattern. A flexible system of fertilization was devised which allowed for high yields in wet years, and reduced financial losses in dry years. Relatively immobile nutrients, such as phosphorus, potassium and sulphur, were applied at rates determined from generalized soil properties and the total nutrient uptake required for a crop to achieve its maximum yield potential in a season with average rainfall. Nitrogen was applied as a series of split applications, which were adjusted during the season according to the degree of water stress observed. This approach was tested in on-farm trials for maize production on nutrient-poor sandy soils in three regions of Zimbabwe, and resulted in larger yields and profits than current recommendations, providing an accept able level of financial return. Small further yield increases were possible, at acceptable returns, when these fertilizer practices were combined with the establishment of tied-ridges by ox-drawn implements, 30 days after planting. By careful estimation of yield potential, and appropriate adjustments to soil fertility, this system of soil management could be applied to other cropping situations and other areas with variable rainfall.

Uso de fertilizantes para condiciones de precitación variable

Field trials conducted previously in Niger have shown that in years when the onset of the rains is 15–20 days earlier than average, the long growing season can be exploited by growing a relay crop of millet (Pennisetum glaucum) and cowpea (Vigna unguiculata). In the trials reported here, the advantages of relay cropping were compared with intercropping with improved management and intercropping under traditional management during the 1989, 1990 and 1991 rainy seasons at the ICRISAT Sahelian Center, Sadore, Niger. The length of the growing season varied from 139 to 150 days over the three seasons. The relay crop produced more dry matter and leaf area and yielded more than the intercrops in all three years, confirming that in years when the onset of the rains is early, relay cropping with millet and cowpea is a better option than growing the same two species as an intercrop. Relay cropping avoids the competitive effects inherent in intercropping systems, while offering the additional advantages of rotating cereals with legumes.

Mijo y caupí en sistemas de relevo y de cultivo simultáneo

Fourteen on-farm sites used in barley fertilizer trials were retained for a second year and, without further fertilizer, sown with a mixed vetch/barley forage crop. Rainfall at the sites ranged from 149 to 451 mm, and linear regression on rainfall accounted for 76% of the variation in hay yield between the sites, with a greater rain-related increase in vetch than barley. Soil nitrogen and phosphorus content at planting time in the second year was significantly affected by the rate of fertilizer application in the previous year. Previous phosphate fertilization tended to increase hay yields, particularly at sites where levels of native available phosphate were poor. However, responses to residual fertilizer nitrogen were small and variable.

Respuesta al N y P residual en el norte de Siria

This paper offers guidelines for successful crop rotation trials derived from experience at ICARDA in the period 1980–1990. Emphasis is placed on the need to determine realistically the likelihood of the long-term, successful completion of the trial and the importance of considering carefully experimental design criteria prior to trial establishment. This involves assessment of what primary and secondary information to collect for the trial, and over what duration, in order to ensure an appropriate return for the large investment generally undertaken in any crop rotation trial.

Principios para ensayos con rotación de cultivos

Reports that an Indonesian soyabean (Glycine max) line (TGm737p) shows greater seed storage longevity than an American cultivar (Bossier), and that the cross between them (TG×536–02D) is intermediate between the two parents in this regard, were investigated by growing all three genotypes under identical conditions (30°C/20°C, 12 h d-1 photoperiod) in a plastics house. Seeds were harvested when moisture content had declined naturally to 14–15% (wet basis), and then stored hermetically at five different moisture contents at 40°C. Analyses of the resultant seed survival curves revealed that the seed lot constant Ki (a measure of potential longevity) was greatest in TGm737p and least in cv. Bossier, the cross being much closer to the American parent in this regard. The negative logarithmic relation between σ (standard deviation of the frequency distribution of seed deaths in time) and moisture content did not differ (p > 0.10) among the three genotypes. Moreover, this relation was close to that predicted by earlier results for four different genotypes. Similarly, there was no significant difference (p > 0.25) among genotypes in the negative semi-logarithmic relation between σ and seed equilibrium relative humidity (rh): the regression slope was equivalent to a doubling of longevity for each 8.0% reduction in rh. Finally, comparison of the negative logarithmic relation between absolute longevity (50% viability period) and seed moisture content confirmed that TGm737p showed greatest longevity (p > 0.005), but failed to show a difference in absolute longevity between cv. Bossier and the cross (p > 0.25). The results confirm that the seed viability equation of Ellis and Roberts (1980a, b) provides a framework within which the seed longevity of different genotypes can be compared. They suggest too that there is considerable doubt as to whether or not the greater potential longevity of the Indonesian line TGm737p has been incorporated successfully into the cross TG×536–02D.

Longevidad de la semilla de soja

The fate of seed of six annual medics produced in spring 1987 and used in the pasture phase of cereal-pasture rotations was followed over four years. The medics were sown at low and high density, and seed set beyond 1987 was prevented by cultivation in 1988, and by herbicides in 1989 and 1990. The breakdown of hard seed was measured in the summer and autumn of each year, and the number of seedlings was counted after the first seasonal rains in the final three years. The survival of seeds buried at three depths was monitored. Seed of Medicago noeana lived longer than that of M. rotata and M. polymorpha. Survival of M. rigidula and M. truncatula seed was intermediate and there was no effect of planting density on survival. Breakdown of hard seed was least in the first year and increased in the second and third years. Within any year breakdown began in late summer or early autumn, beginning earlier as the seed aged. Seed burial enhanced germination in the year following seed set, depressed it in the second and third years, and had little effect in the fourth year. The net effect of seed burial on survival was therefore slight. The results suggest that the use of medics that have harder seeds than currently available cultivars, and medics in which germination is delayed until late in the autumn, will increase the sustainability and productivity of cereal-pasture rotations in semi-arid areas.

Dinámica de semilla de los Medicago spp anuales en Siria

Four peanut cultivars of Spanish or Virginia botanical type and varying time to maturity were grown at a range of plant densities (44 000 to 352 000 plants ha-1) and spatial arrangements under irrigated conditions in sub-tropical southern Queensland, Australia. Total and pod dry matter production of the very early maturing Spanish cultivar Chico showed strong positive responses to increased plant density up to the highest density tested. Responses were less pronounced for the later maturing Spanish cultivar McCubbin and were minimal for the Virginia cultivars Early Bunch and Mani Pintar. All cultivars were insensitive to spatial arrangements. Accumulation of total dry matter, adjusted for the higher synthesis costs of oil and protein during seed filling, was well correlated to intercepted photosynthetically active radiation (PAR). Much of the variation in dry matter production among cultivars and plant populations could be accounted for by the effects of differing leaf area duration on cumulative intercepted PAR. Radiation use efficiency was negatively associated with the canopy extinction coefficient (k) within most sowing dates, and also negatively associated with minimum temperature across sowing dates for all cultivars except Mani Pintar.

Crecimiento y eficacia del use de radiación en el maní

The partitioning of dry matter between vegetative and reproductive yield components was analysed for four diverse peanut cultivars at a range of sowing dates and plant populations in sub-tropical Queensland, Australia. Rates of accumulation of pods (pod addition) varied significantly with both cultivar and sowing date. Within cultivars, much of this variation could be attributed to variation in crop growth rate during the critical pod addition period. The proportion of current assimilate distributed to pods depended on inherent cultivar characteristics and also correlated well with the current crop growth rate relative to the crop growth rate during pod addition (that is, with relative source activity). Neither plant density nor spatial arrangement had any significant effects on patterns of assimilate distribution. All cultivars appeared capable of remobilizing stored assimilate to maintain constant rates of pod yield increase despite fluctuations in crop growth rate which might be expected to produce short term source limitations to pod yield accumulation. The harvest index (HI) increased linearly during the entire pod fill period in all cultivars except the very early maturing Spanish cultivar Chico. Neither density nor spatial arrangement affected rate of increase in HI except in Chico, where increased density produced increased rates of increase in HI. Variation in the rate of HI increase among sowing dates was small.

Distributión de la materia seca en el maní

The diffusion of rice varieties introduced into an area with poor production potential and problems of soil degradation was studied in a farming systems research and extension programme. Two rice varieties, BW. 272–3 and At. 85–1 were identified as appropriate for the area and efforts made to transfer the experimental results to the farmers. BW. 272–3 was widely adopted by farmers in the lower, intermediate and upper areas of the recommendation domain but adoption of At. 85–1 was confined to the intermediate and upper areas. Adoption of rice varieties was mainly governed by the characteristics that allowed a variety to perform well under adverse conditions and by the farm gate price it could command, which depended on grain pericarp colour.

Difusión de nuevas variedades de arroz

Varietal selection is particularly difficult for very heterogeneous environments where farmers have a range of preferences. To address these issues in Rwanda, local bean experts, generally women, are invited to the research station to assess cultivars and to select those they prefer for their plots. Farmers use observed yield as only one of several criteria to predict varietal performance in their more stressed home environments. The varieties they choose are often higher yielding on-farm and are retained longer by farmers than those selected by the breeders. The results of early farmer involvement also compare favourably with those of the standard breeding programme in terms of enhanced genetic diversity and reduced research costs. There are few pre-conditions for developing such a client-driven breeding programme.

Papel del agricultor en la selección de variedades para una estación inaestigación