In experiments with transplanted rice, the first row next to the unplanted alley consistently gave significantly higher grain yields than the centre rows. The increase was progressively higher as the width of the unplanted alley between plots was increased up to 100 cm., but where the increase was very large, yields of the second outermost row were significantly decreased. No evidence was found that border effects reach beyond the second outermost row. Varieties that differed in tillering capacity, plant height, or both responded differently to the unplanted borders.

The cyanide contents were determined quantitatively of the peeled roots of 108 cassava clones, which had been previously selected as low-cyanide by the picrate leaf-test, and compared with the qualitative picrate leaf-test scores used for rapid screening. The correlation coefficient between root tuber cyanide content (dry weight basis) and leaf-test score was r = 0·36 (P < 0·01). The picrate leaf-test, while useful for screening large numbers of clones in early stages of breeding, needs augmenting by more accurate methods. There was no significant correlation between root cyanide content and dry root yield (r = 0·18). The cyanide content varied considerably between roots of the same plant; and plants of the same cultivar grown under the same conditions. Possible improvements in the effectiveness of the cyanide screening procedure are discussed.

Mineral fertiliser is a scarce input for smallholder maize farmers in Malawi. A recent provision of small amounts of subsidised fertilisers by government programmes to farmers throughout Malawi has increased fertiliser access and raised maize production, but fertiliser management and yield responses frequently remain poor. To seek ways to use the fertiliser more efficiently, we analysed the effects of low rates of N (15 or 30 kg N ha−1) and P (9 kg P ha−1) fertiliser in combination with improved weed management on maize yields in experiments on 12 smallholder farms in Chisepo, central Malawi. Several indices of N and P use efficiency were computed from the above-ground crop components and nutrient contents. Maize yield simulations were conducted using long-term rainfall records in the APSIM crop-soil system model. NP fertiliser significantly (p < 0.001) raised maize grain yield from 0.65 to 1.5 t ha−1, and twice-weeding fertilised maize significantly (p < 0.001) raised maize yields by 0.4 t ha−1 compared with weeding once (0.9 t ha−1). The agronomic efficiency of applied fertiliser N (AEN) averaged 19.3 kg grain kg N−1 with one weeding but doubled to 38.7 kg with the additional weeding. The physiological efficiency of applied N (PEN) was 40.7 kg grain kg−1 N uptake. APSIM predicted that similar or larger maize yield responses to 15 or 30 kg N ha−1 can be expected in 8 out of 10 years in areas with similar rainfall patterns to Chisepo. A financial analysis showed that the application of these small amounts of fertiliser was economic even when fertiliser was purchased from the open market, provided the crop was adequately weeded. Participatory assessments helped farmers understand the increased efficiency of fertiliser use possible with additional weeding, although some farmers reported difficulty implementing this recommendation due to competing demands for labour. We conclude that to raise the productivity and sustainability of fertiliser support programmes in Malawi, initiatives should be introduced to help identify and educate farmers on the major drivers of productivity in their systems.

Continuous rice–wheat (RW) cropping with intensive tillage has resulted in land degradation and inefficient use of water in Indo-Gangetic Plains (IGP) of South Asia. Replacement of rice with less water requiring crops such as soybean in RW system and identification of effective strategies for tillage management could result in sustainable cropping system in IGP. A field experiment was conducted for five years on an annual soybean–wheat (SW) rotation in the northwest IGP of India to evaluate effect of tillage, raised bed planting and straw mulch on yield, soil properties, water use efficiency (WUE) and profitability. In soybean, straw mulch reduced soil temperature at seeding depth by about 2.5 °C compared with no mulch. Straw mulch also resulted in slightly reduced water use and slightly higher WUE relative to their respective unmulched treatments. During wheat emergence, raised beds resulted in higher soil temperature by 1.6 °C compared with flat treatments. Bulk density and cumulative infiltration were greater in no-tillage compared with conventional tillage. Soil organic carbon in surface layer increased significantly after five years of experimentation. Soybean and wheat yields were similar under different treatments during all the years of experimentation. Soybean and wheat planted on raised beds recorded about 17% and 23% higher WUE, respectively, than in flat layout. The net returns from SW system were greater in no-tillage and permanent raised beds than with conventional tillage. Both no-tillage and permanent raised bed technologies can be adopted for sustainable crop production in SW rotation in northwest IGP. However, more studies are required representing different soil types and climate conditions for making recommendations for other regions of IGP.

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.

Accurate crop varietal identification is the backbone of any high-quality assessment of outcomes and impacts. Sweetpotato (Ipomoea batatas) varieties have important nutritional differences, and there is a strong interest to identify nutritionally superior varieties for dissemination. In agricultural household surveys, such information is often collected based on the farmer's self-report. In this article, we present the results of a data capture experiment on sweet potato varietal identification in southern Ethiopia. Three household-based methods of identifying varietal adoption are tested against the benchmark of DNA fingerprinting: (A) Elicitation from farmers with basic questions for the most widely planted variety; (B) Farmer elicitation on five sweet potato phenotypic attributes by showing a visual-aid protocol; and (C) Enumerator recording observations on five sweet potato phenotypic attributes using a visual-aid protocol and visiting the field. In total, 20% of farmers identified a variety as improved when in fact it was local and 19% identified a variety as local when it was in fact improved. The variety names given by farmers delivered inconsistent and inaccurate varietal identities. Visual-aid protocols employed in methods B and C were better than those in method A, but greatly underestimated the adoption estimates given by the DNA fingerprinting method. Our results suggest that estimating the adoption of improved varieties with methods based on farmer self-reports is questionable and point towards a wider use of DNA fingerprinting in adoption and impact assessments.

The relation between soil mechanical impedance as a result of soil drying, and root system growth (mass and length density) of rice was investigated in greenhouse and field studies. In a greenhouse experiment, soil drying for 16 days increased mechanical impedance in the 0–20 cm soil layer from near 0 to 2.5 MPa, and decreased root growth by 47% compared to the continuously flooded control. Root length density decreased with decreasing soil moisture and increasing soil mechanical impedance. In a lowland field experiment using a sprinkler irrigation gradient treatment for 19 days during the vegetative growth stage, soil mechanical impedance as low as 0.01 MPa inhibited root growth while values greater than 0.3–0.5 MPa decreased root growth and extension by 75%. The relative loss of potential root growth was continued after reflooding. Root length density, measured at flowering, was linearly related to yield.

Thirty one soils from the Savanna Zone of Nigeria were low in exchangeable potassium, ΔK°, and ΔK°° and had very low potassium buffer capacities. The soils were low in clay, which was kaolinitic. Two soils high in 2:1 layer clay minerals contained higher quantities of labile potassium. Total potassium varied with parent material. Release of non-exchangeable potassium was low. After cropping, wet/dry cycles released exchangeable potassium. Potassium deficiency is expected to occur commonly with intensive cropping in the field.

The effects of three shade densities, NPK fertilizers and fertilizer-pest/disease relationship on Amazon cocoa (Theobroma cacao L.) over a 20-year period are reported. Yields equivalent to the mean yield from the unfertilized plots were achieved in about 3, 4 and 6 years in fertilized plots under the no shade (S0), medium shade (S1) and heavy shade (S2) treatments, respectively. There were three growth phases, each characterized by a different yield trend. No appreciable changes in the exchangeable Ca and Mg were observed during the experiments, but K dropped to 68% of its initial value while the available P of the unfertilized and the S0 plots fell by 38%. Fertilizer recommendations for P and K should take account of overhead shade. The effect of N application was often negative. No direct relationship was observed between fertilizer treatment and the incidence of pest/blackpod/swollen shoot virus (SSV) disease of cocoa. SSV infection was greatest under S0. Cocoa farms in Ghana cannot maintain high yields (above 1000 kg dry cocoa ha−1) beyond 15 years of intensive cropping.

Experiments were made in glasshouses, growth cabinets and growth rooms to study the differential responses of the broad bean to water stress during the vegetative and flowering phases of growth. Plants in the flowering phase proved to be more sensitive to drought than in the vegetative period, and there were different responses (Relative Growth Rate) to temperature at different stages of plant growth. Day length and temperature influenced the position of the earliest flower initials on the stem, and intensity of flower shedding was aggravated by high temperature.

The results of research on the water relations and irrigation need of avocado are collated and reviewed in an attempt to link fundamental studies on crop physiology to irrigation practices. Background information is given on the centre of origin (Mexico and Central America) and the three distinct ecological areas where avocados are grown commercially: (1) Cool, semi-arid climates with winter-dominant rainfall (e.g. Southern California, Chile, Israel); (2) Humid, subtropical climates with summer-dominant rainfall (e.g. eastern Australia, Mexico, South Africa); and (3) Tropical or semi-tropical climates also with summer-dominant rainfall (e.g. Brazil, Florida and Indonesia). Most of the research reported has been done in Australia, California, Israel and South Africa. There are three ecological races that are given varietal status within the species: Persea americana var. drymifolia (Mexican race), P. americana var. guatemalensis (Guatemalan race) and P. americana var. americana (Antillean, West Indian or Lowland race). Interracial crossing has taken place. This paper summarises the effects of water deficits on the development processes of the crop and then reviews plant–water relations, crop water requirements, water productivity and irrigation systems. Shoot growth in mature trees is synchronised into flushes. Flower initiation occurs in the autumn, with flowering in late winter and spring. Flowers form on the ends of the branches. A large heavily flowering tree may have over a million flowers, but only produce 200–300 fruits. Fruit load adjustment occurs by shedding during the first three to four weeks after fruit set and again in early summer. Water deficits during critical stages of fruit ontogeny have been linked to fruit disorders such as ring-neck. Reproductive growth is very resistant to water stress (compared with vegetative growth). Avocado is conventionally considered to be shallow rooted, although roots extend to depths greater than 1.5 m. The majority of feeder roots are found in the top 0.60 m of soil and root extension can continue throughout the year. Leaves develop a waxy cuticle on both surfaces, which is interrupted by stomata on the abaxial surface (350–510 mm−2), many of which are blocked by wax. Stomata are also present on the sepals and petals at low densities (and on young fruit). During flowering, the canopy surface area available for water loss is considerably increased. Stomatal closure is an early indicator of water stress, which together with associated changes in leaf anatomy, restricts CO2 diffusion. There have only been a few attempts to measure the actual water use of avocado trees. In Mediterranean-type climates, peak rates of water use (in summer) appear to be between 3 and 5 mm d−1. For mature trees, the crop coefficient (Kc) is usually within the range 0.4–0.6. The best estimate of water productivity is between 1 and 2 kg fruit m−3. Soil flooding and the resultant reduction in oxygen level can damage roots even in the absence of root rot. Avocado is particularly sensitive to salinity, notably that caused by chloride ions. Rootstocks vary in their sensitivity. Both drip and under-tree microsprinklers have been/are successfully used to irrigate avocado trees. Mulching of young trees is a recommended water conservation measure and has other benefits. A large proportion of the research reviewed has been published in the ‘grey’ literature as conference papers and annual reports. Sometimes, this is at the expense of reporting the science on which the recommendations are based in peer-reviewed papers. The pressures on irrigators to improve water productivity are considered.

Pearl millet was grown on stored water at Niamey, Niger, using three row spacings. Water extraction based on neutron probe readings was compared with crop transpiration using a porometer and allied measurements. Between 23 and 52 days after sowing, plants at the narrow and medium spacings used about 77 and 100 mm of water, respectively, and those at the wide spacing used between 59 and 75 mm. Estimates of seasonal crop evaporation from leaf resistances and from the green leaf area index (GLAI) of the crops were 103, 130 and 123 mm for the narrow, medium and wide spacings, respectively. The water use per unit of dry weight produced was similar for both narrow and medium spacings but water was used more efficiently in the wide spacing. Dry weight increased in proportion to intercepted radiation with the same efficiency (1·3 g MJ−1) irrespective of spacing.

Physiomorphological differences between crops in binary mixtures can influence grain yield. A two year field study considered intra-row mixtures of two sorghum varieties, one short (KSV4) and one tall (Yarwasha), with cowpea at Maiduguri in Nigeria. The performance of the component crops depended largely on the relative proportion of cowpea and sorghum in the mixtures. Comparison of actual and expected land equivalent ratios suggested that mutual compensation and cooperation prevailed within mixtures in most instances. The system productivity index of mixtures where the proportion of sorghum: cowpea was 1:3 showed that their yield stability was greater than that of other sorghum-cowpea mixtures.

Combination de sorgol caupi en la Sabana del Sudan

The results of research on the water relations and irrigation needs of olive are collated and summarised in an attempt to link fundamental studies on crop physiology to irrigation practices. Background information on the ecology of the olive (it is native to the coastal areas of the eastern Mediterranean) and on crop development processes are presented, followed by reviews of the influence of water stress on gas exchange (stomatal conductance, photosynthesis and transpiration), crop water requirements, water productivities and irrigation systems. The olive has many attributes that help to protect it against drought, including the morphology of the leaf, and the capacity to conserve water under conditions of high evaporative demand through stomatal closure, osmotic regulation and resistance to cavitation. The concept of ‘deficit irrigation’ has been the subject of much research. Although vegetative growth is restricted, there is no convincing evidence that ‘sustained deficit irrigation’ or ‘regulated deficit irrigation’ or ‘partial root zone drying’ offer any advantages over conventional practices. Water productivities are very variable and difficult to reconcile due, in part, to biennial bearing, tree-to-tree variability and differences in tree population densities. Similarly, no clear consensus has emerged on how best to exploit the sensitivity of trunk expansion to water availability in irrigation scheduling. As production methods for this historical crop are intensified (high-density hedgerows, irrigated and mechanized orchards), so will the need to understand the role that water plays in the production processes become ever more critical.

Photosynthetic rates were monitored during the warm dry season in tea Clone 6/8 in a line-source irrigation × fertilizer experiment in the Southern Highlands of Tanzania. Irrigation and fertilizer increased photosynthetic rate both by enhancing photosynthetic rate per unit leaf area (A) in healthy leaves and by increasing the proportion of sunlight intercepted by photosynthetically efficient leaves. Irrigation-induced increases in A could be accounted for by increases in stomatal conductance (g) and associated reductions in leaf temperature. Fertilizer at an annual application rate of 225 kg N ha-1 caused increases in A associated with increases in g and improved responses to ambient CO2 concentration and illuminance (photon flux density, PFD). However, a further increase in fertilizer application rate to 375 kg N ha-1 a-1 decreased A in spite of increasing g. Light-saturation of photosynthesis occurred only at the higher fertilizer application rate. In unfertilized tea or tea fertilized at the lower rate, A decreased at a PFD of between 1400 and 2000 μmol m-2 s-l. These results are discussed in terms of the relation between photosynthesis and yield in tea.

Fotosíntesis en el té

Field measurements with a pressure chamber showed that the water potential of tea shoots was more closely related to the atmospheric saturation deficit than to the amount of water in the soil. Records for shoot growth and weather then revealed that, within a narrow range of mean temperature, the weekly rate of shoot extension was inversely related to mean saturation deficit measured at 1400 h. During periods when saturation deficit did not rise above 20 mbar, the rate of shoot extension varied linearly with mean temperature above a base temperature of 12.5–13.0°C. These correlations suggested that seasonality of shoot growth in tea can be explained largely by the independent effects of temperature and humidity. The effect of mean air temperature on the rate of shoot extension was confirmed in a glasshouse built over an established crop.

The incidence of rotting among three varieties of yams (Dioscorea rotunda, D. alata and D. cayenensis), stored in a yam barn at Ibadan for about four months, was between 0·6 and 68·5 per cent. The micro-organisms which caused rotting on experimental re-inoculation into yam tubers were the fungi, Botryodiplodia theobromae, Penicillium sp., Marcophomina phaseoli, Hendersonula toruloidea and a motile gram-negative orange-red pigmented bacterim, probably Serratia sp. This is the first record of the ability of M. phaseoli, and H. toruloidea to cause rotting in yam.

Farmers in the Lake Alaotra region of Madagascar are currently evaluating a range of conservation agriculture (CA) cropping systems. Most of the expected agroecological functions of CA (weed control, erosion control and water retention) are related to the degree of soil cover. Under farmers’ conditions, the grain and biomass productivity of these systems is highly variable and the biomass is used for several purposes. In this study, we measured biomass production of cover crops and crops in farmers’ fields. Further, we derived relationships to predict the soil cover that can be generated for a particular quantity of mulch. We used these relationships to explore the variability of soil cover that can be generated in farmers’ fields, and to estimate how much of the biomass can be removed for use as livestock feed, while retaining sufficient soil cover. Three different kinds of cropping systems were investigated in 91 farmers’ fields. The first two cropping sequences were on the hillsides: (i) maize + pulse (Vigna unguiculata or Dolichos lablab) in year 1, followed by upland rice in year 2; (ii) the second crop sequence included several years of Stylosanthes guianensis followed by upland rice; (iii) the third crop sequence was in lowland paddy fields: Vicia villosa or D. lablab, which was followed by rice within the same year and repeated every year. The biomass available prior to rice sowing varied from 3.6 t ha−1 with S. guianensis to 7.3 t ha−1 with V. villosa. The relationship between the mulch quantity (M) and soil cover (C) was measured using digital imaging and was well described by the following equation: C = 1 − exp(−Am × M), where Am is an area-to-mass ratio with R2 > 0.99 in all cases. The calculated average soil cover varied from 56 to 97% for maize + V. unguiculata and V. villosa, respectively. In order to maintain 90% soil cover at rice sowing, the average amount of biomass of V. villosa that could be removed was at least 3 t ha−1 for three quarters of the fields. This quantity was less for other annual or biennial cropping systems. On average the V. villosa aboveground biomass contained 236 kg N ha−1. The study showed that for the conditions of farmers of Malagasy, the production and conservation of biomass is not always sufficient to fulfil all the above-cited agroecological functions of mulch. Inventory of the soil cover capacity for different types of mulch may help farmers to decide how much biomass they can remove from the field.

Intercropping cereals with legumes for forage or food production is extensively used as a cropping practice in many parts of the world. A 2-year field study was conducted using common vetch (Vicia sativa), winter wheat (Triticum aestivum), triticale (× Triticosecale), barley (Hordeum vulgare) and oat (Avena sativa) sole crops as well as common vetch intercrops with each of these cereals in a 35:65 cereal:common vetch ratio based on seed numbers, to determine their ability to compete with sterile oat (Avena sterilis spp. sterilis). At nine weeks after planting, fewer sterile oat plants emerged in common vetch sole crop than in cereal sole crops. Intercropping of cereals with common vetch generally did not affect sterile oat stem number and biomass compared with cereal sole crops. At harvest, cereal sole crops provided greater total dry biomass (DB) than the common vetch sole crop. However, triticale and oat produced more DB than winter wheat and barley. In most cases intercropping reduced total DB compared with cereal sole crops. The results of this study indicated that intercropping of the four winter cereals with common vetch did not provide any significant competitive advantage against sterile oat. However, common vetch sole crop showed the greatest suppressive ability against sterile oat among the sole crops or intercrops studied.

In the Sahelian zone of Niger, there is a need to develop guidelines for integrated nutrient management, which relies on the potential nutrient sources of manure, pearl millet residue and mineral fertilizers. A fully factorial on-station experiment was conducted during the 1994 and 1995 rainy seasons at Sadoré, Niger, combining application of: (i) broadcast millet residue (300, 900 and 2700 kg ha−1), (ii) broadcast cattle manure (300, 900 and 2700 kg ha−1) and (iii) mineral fertilizer (unfertilized control, 15 kg N ha−1 + 4.4 kg P ha−1 and 45 kg N ha−1 + 13.1 kg P ha−1). Manure and fertilizer increased millet yields in both years whereas residue was effective in 1995 only. The effect of manure and residue were additive, as was the effect of manure and fertilizer but only up to 50 kg N ha−1. However in 1995, the response to fertilizer was approximately doubled in the presence of 900 or 2700 kg residue ha−1 compared to fertilizer with 300 kg ha−1 residue, indicating a strong synergistic effect. This synergistic effect was reflected in the partial factor productivity of nitrogen and phosphorous in both years. Two treatment combinations stand out as particularly relevant based on yield, partial factor productivity and nutrient balance criteria: 2700 kg manure ha−1 combined with (i) 300 kg residue and no fertilizer (95% grain yield increase); (ii) 900 kg residue ha−1 and 15 kg N + 4 kg P ha−1 (132% grain yield increase). There is a need for similar, long-term experiments to confirm the present results.